Advances in atomic emission, absorption and fluorescence spectrometry, and related techniques

Contents

• 1 Sample introduction
  • 1.1 Preconcentration
  • 1.2 Chemical vapour generation
  • 1.3 Nebulization
  • 1.4 Solid sampling
  • 1.5 Electrothermal vaporization

• 2 Instrumentation
  • 2.1 Spectrometers
  • 2.2 Sources and atom cells
  • 2.3 Detectors
  • 2.4 Background correction
  • 2.5 Data acquisition and control

• 3 Fundamentals
  • 3.1 Plasmas
  • 3.2 Flames
  • 3.3 Furnaces

• 4 Laser-based analytical atomic spectrometry
  • 4.1 Lasers as energy sources
  • 4.2 Lasers as sources of intense monochromatic radiation

• 5 Chemometrics

• 6 Coupled techniques for speciation
  • 6.1 Capillary electrophoresis
  • 6.2 Gas chromatography
  • 6.3 Liquid chromatography

• References

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Abstract

This Atomic Spectrometry Update is a new review resulting from an amalgamation of the reviews formerly appearing under the titles 'Atomic Emission Spectrometry' and 'Advances in Atomic Absorption and Fluorescence Spectrometry and Related Techniques'. An attempt has been made to address only novel and interesting developments so less emphasis has been placed on applications, which are covered in greater detail in the applications reviews, and more on technical developments. The structure of the review has been changed somewhat to keep pace with the evolution of the field. The three primary techniques of atomic absorption, emission and fluorescence spectrometry are all dealt with under each individual heading, so that 'Sample Introduction', for example, deals with sample introduction methods pertaining to all three techniques. There are now separate sections on 'Laser-based Analytical Spectrometry' and 'Coupled Techniques for Speciation Studies' to reflect the growing importance of these fields. It is hoped that these developments will result in a more integrated overview of developments in these related techniques, and any comments on the merit of this approach, or suggestions for 'fine tuning' would be welcome.

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1 Sample introduction

1.1 Preconcentration

The published articles or conference presentations that have been included in this section have been divided into two broad categories. The first of these contains a survey of procedures in which the sample pretreatment involved either: discrete sampling; continuous flow; segmented flow; use of fluid flow characteristics for mixing; controlled residence times; mass transfer between phases (solid phase extraction, precipitation or liquid–liquid extraction); or direct introduction of the liquid phase into the spectrometer. This first section is simply titled 'flow injection'. The second section contains preconcentration procedures that do not fall within this rather broad definition of FI.

1.1.1 Flow injection. Developments in the last two or three years in the use of FI techniques with ETAAS have been reviewed.¹ Topics covered included air-segmentation and new procedures for precipitation and liquid–liquid extraction (LLE) as well as the use of the knotted tubular reactor as a sorption medium. Several other English language reviews have appeared^2–5 as well as ones in Russian,⁶ Chinese⁷ and Japanese.⁸ The use of FI procedures with AAS in dissolution testing of oral dosage forms has been reviewed.⁹ Stability constants have been determined¹⁰ based on FI-ion exchange with FAAS.

An FI microsampling system for the determination of Fe in soils by FAAS has been developed.¹¹ The LOD was 0.4 µg ml⁻¹ and the throughput 300 h⁻¹. Copper was determined in beer¹² at 30 samples h⁻¹. Honey samples were diluted in a continuous flow manifold¹³ with a solution containing HCl and lanthanum for the determination of Ca, Mg and Zn by FAAS.

The majority of the published papers concerning FI with atomic spectrometry in this review period are concerned with some aspect of preconcentration and/or matrix separation. Precipitated metal hydroxides were collected on zirconia spheres in a micro-column,¹⁴ re-dissolved in 70 µl of dilute HNO₃ and determined directly by FAAS. Detection limits of between 0.1 and 0.5 µg l⁻¹ for Cu, Ni, Pb and Zn were obtained. Zinc was preconcentrated¹⁵ as the thiocyanate complex on polyurethane foam (packed into a micro-column) and determined by FAAS following elution with water–acetone–nitric acid (68 + 30 + 2). For 3 min loading, the LOD was 0.8 µg l⁻¹. Liquid–liquid extraction is less popular than solid phase extraction, probably because of the difficulty of phase separation; however, a few LLE methods have been described. Chinese workers determined (a) Pb in biological specimens¹⁶ by extraction of the iodo-complexes into IBMK, and (b) Cr and Ni in human hair¹⁷ by extraction of the DDC complexes into the same solvent. In both cases quantification was by FAAS. In a 'wetting film' system the oxine complex of Cu was extracted¹⁸ into a film of IBMK on the walls of a PTFE reactor followed by back extraction into a slug of HNO₃ solution sandwiched between two air segments. Even though the eluent was delivered to the FAA spectrometer at 0.6 ml min⁻¹, the throughput was 30 h⁻¹ with an enrichment factor of 43 for an LOD of 0.2 µg l⁻¹. Four parallel hollow fibres were impregnated¹⁹ with bis(2-ethylhexyl)phosphate in kerosene for the preconcentration of Pb from an aqueous formate buffer solution. Again, elution was with dilute HNO₃. Wang and co-workers described²⁰ a procedure in which metals were retained by an iminodiacetate–ethylcellulose membrane mounted on either side of the flow channel. The eluent was 300 µl of dilute HNO₃ for the determination of Cd, Co, Cu, Mn, Ni, Pb and V by FAAS or ICP-OES with LOD between 0.2 and 1 µg l⁻¹ and enrichment factors from 7 to 18. Chelation was also used to retain Cd²¹ on a poly(aminophosphonic acid) resin for determination in mussels. The solution LOD was 0.6 µg l⁻¹ for a sample volume of 3.4 ml with preconcentration factors ranging from 16 to 47 for sample volumes of 3.4–10 ml. The eluent was dilute HCl. The procedure was also applied in the determination of Co in steel.²² The analyte was retained at pH 2 and eluted with dilute HNO₃. Several groups of Chinese workers have devised preconcentration methods based on chelation with a solid phase extractant packed into a microcolumn: these include the determination of Pb in nickel sulfate by retention on YZ 50102,²³ the determination of Ag in geological materials by retention on triphenylphosphine-loaded glass beads,²⁴ the determination of Cu, Fe^III and Ni in caustic soda by retention on 'D401 chelating resin',²⁵ and the determination of copper by retention of the complex with 8-hydroxyquinoline-5-sulfonic acid on an anion-exchange resin, D-290.²⁶

Holcombe and coworkers compared²⁷ the performance of poly(L-cysteine) with that of 8-hydroxyquinoline. Both chelating agents were immobilized on controlled pore glass. The analytes were Cd, Cu and Pb and the matrices were synthetic sea-water, cobalt and nickel and Seawater CRM. Recoveries of 50 µg l⁻¹ of Cd and Pb from synthetic sea-water were quantitative for both resins; however, for solutions containing 10 000-fold excess of cobalt and nickel, the recoveries of Cd and Pb were low with the oxine material, but quantitative for the poly(L-cysteine) material. Neither material gave acceptable recovery for Cu. For the determination of Cd and Pb spikes in the CRM, the recoveries were variable. Recoveries of Cd were high, though that of Pb was acceptable with CASS-1 but low for NASS-2. Breakthrough curves were used to investigate the nature of the various binding sites on the materials and the various binding constants and capacities were estimated. Some preliminary results²⁸ for the behaviour of poly(L-aspartic acid) immobilized on gold-plated minigrids have been obtained. Sea-water was the matrix for one of the relatively few sorbent extraction FAAS procedures published in this review period.²⁹ The analytes, Co, Fe, Mn, Ni and Zn, were retained as the complexes with 5,7-dichloro-8-hydroxyquinoline on C₁₈ silica and eluted with acidified (pH 2) methanol. Sensitivity enhancements of between 60 and 80 were obtained with LOD ranging from 0.5 to 4 µg l⁻¹ for a 1 min loading time at 7 ml min⁻¹. The procedure was suitable for the determination of Mn and Zn in sea-water samples, but the concentrations of Co, Fe and Ni were too low to be determined. There was no discussion of the effect of sample volume on the LOD.

In the determination of Cr^III and Cr^VI,³⁰ the Cr^VI was retained as the APDC complex on the interior surface of a PEEK tube and then eluted with IBMK. The Cr^III was retained from a solution of potassium hydrogen phthalate on C₁₈; Cr^VI was not retained, but could be preconcentrated downstream via the APDC–PEEK tubing procedure. The Cr^III was eluted with methanol. For 5 ml sample volume the LOD were 0.9 and 0.5 µg l⁻¹ for Cr^III and Cr^VI, respectively. Preconcentration, following ion chromatographic separation, on a solid-phase extraction cartridge³¹ with AAS detection allowed the quantification of both cations (Ca and Mg) and anions (halides, nitrate and sulfate), apparently. Cyclamate was determined³² by an indirect continuous flow FAAS procedure in which oxidation to sulfate was followed by precipitation of the lead salt. Following collection on a filter and washing with ethanol, the precipitated lead sulfate was dissolved in ammonium acetate solution. The Ca and Mg content of brines was determined³³ by a combined low- and high-pressure system. The metals were retained as ion pairs of the xylenol orange chelates with tetrabutylammonium hydroxide on C₁₈ in the low pressure system and eluted with methanol and delivered to the spectrometer via a hydraulic high pressure nebulizer.

The Ni and Sn content of brass was determined³⁴ by a procedure in which the metals were dissolved electrolytically in a flow system with collection of the electrolyte in the autosampler cup of an AA spectrometer with ETA. Apparently a sample could be analysed every 2 min. The procedure was used to determine³⁵ Al in aluminium alloys, and Cu and Zn in brass and bronze, and has been further developed³⁶ for the determination (in metal alloys) of Cu, Pb and Zn by FAAS and Fe, Ni and Sn by ETAAS.

Adams and co-workers have devised a number of preconcentration procedures for use with ETAAS. Blood was analysed³⁷ for Cd and Pb; the complexes with ammonium diethylphosphorodithioate were retained on the inner walls of a knotted PTFE reactor followed by elution with 35 µl of methanol. The determinations were made with a pyrolytic graphite tube pre-treated with iridium. The LOD were 0.2 and 2 ng l⁻¹ for Cd and Pb, respectively. Platinum was determined³⁸ in the same matrix by an analogous procedure in which the analyte was retained as the APDC complex. For a 90-s preconcentration at 8.8 ml min⁻¹, a preconcentration factor of 112 and an LOD of 10 ng l⁻¹ were obtained. The performance characteristics of the procedure were improved³⁹ by pre-coating the reactor with the chelating agent. This avoids the need for reagent addition, there are no losses to tubing walls outside the reactor, there is no need for a pre-fill step between samples of different concentrations and it allows for better optimization of the processes of reagent sorption and analyte preconcentration. This new procedure was applied³⁹ to the determination of Cu and Mn in some biological RMs. The chelating reagent was 1-phenyl-3-methyl-4-benzoyl-5-one, which was first pumped through the reactor as a 0.05% solution for 30 s at 2.5 ml min⁻¹. The excess was removed by air pumped at 5 ml min⁻¹ for 15 s. The sample was delivered to the reactor at 2–2.5 ml min⁻¹ for 30 s and the liquid was flushed out with air. The adsorbed chelates were eluted with 30–40 µl of methanol delivered by an air carrier directly to the pyrolytic graphite-coated atomizer. The enhancement factors were 34 and 21 and the LOD were 6 and 5 ng l⁻¹, for a sampling frequency of 22 h⁻¹. Single cell protein, cod muscle and freeze-dried animal blood were accurately analysed.

A microcolumn of Muromac-A-1 chelating resin was incorporated⁴⁰ into the autosampler capillary for the preconcentration of Cd, Co and Ni from sea-water matrices. Sample volumes of 800–1800 µl were loaded through the resin and the retained analytes eluted with 20% HNO₃; LOD were 0.1, 7 and 33 ng l⁻¹ for Cd, Co and Ni, respectively. Three saline water RMs (NASS-4, CASS-3 and SLEW-1) were accurately analysed. A somewhat similar procedure was subjected⁴¹ to a Simplex optimization procedure. The microcolumn was packed with silica gel functionalized with 1-(di-2-pyridyl)methylene thiocarbonohydrazide, the analyte was Ni and the eluent 2 M HNO₃. The optimized procedure had an LOD of 60 ng l⁻¹ and a throughput of 36 h⁻¹ for 60 s preconcentration.

A solid phase extractant, PbO₂, highly selective for lead consisting of a crown ether macrocyle immobilized on silica gel was used in the analysis⁴² of some biological RMs. The material retained Pb over a wide range of acidities (0.08–3 mol l⁻¹ HNO₃) and the only interferences were from barium, potassium and strontium, whose ionic radii are similar to that of Pb. For a loading time of 20 s at 3 ml min⁻¹, with elution into 46 µl of 0.03 mol l⁻¹ EDTA (delivered by an air carrier to the furnace) an enhancement factor of 23 and an LOD of 2 ng l⁻¹ were obtained. The retention efficiency was 70% and the throughput was 23 h⁻¹. For the determination of Co⁴³ a column of fullerene (C₆₀) was used to retain the APDC complex. An autosampler cup was used as the interface between the FI system and spectrometer. The analyte in a 25-ml sample volume was eluted with 500 µl of IBMK, delivered with a nitrogen carrier, and the LOD was 8 ng l⁻¹. A similar procedure has been applied⁴⁴ for the determination of Bi in steels and aluminium alloys. The analyte was retained on activated carbon as the diethyldithiophosphoric acid complex. The aluminium or iron complexes were washed from the column with 500 µl of dilute HCl and then the analyte complex was eluted to an autosampler cup with 500 µl of ethanol. The enhancement factor for 10 ml of sample (loaded at 2.5 ml min⁻¹) was initially given as 37 and the LOD was 50 ng l⁻¹. When the work was published,⁴⁵ the enhancement factor was corrected to 14. Retention of Al and Cu on a microcolumn of Chelex-100⁴⁶ followed by elution with 1 ml of dilute HNO₃ was used in a procedure for the analysis of dialysis concentrates. For the determination of Cd in sea-water,⁴⁷ a column of C₁₈ retained either the complex with 4-(2-pyridylazo)resorcinol or 2-(2-pyridylazo)-5-dimethylaminophenol. A 50 µl portion of the methanol eluent was injected into the graphite furnace. The LOD was between 2 and 4 ng l⁻¹. In a somewhat similar procedure, a C₁₈ column retained⁴⁸ the APDC complex of Cd from a sea-water matrix. The analyte was eluted with 80 µl of methanol for a LOD of 42 pmol in 2 ml (0.2 ng l⁻¹). Chinese workers⁴⁹ determined Cd, Cu and Pb retained as their 5-sulfo-8-hydroxyquinoline complexes on a C₁₈ column. The analytes were eluted with a 180 µl mixture of HCl and HNO₃ and a 30 µl sub-sample was transferred by an air carrier to the atomizer. The enrichment factors were 22, 28 and 26 and the LOD were 0.7, 4.2 and 5.4 ng l⁻¹ for Cd, Cu and Pb, respectively. Burguera et al. devised a flow-based procedure⁵⁰ for the determination of Bi in blood in which the sample was first digested in a flow-through reactor in a microwave oven, the analyte being separated by precipitation with tin(II) from alkaline solution and collected in a knotted coil in an ultrasonic bath, and finally the precipitate was dissolved in HNO₃ and a 20 µl sub-sample was introduced into the atomizer. The LOD was 0.4 µg l⁻¹. Krug et al.^51,52 preconcentrated Pb electrochemically on a W-coil atomizer, which was temporarily functioning as the cathode in a flow-through electrolysis cell. For a 2-min preconcentration at 1 ml min⁻¹, an enrichment factor of 25 and an LOD of 0.2 µg l⁻¹ were obtained.

Martinez and co-workers have developed FI-SPE procedures for use with ICP-OES.^53,54 Bismuth was determined⁵³ in urine by retention of the 8-hydroxyquinoline complex on an Amberlite XAD-7 column, with elution by HNO₃, with an LOD of 30 ng l⁻¹ (100 ml solution) by use of an ultrasonic nebulizer. However, for the determination of Pb in tap water,⁵⁴ retention of the APDC complex on a knotted reactor with dissolution in 4 mol l⁻¹ HCl was employed. For a 10-ml sample the LOD was 200 ng l⁻¹ (again ultrasonic nebulization was employed). An iminodiacetate–agarose adsorbent, IDA Novarose, was used⁵⁵ to accumulate Cd, Co, Cr, Cu, Fe, Mn, Ni and Zn at pH 4–8, prior to detection by ICP-OES. The retention of the trivalent cations (Fe and Cr ) was slow, whereas the divalent cations could be loaded at flow rates up to 80 ml min⁻¹. The procedure was applied to river, tap and lake waters.

Chinese workers⁵⁶ were able, apparently, to glue Au to a PVC nylon 6 resin microcolumn, prior to elution with thiourea and detection by ICP-OES. Following digestion in 30 ml of 50% aqua regia, 3 ml of 10 g l⁻¹ animal glue were added and, after making up to 100 ml, a sub-sample was injected in the FI manifold for direct transport to the column. The LOD was 2 µg l⁻¹.

Two solid phase extraction columns in series⁵⁷ allowed cationic (on Chelex 100) and anionic (on AG-1 X-8) species of Cu and Mn to be preconcentrated. Following elution with dilute HCl the elements were determined by ICP-OES. The procedure was applied to the speciation of these elements in milk. On the addition of an acetate buffer, casein and other macromolecules precipitated; analysis of the filtrate and precipitate allowed the casein-bound metal content to be determined.

An FI system was used⁵⁸ to introduce a 25-µl slurry sample in an air carrier down a glass tube inserted into an axial hole in a carbon electrode into an arc or spark. Sediment samples were ground and sieved to particle sizes of less than 38 µm and slurried in nitric acid with lanthanum oxide in an ultrasonic bath. The analyte was Al, and 40 determinations per hour were possible against aqueous standard solutions. Chinese workers⁵⁹ used FI as the interface between capillary isotachophoresis and ICP-OES. Unlike capillary electrophoresis, which has a flow rate of 0.02–1 µl min⁻¹, the eluent from the isotachophoresis system flows at about 1 ml min⁻¹, which makes interfacing with the spectrometer relatively simple. The system was used to separate and determine Cu and Cu–EDTA. The combination of ion chromatography with plasma source spectrometric detection has been critically compared⁵ with other techniques such as NAA, GD-MS and ETV-ICP-MS.

1.1.2 Off-line preconcentration. Publications in this section have been divided into two broad categories: the first of these contains descriptions of procedures that would be relatively easy to interface directly with an atomic absorption or emission spectrometer, and the second contains descriptions of procedures that would be difficult to interface with such a spectrometer. Within each of these broad categories, the material is sub-divided into sections dealing with FAAS, ETAAS and ICP-OES.

Baker's yeast (Saccharomyces cerevisiae) immobilized on sepiolite (the calcareous internal shell of the cuttlefish) was used⁶⁰ to preconcentrate Fe and Ni. Sample volumes of up to 500 ml were processed at 2.5 ml min⁻¹. Elution was with 1 mol l⁻¹ HCl and the LOD were 60 and 90 µg l⁻¹ for Fe and Ni, respectively. A brass SRM was analysed. The procedure has also been applied⁶¹ to the determination of Cd, Cu and Zn in river and sea-waters. The cupferron complexes of Cd and Pb were collected⁶² on activated carbon in a batch procedure for the analysis of urine. After filtering and drying the retained metals were dissolved in dilute HNO₃ and determined by FAAS with the aid of a slotted tube atom retarder. The LOD were 0.03 and 0.3 µg l⁻¹ for Cd and Pb, respectively, for a 210 ml sample and 1.5 ml HNO₃ solution. A melamine–urea resin was used⁶³ in a Cr speciation procedure. The resin only retains Cr^VI, and thus this species can be preconcentrated and determined after elution with sodium acetate solution. Following oxidation with H₂O₂, total Cr was determined by the same procedure and thus Cr^III was found by difference.

Chinese workers have described a number of procedures including: (a) solid phase extraction on polyaminophenol resin to accumulate iron;⁶⁴ (b) solid phase extraction on dithizone-loaded silica to concentrate Cu; (c) the co-precipitation of Cd, Co, Mn and Ni with either copper pyrrolidinedithiocarbamate (PDC) or magnesium 8-hydroxyquinoline; and (d) the collection of Pb in soy sauce on activated carbon. Vietnamese workers described⁶⁵ a procedure for the determination of Au by FAAS following either precipitation with tellurium or extraction from an acid solution into IBMK. A liquid–liquid extraction procedure in which the analytes (Fe, Mo, Sn and V) were extracted from acid solution in the presence of methylenedisalicylohydroxamic acid into IBMK containing tributyl phosphate was used to analyse⁶⁶ food, sediment and crude petroleum.

A robotic sample pre-treatment procedure has been devised⁶⁷ which not only preconcentrates the analyte (Cu) as the PDC chelate on the interior of a knotted reactor but also performs sample weighing and digestion. The procedure was applied to the analysis of environmental and biological materials by ETAAS. Preconcentration on Dowex 1X8 or Chelex-100 has been used⁶⁸ to preconcentrate trace elements in high purity gallium, arsenic and arsenic oxides. The desorbed elements were determined by ETAAS, FAAS, NAA and ICP-OES. A diethylaminoethylcellulose anion exchange resin was used⁶⁹ to collect Cr^VI while other cations were collected by an iminodiacetate resin. The eluted metals (Cd, Co, Cr, Ni and Pb) were determined in some pharmaceutical substances by ETAAS or TXRF. Mineral waters were analysed⁷⁰ for Cd, Co, Ni, Pb and V by retention on a cellulose material modified with 8-hydroxyquinoline-5-sulfonic acid. The analytes in a 20-ml sample volume were eluted into less than 1 ml and determined by ETAAS. The LODs were 4, 100, 40, 60 and 200 ng l⁻¹ for Cd, Co, Ni, Pb and V, respectively. Elution with IBMK was used⁷¹ to remove Cd, Co, Cr, Cu, Fe, Mn, Ni and Pb from a column of immobilized sodium DDC or APDC with direct introduction of the solvent into the atomizer. The LOD ranged from 40 to 300 ng l⁻¹. The same solvent was used⁷² to elute organo-Sn compounds from an Amberlite XAD-2 column impregnated with tropolone. The LOD was 10 ng l⁻¹. Polyethylene powder conditioned with 1-(2-pyridylazo)-2-naphthol retained⁷³ the complexes of Cd, Cu, Pb and Zn prior to elution with perchloric acid solution. The procedure was applied to the analysis of haemodialysis concentrates. Japanese workers (describing their results in a Chinese journal) showed: (a) that In, Rh, Ru, Sr and V could be collected⁷⁴ as precipitates with chitosan and that the LOD for Ru was 60 ng l⁻¹; and (b) that Cu and Pb could be collected⁷⁵ as the PDC complexes on a miniature membrane filter which was then dissolved in methylcellosolve prior to analysis by ETAAS.

Polyaniline⁷⁶ extracted Cd, Cu, Pb and Sb from acid digests to which potassium iodide had been added in a procedure whose goal was to separate the analytes from potentially interfering matrix components (the volume of eluent was greater than the sample volume). The analytes were eluted with dilute HNO₃ and determined by ICP-OES; the recovery for Sb was about 75%. Coal fly ash (CRM) and a sea plant material were analysed. For the determination of Y, Bi, Th, U and other REE in steel,⁷⁷ all the metallic components of the sample were retained from a solution containing oxyethylidenediphosphonic acid (OEDPPA) on a column of tetraphenylmethylenediphosphine oxide on macroporous styrene–divinylbenzene. The iron was removed by rinsing with acid solution (0.5 mol l⁻¹ HNO₃), followed by the analytes with 0.5 mol l⁻¹ OEDPPA at pH 4.5 for determination by ICP-OES or ICP-MS. Recoveries were >80% for all analytes except Th, for which the recovery was 65%. The 'metalfix chelamine resin' was used⁷⁸ to retain Cd, Ni and Pb from pH 4 solutions with elution by 1 mol l⁻¹ HCl. The eluent was analysed by ICP-OES and the procedure was applied to the analysis of fir wood. In the determination of organo-P (as phosphino-polycarboxylates) in oil production waters, silica-immobilized C₁₈ was used⁷⁹ to preconcentrate the analyte species and separate inorganic P. The analyte species were eluted with a boric acid buffer at pH 9 and determined by ICP-OES and ICP-MS with either cross-flow or ultrasonic nebulization (for which the LOD was about a factor of 2.5 better—20 µg l⁻¹ for ICP-OES and 0.5 µg l⁻¹ for ICP-MS). Japanese workers have devised procedures for: (a) the preconcentration,⁸⁰ by a factor of 1000 at µg l⁻¹ concentrations, of As on a column consisting of the iron(III) complex of lysine polyacetic acid; and (b) the preconcentration of Cd, Co, Cu, Ni, Pb, V and Zn,⁸¹ by factors of about 100 from sea and river waters, on a column containing xanthurenic acid immobilized on silica gel. In the former procedure, As was determined by ICP-OES, and in the latter both ICP-OES and ETAAS were used.

The liquid–liquid extraction of Pb⁸² from an acid solution containing zinc hexamethylenedithiocarbamate into 2,6-dimethylheptan-4-one was used for sample pre-treatment in the analysis of tap and ground water samples by FAAS. The Pb was back-extracted into an aqueous acid solution containing copper. The LOD was 70 ng l⁻¹. For the determination of Zn in sugar by FAAS,⁸³ the analyte was extracted as the xanthate complex into a surfactant phase and then back-extracted as the EDTA complex when the solution was cooled. A column of Amberlite XAD-4⁸⁴ retained the complexes of Cd, Co, Cu, Ni and Pb with 1-nitro-2-naphthol in a procedure for the analysis of chemical grade potassium salts by FAAS. Total Hg in drinking water and methylmercury in air were determined⁸⁵ by an ETAAS procedure in which the complexes with 2,3-dimercaptopropane-1-sulfonate was retained on Sep-Pak C₁₈ cartridges. After elution with methanol, a sub-sample of 50 µl was introduced into the furnace. The LOD was 50 ng l⁻¹. The complexes were eluted with HNO₃ in acetone, which was evaporated to near-dryness and the residue taken up in HNO₃. The LOD ranged from 20 to 60 ng l⁻¹. Chinese workers developed a procedure for the indirect determination of Si in a cobalt compound by extraction of the heteropolymolybdate into IBMK. After washing the organic layer to remove excess isopolymolybdate, the molybdenum was determined by FAAS with direct introduction of the organic solvent. The interference from phosphorus was overcome by a prior extraction of the phosphomolybdate into ethyl acetate. The LOD for Si was 0.2 mg l⁻¹. Also in the Chinese literature are descriptions of: (a) a flotation procedure⁸⁶ for the determination of Ag, Cd, Cu and Pb in which the metals were collected from a mixture of potassium iodide, methylene blue and toluene; (b) a procedure for the preconcentration of Cu on microcrystalline naphthalene loaded with dithizone;⁸⁷ and (c) a procedure for the determination of Cd by co-precipitation with copper sulfide. In the first procedure, the analyte was eluted with DMF, which was introduced directly into the flame. Reductive precipitation with selenium as the collector⁸⁸ formed the basis of a method for the determination of Au, Pd, Pt and Rh in geochemical samples by FAAS and ICP-OES.

Complexation and sorption on activated carbon⁸⁹ was used in a procedure for the determination of Bi and Mo by ETAAS. The complexing agent was dithiophosphoric acid O,O-diethyl ester and the analytes were retained on a bed of activated carbon on a filter paper. After drying, mixing with HNO₃, drying and mixing with dilute HNO₃, a 20 µl subsample was transferred to the atomizer. The procedure was applied to the analysis of steels, and the possible interference from the iron was overcome by reduction to iron(II) with ascorbic acid. The same collection procedure was used⁹⁰ for the preconcentration of As from natural waters as the heteropolymolybdate. After collection by filtration, the 100 mg of carbon was suspended in 5 ml of 0.1 mol l⁻¹ acetic acid containing 0.02% of Zr and 10 µl was transferred to the furnace for measurement at 197.2 nm. The LOD was 20 ng l⁻¹ for a 1 l sample. A Chinese researcher has described⁹¹ a procedure for the determination of Tl by retention of Tl^III on a tributylphosphate resin. The analyte was oxidized with a mixture of iron(III) and H₂O₂ and eluted with a solution of ammonium sulfite and ascorbic acid. The LOD was 3 µg l⁻¹.

Coprecipitation with dithiophosphates was used⁹² to preconcentrate As, Cu, Pb and Se from digests of biological materials for determination by ICP-OES. The LOD ranged from 5 to 400 µg kg⁻¹. Chinese workers⁹³ separated Au by precipitation of the complex with malachite green on microcrystalline naphthalene which was collected, packed into a column and the Au eluted with 6 mol l⁻¹ HNO₃. The eluent was evaporated to near-dryness and the residue taken up in HCl for determination by ICP-OES. A somewhat similar procedure⁹⁴ has been used for the determination of Zn. The 1-(2-triazolyazo)-2-naphthol complex was retained on a Sep-Pak C₁₈ cartridge and then eluted with ethanol. The ethanol was evaporated and the residue taken up in a mixture of HNO₃ and H₂SO₄ and the Zn determined by ICP-OES with an LOD of 20 µg l⁻¹. For the determination of Cd, Co, Cu, Fe, Ni, Pb and Zn in saline matrices (haemodialysis concentrates) the analytes were extracted as the complexes with 1-pyrrolidinedithiocarbamic acid onto 30 µm Amberlite XAD-2 resin. After filtering and washing, the resin was slurried with 1% Triton X-100 solution and introduced directly into the spectrometer for determination by ICP-OES. Solid phase extraction has been combined with liquid–liquid extraction⁹⁵ for the determination of Sc in red mud down to an LOD of 10 µg kg⁻¹. The analyte was first retained on a column of Dowex 50W-X8 from which several other components were eluted first before Sc and Y and other lanthanides. The eluate was extracted with di(2-ethylhexyl)phosphoric acid and the Sc back-extracted with NaOH solution for determination by ICP-OES.

The As content of air was determined⁹⁶ by absorption in acid and then evaporation to dryness in the presence of graphite powder, which was then analysed by AES.

Some preliminary results for the determination of organohalogen compounds in waters have been presented.^97,98 Both methods are based on the use of a solid phase extraction procedure (either on activated charcoal or a polyacrylate fibre) followed by detection in a helium MIP by OES.

1.2 Chemical vapour generation

Tsalev has reviewed recent developments with AAS detection.⁹⁹ Applications to speciation by both chromatographic (HPLC and GC) and non-chromatographic procedures are included. Sanz-Medel et al. have reviewed¹⁰⁰ the use of organized surfactant assemblies (micelles, vesicles) and emulsions in procedures for AAS. Topics covered include the generation of volatile hydrides and ethylides, CV Hg generation and the 'synergic effects of using vesicles to improve the separation of reversed-phased HPLC and the detectability of AAS by on-line vesicular hydride generation'.

1.2.1 Fundamental studies in hydride generation. Experimental conditions for the determination of As by AFS using FI-HG were optimized¹⁰¹ based on mathematical modelling. The use of AFS restricted the range of parameters that could be varied, as the borohydride solution flow rate and concentration were fixed to ensure a constant generation of H₂. Furthermore, in those experiments where an argon purge gas was added prior to the gas–liquid separator, the total flow of argon to the atomizer was kept the same. It was concluded that the optimal sensitivity and throughput were obtained with minimal GLS headspace volume and maximum carrier solution flow rate. For the HG-AAS determination of Se, optimum conditions were found with the aid of two 2⁴ experimental designs.¹⁰² Helium (400 ml min⁻¹) was used as the purge gas, and both H₂ (300 ml min⁻¹) and O₂ (40 ml min⁻¹) were added. The LOD was between 0.7 and 2 ng, but the sample volume was not specified (nor was the nature of the GLS). In the determination of Sb by FI-HG-AFS¹⁰³ it was found that a cooled GLS (10 °C) with reduced headspace gave 'taller and narrower' signal peaks and also stabilized the baseline. Other innovative features of the procedure included the addition of both a surfactant (Triton X-100) and a silicon antifoam agent, which eliminated matrix effects (samples included urban particulate matter, tap and sea waters).

Carrero and Tyson have further developed¹⁰⁴ their procedure for generating hydrogen selenide from the surface of an anion-exchange resin. Sample solution containing Se^IV was first passed through the resin to preconcentrate the analyte, then borohydride was passed through to load a controlled amount of the reagent, and finally acid was passed to generate the hydrogen selenide which was subsequently trapped on the interior surface of a graphite furnace atomizer. The entire procedure was automated with a FIAS 200 FI system. For a 20 ml sample volume, the LOD was 4 ng l⁻¹. There have been several reports of the use of nebulizers and spray chambers as HG devices and GLS for ICP-OES determinations. Tao and Sturgeon¹⁰⁵ modified a Meinhard-type nebulizer by the insertion of a capillary tube into the sample introduction channel through which the acidified sample was introduced. The borohydride solution was introduced via the conventional channel and the solutions mixed just prior to nebulization. A Scott-type spray chamber was used. The very short residence times in the solution phase allowed the determination of Se in the presence of 5% nickel, 2.5% cobalt and 20 mg l⁻¹ copper. The LOD was 2 µg l⁻¹ and the method was validated by the analysis of a nickel oxide CRM. The procedure was also used with ETAAS with in-atomizer collection of the hydrogen selenide. Chinese workers¹⁰⁶ described a 'multi-functional cyclone nebulizer–hydride generator' for the determination of As, Hg, Pb and Sb and¹⁰⁷ a combined HG-ultrasonic nebulizer based on a CETAC U-5000AT device. Both procedures were used in conjunction with ICP-OES. A moving-bed HG device, also for ICP-OES, has been devised.¹⁰⁸ Solid borohydride and an organic acid were co-immobilized on a moving strip onto which the sample solution was directed. The device was used in the determination of As, Se and Sb in a Chinese tea. It has been shown¹⁰⁹ that H₂Se can be collected (at 200 °C) on a gold wire in a manner analogous to the amalgam trapping of mercury. When heated (600 °C) in a stream of H₂, H₂Se was released and detected by AFS. Electrochemical HG has been used for the determination of As by ETAAS¹¹⁰ and quartz tube atomization AAS.¹¹¹ In the former procedure, the generated hydride was collected for 2 min and then transported to the atomizer to give an LOD of 20 ng l⁻¹ for a 200 µl sample volume. The atomizer was pre-treated with iridium, and 400 firings were possible without any change in performance. For the latter procedure, an LOD of 10 ng l⁻¹ (continuous flow made) was obtained with a throughput (FI mode) of 36 h⁻¹. To study the mechanism of atom formation of As in a flame, Tesfalidet et al.¹¹² constructed a miniature H₂–O₂ burner inside the cavity of an electron spin resonance spectrometer. Radical recombination in the flame was identified as the atomization mechanism. The procedure was also used to study the spatial distribution of H radicals in a quartz tube atomizer. The spatial distribution of Sb atoms in such an atomizer has been measured¹¹³ in cross-section by using a charge coupled device detector. In an unheated flame-in-tube atomizer, the highest free atom density was at the centre and the distribution was not influenced by the purge gas and O₂ delivery rates, but was significantly affected by the position of the O₂ delivery capillary tip. In an externally heated tube the atom distribution was more homogeneous. At high analyte concentrations (into the roll-over region of the calibration), the atom density was higher both near the walls and at the centre. The presence of polyatomic particles on whose surface free atom decay was induced was considered responsible. Chinese workers, who studied the atomization of As¹¹⁴ in a low-temperature atomizer (25–200 °C) with AFS detection, concluded, from experiments with a dual generation system (one for AsH₃ and one for H₂), that the atomization mechanism involved collision with hydrogen radicals.

Chinese workers have also investigated¹¹⁵ the role of organic acids in the HG determination of Pb. In a medium containing potassium dichromate as oxidant, the interference from the resulting Cr^III was overcome by the addition of lactic and malonic acids. A holed T-tube atomizer (flame heated) has improved¹¹⁶ the tolerance for interference by other hydride-forming elements in the determination of As, Bi, Sb and Se compared with those observed in a conventional flame-heated quartz tube atomizer. Five 2 mm holes were evenly spaced along the length of the atomizer. Presumably the holes were in the bottom of the tube facing the flame. With regard to LOD, the only significant decrease was for As, for which the LOD in the holed tube of 0.2 µg l⁻¹ compared with 0.06 µg l⁻¹ for the conventional tube. The linear ranges were improved for all elements except Sb, which was the element that showed least improvement in interference tolerance, being more tolerant only to Se. Chinese workers investigated¹¹⁷ the interference of tellurium on the HG-AFS determination of Se and showed that the interference could be removed by the addition of Fe^III. Camero and Sturgeon¹¹⁸ trapped the hydrides of As, Se and Sb on the interior surface of graphite furnace atomizer by electrostatic deposition. The generation/collection efficiencies were estimated to be 80, 71 and 62%, respectively, values which seem a little lower than those for collection on an iridium-coated surface.

Workers at Xiamen University have a number of interesting developments to report.¹¹⁹ First, they confirm the formation of ZnH₂ (see also ref. 120). Second, they report the generation of CoH₂ and NiH₂, and third they report an enhancement in sensitivity in the determination of Cd, Tl and Zn by HG-ICP-OES by the presence of Co, Ni and Te. The degree of enhancement was increased as the length of tubing between the GLS and the spectrometer was increased. This is in contrast to the behaviour observed in the absence of the enhancers.

Several publications have described the results of efforts to determine more than one hydride-forming analyte per sample. The problems to be overcome include: (a) the difficulty of adjustment of the various oxidation stages, particularly the production of Se^IV and As^III following oxidative sample pretreatment; and (b) the mutual interferences in both generation and atomization stages. In a procedure entitled 'simultaneous determination of As, Hg, Se and Sb',¹²¹ the various hydrides were generated batchwise and trapped cryogenically (liquid nitrogen). The trapped species were simultaneously evaporated by electrical heating and transported to a He MIP for quantification by OES. The procedure was also applied to the determination of Bi, Pb and Sb.¹²² A similar concept has been described for the determination of As, Sb, Se and Sn by gas-phase molecular absorption spectrometry. First Sn hydride was generated at low acidity, then the hydrides of As, Sb and Se were generated at higher acidity. All hydrides were trapped in a liquid nitrogen cryotrap and then volatilized simultaneously. A diode array spectrometer was used and multiple linear regression was applied to deconvolute the spectra obtained. The wavelengths of maximum absorption were 190, 198, 22 and 194 nm, respectively, and the LOD were 50, 20, 100 and 1000 µg l⁻¹. Russian workers¹²³ claimed to have determined As, Hg and Se simultaneously by continuous flow HG-ICP-OES. Samples (skim milk powder and freeze-dried bovine liver) were decomposed by HNO₃ at 160 °C under pressure. The LOD were 0.2, 1 and 0.06 µg l⁻¹ for As, Hg and Se, respectively. For the determination of As, Bi, Ge, Sb, Se, Sn and Te in 30% zinc sulfate solution, Rigby and Brindle¹²⁴ divided the analytes into two groups. In the first, Se and Te were determined following heating in the presence of HCl for 25 min; in the second, As, Bi, Ge, Sb and Sn were determined after the addition of L-cysteine and L-histidine (and some HCl). The hydrides were generated in a continuous flow (CF) system with determination by ICP-OES. Detection limits in the original sample solution were 3, 3, 8, 2, 3, 2 and 38 µg l⁻¹ for As, Bi, Ge, Sb, Se, Sn and Te, respectively. For the determination of As, Bi, Sb, Se and Te in nickel, Feng and Fu¹²⁵ also divided the analytes into two groups. The samples were dissolved in HNO₃ and the analytes co-precipitated with lanthanum hydroxide, filtered and redissolved in HCl. In the first group, As and Sb were determined following the addition of HCl, thiourea and ascorbic acid. In the second group, Bi, Se and Te were determined after the addition of HCl. The analysis was performed by HG-AFS, giving LOD in nickel of 0.1, 0.4, 0.2, 0.1 and 0.1 mg kg⁻¹ for As, Bi, Sb, Se and Te, respectively. A HG-ETAAS procedure with trapping on an iridium coating has been developed for the determination of As, Sb and Se.¹²⁶ The hydrides were generated in a CF system in a 2.5 m titanium reaction loop. The procedure was applied to the analysis of a number of environmental and clinical materials; in the determination of 1 mg kg⁻¹ of Sb in soil, up to 600 mg kg⁻¹ of As did not interfere. A procedure for the determination of As and Se in soluble coffee by ICP-OES has been developed.¹²⁷ Following oxidative pretreatment, the Se^VI was reduced to Se^IV with HCl; presumably the As remained as As^V. Steel was analysed for Se and Te by HG-AFS¹²⁸ following acid decomposition and removal of potentially interfering cations by cation exchange. Several instrumental developments for the determination of multiple hydride forming elements have been described.^107,129

As, Hg and Se were extracted¹³⁰ from coal by sub-critical water prior to determination by chemical vapor generation AFS. Arsenic and Se were determined¹³¹ in foodstuffs, after microwave assisted digestion in HNO₃, by AAS. Selenium was reduced with HCl (after removal of the nitric acid by heating to dryness); arsenic was reduced with hydroxyammonium chloride, potassium iodide and ascorbic acid. Chinese workers¹³² have developed a method for the determination of As and Hg in biological samples and patent medicines by HG-AFS. The pre-treatment conditions were strongly oxidizing.

1.2.2 Generation of other volatile compounds. Lopez-Molinero and co-workers have devised procedures for the determination of As after volatilization as the trifluoride,^133,134 the chloride¹³⁵ and the bromide.¹³⁶ Both AAS¹³⁵ and ICP-OES^133,134,136 have been used for quantification. It is not clear what the advantages over HG might be—possibly greater freedom from matrix interferences, as the LOD would appear to be much poorer than those obtained with HG. The reactions are carried out in a batch mode in concentrated H₂SO₄. It has been shown that Ge can be determined via the tetrachloride^136–138 by ICP-OES, this time in a continuous flow procedure. The LOD was 0.2 µg l⁻¹. Chloride in water was determined¹³⁹ by volatilization of the chlorine formed on reaction with permanganate in sulfuric acid medium with quantification by MIP-OES. The LOD was 10 µg l⁻¹. A somewhat similar procedure was used for the determination of both chloride and organochlorine compounds in waters,¹⁴⁰ except that the final determination was by either dc or rf GD-OES. The LOD were between 0.1 and 0.5 µg l⁻¹. Organochloride and organobromide compounds were determined by MIP-OES¹⁴¹ after conversion to hydrogen chloride or bromide and trapping in NaOH solution. Organic species were separated from inorganic species by passage through a column of activated charcoal. The LOD were 8 and 3 µg l⁻¹ for Br and Cl, respectively. The ratio of ¹⁵N∶¹⁴N in soils has been determined by OES.¹⁴² Ethylation has been used¹⁴³ as the basis of the determination of Bi by FI-AAS. The LOD was 0.8 µg l⁻¹ and the procedure was successfully applied to the analysis of urine, for which only a simple 1 + 1 dilution was needed as sample pre-treatment.

1.2.3 Vapour generation of individual elements.
1.2.3.1 Arsenic. Both As^III and As^V were determined in sea-water¹⁴⁴ after preconcentration on activated alumina as the complexes with quinolin-8-ol-5-sulfonic acid by HG-AAS. The retained species were eluted with HCl; As^V was not reduced. The LOD were 0.05 and 2 µg l⁻¹, respectively for a 2 ml sample volume. Chinese workers have reduced As^V with L-cysteine for determination by HG-AAS;¹⁴⁵ reduced As^V with KI and ascorbic acid for the analysis of foods and beverages;¹⁴⁶ and used target factor analysis for the determination of four As species in waters.¹⁴⁷ Muinoz et al. devised¹⁴⁸ a microwave-assisted distillation procedure for the separation of inorganic As species from seafood products prior to determination by HG-AAS. The concentrations found ranged between 0.05 and 1 mg kg⁻¹. The As contents of a large number of water samples from the US, China and Canada have been determined¹⁴⁹ by HG-ETAAS with in-atomizer trapping on a palladium-coated cuvette. For a 25-ml sample the LOD was 0.3 ng l⁻¹.

Both AFS and AAS were evaluated for the determination of As in wine and beer by HG.¹⁵⁰ The only pre-treatment was de-gassing and sample solutions were injected directly in the 6 mol l⁻¹ HCl carrier in the FI manifold. The LOD was 0.3–0.5 µg l⁻¹. Chinese workers digested the samples under auto reflux for the analysis of oil by HG-AFS,¹⁵¹ and passed the hydrides through a solution of permanganate to selectively remove stibine in the determination of As in antimony trioxide.¹⁵²

Inorganic As species were selectively determined by HG-ICP-OES.¹⁵³ By control of the solution acidity, only As^III was determined; As^V was then reduced with L-cysteine (for 12 h) and total As was determined. Chinese workers determined As in 67 food coal-tar dyes by HG-ICP-OES,¹⁵⁴ and As in sediments after co-precipitation with aluminium hydroxide.¹⁵⁵ In the former study HCl and KI were used to reduce As^V, whereas, in the latter study, only HCl was used.

The generation of volatile halides has also been used as the basis for the separation of As from unwanted matrix components. The generated species include the fluoride,¹³⁴ the bromide¹³⁶ and the chloride.¹³⁵

In a study of As in urine, HG was used¹⁵⁶ though the type of spectrometry was not specified. The investigators found elevated concentrations in the urine of some subjects which could not be assigned to a high consumption of seafood and they concluded 'that additional factors relevant in the exposure to As are still unidentified'.

Arsenic species have been separated by HPLC with HG-AAS detection.¹⁵⁷ In this study, UV photolysis was found to be superior to microwave-assisted decomposition in the post-column reaction scheme to convert non-hydride active forms of arsenic to hydride active forms.

1.2.3.2 Bismuth. A comprehensive study of reaction media for the determination of Bi by HG-ICP-OES has been undertaken.¹⁵⁸ Tartaric acid was found to be the most effective medium in terms of efficiency of generation and control of interferences. The LOD was 0.3 µg l⁻¹ and the method was validated by the accurate analysis of some RM (water and silicate) and was applied to the analysis of iron ore and coal fly ash. Interferences from a number of transition metals and other hydride forming elements (and mercury) were evaluated. Chinese workers¹⁵⁹ generated bismuthine from a slurry of the sample (geological materials mixed with aqua regia) on the addition of 0.8% sodium borohydride solution. The LOD was 60 µg l⁻¹ for determination by AFS. The interference from copper was eliminated by the addition of solid thiourea. Both copper and nickel interferences were considerably reduced on masking with 1-(2-thiazolylazo)-p-cresol.¹⁶⁰ A batch generation system was used with detection by AAS. Urine was analysed by AAS¹⁴³ following ethylation of the Bi. The LOD in urine was 2 µg l⁻¹.
1.2.3.3 Cadmium. To remove interferences in the determination of Cd in waste waters and sewage sludge by HG-AAS with room temperature atomization Munos-Olivas and Camara¹⁶¹ added KCN (0.5%) to the borohydride solution. This must have produced considerable amounts of potentially hazardous HCN on merging with the acid carrier stream. The LOD was 50 ng l⁻¹.
1.2.3.4 Germanium. The effects of various acids on the determination of Ge by HG-ICP-OES have been evaluated.¹⁶² It appears that tartaric might have been the best of the various acids investigated (hydrochloric, citric, oxalic, acetic and sulfosalicylic) in terms of interference tolerance. The LOD were all in the range 0.1–0.3 µg l⁻¹ regardless of which acid was used. Chinese workers also investigated the optimum conditions for HG,¹⁶³ this time for AFS detection. They found that phosphoric acid, or a mixture of phosphoric acid and a mineral acid, gave the best generation efficiency and freedom from interferences. The gas-phase interference from As was eliminated by passing the vapors through a column of HgCl₂. The LOD was 0.4 µg l⁻¹. The technique of HG-AFS was also used in a speciation procedure¹⁶⁴ in which inorganic Ge and carboxyethylgermanium sesquioxide were determined in health drinks. As was discussed above, Ge has been determined by ICP-OES after volatilization as the chloride.^137,138 The authors found that the LOD for HG and chloride generation (CG) were similar (around 0.2 µg l⁻¹) but that CG was much more selective.
1.2.3.5 Lead. Brindle and co-workers determined Pb in calcium carbonate materials (coral) by HG-DCP-OES.¹⁶⁵ Ferricyanide was added as oxidant to aid in the production of Pb in the +4 oxidation state. The LOD was 0.7 µg l⁻¹. A specially designed gas–liquid separator was used. Chinese workers^166,167 also used ferricyanide in a procedure for the determination of Pb by HG-AFS with an LOD of 0.2 µg l⁻¹. The procedure was applied to the analysis of geochemical RM. A mixture of phenanthroline, potassium thiocyanate and oxalic acid was used to mask interferences. Maleki et al. described¹⁶⁸ a procedure for the generation of plumbane on injection of a sample solution into a reaction vessel containing solid borohydride and solid tartaric acid in a heating block at 65 °C. The gases evolved were swept by a stream of nitrogen into one end of a flame-heated tube in an AA spectrometer. The LOD was a modest 4 µg l⁻¹.
1.2.3.6 Antimony. A FI-HG-AAS procedure was developed for the determination of Sb in plant materials¹⁶⁹ in which the Sb^V produced by digestion with oxidizing acids was reduced to Sb^III by KI–ascorbic acid. The LOD was 0.01 µg l⁻¹. The sample digestion procedure was further developed¹⁷⁰ for the analysis of lipid-rich materials. The method was validated by the analysis of bovine liver and pig kidney CRM and applied to the determination of Sb in pigeon eggs, bream and deer livers. The LOD, based on the dry powder, was 7 ng kg⁻¹. Dietz et al.¹⁷¹ cryogenically trapped the stibine produced from a batch reactor, containing acetic acid, into which the aqueous sample and aqueous alkaline borohydride were separately introduced. The hydride was re-volatilized in a stream of N₂ and the Sb detected by MIP-OES. The LOD was 0.09 µg l⁻¹. A procedure for the determination of Sb in brass has been devised.¹⁷² Potentially interfering matrix elements (cobalt, copper, iron and nickel) were precipitated as the hydroxides.
1.2.3.7 Selenium. There is still considerable interest in the development of methods for the measurement of Se in a variety of materials by HG with atomic spectrometry. As it is impossible to generate hydrogen selenide from Se^VI, any strongly oxidizing sample pre-treatment has to be followed by a reduction step in which the Se is reduced to Se^IV. For the determination of Se in urine a microwave-assisted procedure with nitric acid and hydrogen peroxide was developed.¹⁷³ Fumes of NO_x, which cause a spectral interference in the subsequent HG-AAS determination, were eliminated by the addition of urea. Meat was digested in a mixture of nitric, sulfuric and percholoric acids,¹⁷⁴ steel was dissolved in nitric, hydrochloric and perchloric acids,¹²⁸ and hair was digested in nitric and perchloric acids.¹⁷⁵ For each of these procedures, the Se^VI was reduced to Se^IV by heating in the presence of HCl. Li and co-workers noted¹⁷³ that organoselenium compounds were partially decomposed by this process, and concluded that a simple procedure for distinguishing inorganic selenate from inorganic selenite in the presence of organoselenium compounds was not possible. Presumably, distinction between borohydride active 'inorganic' forms of Se and borohydride unreactive 'organoSe' would not be possible either. A procedure for the selective determination of selenite has been developed¹⁷⁶ in which six different extraction solutions were evaluated and the Se was preconcentrated and separated by retention on an anion-exchange resin. An anion-exchange resin was used¹⁷⁷ to remove the interference from copper (retained as anionic chloro complexes) and to preconcentrate the Se as selenotrisulfide. Chinese workers¹⁷⁸ devised a double HG method in which the Se was converted to hydrogen selenide from a relatively large sample volume and trapped in a relatively small volume of alkaline hydrogen peroxide solution, from which hydrogen selenide was generated and detected by AFS. The LOD was 6 ng l⁻¹. By trapping H₂Se on a hot gold wire¹⁰⁹ Se was preconcentrated to give an LOD of 5 ng l⁻¹. The trapped hydride was released at 600 °C into a stream of H₂ and detected by AFS. Soluble Se-species in agricultural drainage waters and soil sediment extracts were distinguished on the basis of resistance to oxidation.¹⁷⁹ Selenium(IV) was determined by AAS following HG from the sample solution with no pre-treatment. OrganoSe was determined by HG-AAS after oxidation with alkaline persulfate, and finally total Se was determined after oxidation with 30% H₂O₂ at 90 °C, followed by reduction to Se^IV by persulfate (20 min at 90 °C). The reduction of Se^VI by a reagent that normally behaves as a strong oxidant is an interesting finding.
1.2.3.8 Tin. A FI-HG-AAS procedure was developed for the determination of Sn in sea-water,¹⁸⁰ with a LOD of 0.1 µg l⁻¹ for a 500 µl sample.
1.2.3.9 Tellurium. Steel samples were decomposed with nitric, hydrochloric and perchloric acids.¹²⁸ Following precipitation with hydroxide and re-dissolution in HCl, the sample solution was passed through a strong cation-exchange resin. The eluent was mixed with HCl and boiled for a few minutes and the Ge (and Se) were determined by HG-AFS.
1.2.3.10 Mercury. Russian¹⁸¹ and Czech¹⁸² workers have reviewed the determination of Hg. The first of these concentrates on the AA determination in soils, while the second is concerned with the determination of organoHg compounds in various environmental samples. Russian workers have also reported¹⁸³ on the determination of Hg in biological materials from cadavers. A new mussel tissue SRM has been issued.¹⁸⁴ The CV-AAS determination of Hg was used in support of studies of the efficiency of polymer-enhanced ultrafiltration,¹⁸⁵ a new technique for the removal of heavy metals from aqueous solutions. Provided that 10% HCl was added, the presence of the water-soluble polymer polyethylimine did not cause any interference. A procedure for the characterization of thiol (–SH) binding groups has been developed¹⁸⁶ in which free Hg^II and thiol-bound Hg^II were distinguished on the basis of their reactivity towards borohydride solutions of different concentrations. Quantitative reduction of Hg^II to Hg⁰ takes place with a specific amount of sodium tetrahydroborate according to the 'stoichiometric reaction of mercury with alkaline NaBH₄', but the reduction of Hg^II–thiol complexes may require up to 6 orders of magnitude molar excess of borohydride. The mercury was determined by AFS. On the other hand,¹⁸⁷ the 'sulfide interference' was overcome by treating the sample solution with solutions of sodium hydroxide, sodium hypochlorite (NaClO) and copper sulfate, added in that order, followed by the addition of 10% tin chloride in 0.5 mol l⁻¹ sulfuric acid. The determination was by AAS. A sequential injection AAS procedure has been developed¹⁸⁸ in which borohydride was used as the generating agent. Small volumes of sample in low acidity HCl solution (0.05 mol l⁻¹) and reagent (15–30 µl, 0.2–1%) separated by air segments were first drawn into the holding coil. Then vapor generation was initiated on reversing the flow and flushing the solutions through a flow-through gas–liquid separator. The LOD was 0.1 µg l⁻¹ and the consumption of reagents, compared with the FI procedure, was decreased by a factor of 25. Preliminary information¹⁸⁹ about a new miniaturized AFS system in which the thermally decomposed samples were introduced from a metal ETV device has been provided. An MIP-AES system with amalgam trapping preconcentration is being developed.¹⁹⁰ Russian workers have developed¹⁹¹ an amalgam trapping device consisting of a column of silica or alumina granules coated with sponge gold.¹⁹¹

Several studies have focused on the extraction/digestion procedure for the determination of total Hg. Three different acid extraction procedures were evaluated¹⁹² for the CV-AAS analysis of soils. Some of the extracts were exposed to UV radiation for up to 4 h. Mercury vapour was generated by the addition of 3% sodium borohydride solution to the sample solution diluted 1 + 1 with HCl. For the analysis of sediments, mercury was solubilized by microwave-assisted extraction in a sealed vessel with nitric acid.¹⁹³ Borohydride solution was again used, but only at a concentration of 0.2 %, with detection by AAS. Coal was analysed¹⁹⁴ following microwave digestion in aqua regia by FI-CV-AAS and FI-CV-ICP-MS. The results were compared with those obtained with a LECO pyrolysis system and by NAA. The LOD were 80 ng l⁻¹ and 6 ng l⁻¹ for AAS and ICP-MS, respectively. Urine was analyzed¹⁹⁵ by a procedure in which bromate, bromide and HCl were added prior to digestion in a flow through microwave system. Quantification was by AAS with the PerkinElmer FIMS (flow injection mercury system). The LOD was around 50 ng l⁻¹. For the determination of Hg in blood and plasma, room temperature bromination was used after overnight digestion or heating for 4 h. Elemental Hg was produced by reduction with tin(II) and quantification was by AFS, with a throughput of 20 h⁻¹. The LOD were 0.5 and 0.9 nmol l⁻¹ for plasma and blood, respectively. To avoid some sources of systematic error in the analyses of biological materials, a single vessel procedure has been developed.¹⁹⁶ The samples were digested with a mixture of nitric and sulfuric acids; the Hg vapour generated on the addition of Sn^II was quantified by AAS. A similar procedure has been used¹⁹⁷ for the analysis of bovine kidney. Following the acid digestion, 30% H₂O₂ was added. The final determination was by AAS. Chinese workers¹⁹⁸ analysed foodstuffs by first mixing the solid sample with vanadium pentoxide and nitric acid and leaving overnight. Then sulfuric acid was added and the mixture heated to 140 °C. Potassium permangante was then added until a pink coloration was observed, and finally the excess permanganate was removed with hydroxyammonium chloride. Quantification was by AAS. A somewhat similar procedure was used¹⁹⁹ by other Chinese workers for the determination of Hg in household waste materials A microwave assisted digestion procedure has been devised²⁰⁰ for the analysis of foliage by CV-AFS. Water samples²⁰¹ were analysed (AAS) by direct introduction into an FI system in which the sample carrier of 3% HCl merged with 3% SnCl₂ in 10% HCl. Total Hg in hydrocarbons and natural gas condensate has been determined by CV-AFS.²⁰² Samples were vaporized at 400 °C and all mercury species were collected on a gold trap at 200 °C. On heating to 900 °C metallic Hg was released. The procedure was applicable to the determination of mercury chloride, methylmercury chloride, ethylmercury chloride, phenylmercury chloride, dimethylmercury, diethylmercury, and diphenylmercury. Atmospheric particulates were analyzed²⁰³ for their mercury content by pyrolysis-gold amalgamation-thermal desorption-AFS. It was found that results for denuder-based methods were higher than those of a conventional filter procedure, as mercury-bearing gold particles were flaking off the gold-coated denuder surfaces.

In an effort to improve the LOD of an ICP-OES procedure for the determination of Hg, the sample solution was preconcentrated²⁰⁴ by retention of the Hg on an anion-exchange column loaded with 1,5-bis[(2-pyridyl)-3-sulfophenylmethylene]thiocarbonohydrazide. The Hg was eluted with 2 mol l⁻¹ nitric acid and merged with a steam of tin(II). The LOD was 4 µg l⁻¹, which is not as low as can be achieved by AAS or AFS. For the determination of Hg in cosmetics,²⁰⁵ an APDC complex was retained on a C₁₈ solid phase extractant by reaction with tin(II) and detection by AFS. The LOD was single digit µg l⁻¹. Treble et al. have devised²⁰⁶ a batch procedure in which the mercury was retained from a solution containing iodide on a cation-exchange resin impregnated with quinine. The mercury was desorbed by thiourea in HCl and Hg vapor was generated on the addition of alkaline potassium borohydride. The LOD was 1 µg l⁻¹.

Some conflicting results have been reported for the determination of inorganic-Hg (i-Hg) and methylmercury (m-Hg) based on the reactions with borohydride and tin(II). Burguera and co-workers described a procedure for the determination of i-Hg and total-Hg in urine.²⁰⁷ The basis of the method is that i-Hg is determined by CVAAS after the addition of tin(II) and total-Hg is determined following the microwave-assisted oxidation of organoHg with persulfate in a FI system. The LOD was 0.1 µg l⁻¹. Thus, the authors assumed that organoHg compounds do not react with Sn^II to give mercury vapour. These workers also applied the procedure for the determination of the same species in fish egg oil.²⁰⁸ Samples were handled as emulsions, though this time the Hg vapor was generated with borohydride, and thus the authors assumed either that this reagent produced a species which did not absorb at 253.7 nm or that borohydride did not react with organoHg compounds. A similar concept was used by Rio-Segade and Bendicho²⁰⁹ for the analysis of biological and environmental samples. Persulfate in sulfuric acid was used as the oxidant and the mercury vapour was generated on the addition of Sn^II. Distinction between i-Hg and total-Hg was based on heating the reactants, in a CF system, to 85 °C. At this temperature organomercury compounds were decomposed to i-Hg, whereas at room temperature only i-Hg reacted with Sn^II. These authors also claimed²¹⁰ to be able to determine m-Hg, which had been selectively extracted (ultrasound assisted) from fish tissue by 2 mol l⁻¹ HCl, by forming Hg vapour on the addition of borohydride. Treatment with 5 mol l⁻¹ HCl extracted i-Hg from which Hg vapor was generated by adding Sn^II. They also claimed in a later publication²¹¹ that Hg⁰ was generated by the post-column merging of a stream of borohydride (0.01%) with the column eluent (10 mmol l⁻¹ tetrabutylammonium bromide and 25 mmol l⁻¹ sodium chloride in 60% methanol) containing separated i-Hg and m-Hg. While it appears to be agreed that Sn^II generates Hg⁰ only from i-Hg, it is not clear whether borohydride generates Hg⁰ from organoHg compounds (specifically methylmercury). Work in progress (C. D. Palmer and J. F. Tyson²¹²) supports the hypothesis that methylmercury hydride is produced when borohydride reacts with m-Hg and that this compound is stable with respect to decomposition to Hg⁰ at room temperature in a quartz tube atomizer and so no signal is observed when borohydride reacts with m-Hg. Chwastowska et al.²¹³ claimed to be able to determine both i-Hg an alkyl-Hg following 'reduction by Sn^II'. The species were separated from the sample matrix with the aid of resin loaded with 2-mercaptobenzothiazole. The resin retained both species, which were subsequently eluted with a solution of thiourea in HCl. Inorganic-Hg was first determined, and then cadmium was added to displace Hg from organoHg compounds which reacted with Sn^II to give Hg⁰. This is a modification of the procedure originally described by Magos.²¹⁴ The LOD was 10 ng l⁻¹. Tao et al. used²¹⁵ a TMAH sample dissolution procedure for the determination of i-Hg in biological tissues. The i-Hg was released by the addition of L-cysteine and reduced to Hg⁰ on the addition of Sn^II. The LOD was 0.1 µg l⁻¹.

1.3 Nebulization

As usual, there has been a large contribution to the literature concerning sample introduction in this review period. Among the papers that have reviewed or given an overview of sample introduction is one by Montaser et al.²¹⁶ containing 202 references, that gave the basic concepts of sample introduction for ICP spectrometry. A paper that challenged many of the 'accepted' theories has been presented by Olesik.²¹⁷ Instead of regarding the spray chamber as a droplet size filter that, when used in conjunction with a pneumatic nebulizer, contributes to the very low sample transport efficiency (often 1–2%), he claimed that the low transport is caused by many other processes including droplet–droplet collisions and coagulation and evaporation of aerosol and solution from the spray chamber walls. Since larger droplets have higher momentum, they cannot follow the gas flow and hence impact on the side of the spray chamber. Therefore, by decreasing the size of the droplets by either using a lower uptake rate, by using a solvent that is more volatile than water, or by heating the aerosol to cause evaporation, analyte transport rates could approach 100%. Other factors that affect the transport rate include acid or salt matrices that alter the extent of evaporation. A variety of nebulizer–spray chamber assemblies were assessed. A more applications-based review has been produced that concentrated on biota analysis by flow injection (FI) coupled with atomic spectrometry.²¹⁸ The review contained 95 references and information about the type of samples analyzed and summaries of the procedures.

The introduction of liquids to microwave induced plasmas (MIP) has continued to attract considerable attention. This has been achieved using a variety of sample introduction devices. An ultrasonic nebulizer (USN) was used to introduce water samples at a rate of 35 µl min⁻¹ into an argon MIP operated at 150 W.²¹⁹ Figures of merit including linear range, precision, LOD and accuracy were reported. The LOD for 31 elements were between 0.3 and 1000 µg l⁻¹ and the accuracy was verified by the analysis of certified water samples. The same sample introduction system has been used to introduce acid digests of a reference material (pine needles) into a vertically positioned, axially viewed, aerosol-cooled plasma.²²⁰ The torch cooling system for the aerosol generation and circulation was described in detail. A hydraulic high pressure nebulizer (HHPN) used in conjunction with a desolvation unit has been used to introduce different Cr species eluting from a high-performance liquid chromatography (HPLC) column.²²¹ The sample was introduced to a BST Rutin C₁₈ RP column and the mobile phase was 15% methanol, 0.1 mM tetrabutylammonium acetate, 0.1 mM ammonium acetate and 1 mM phosphoric acid. The linear range covered two orders of magnitude and the absolute LODs were 13 ng for Cr^III and 18 ng for Cr^VI. Severe interferences from easily ionizable elements were observed during the analysis of real samples. A thermospray nebulizer constructed in-house, used in conjunction with a desolvation device, also made in-house, has been described.²²² The design and construction of both devices was reported in detail. Under optimum operating conditions of 100 W forward power, a support gas flow rate of 0.75 l min⁻¹, carrier gas flow rate of 0.8 l min⁻¹, observation height 8 mm above the top of the torch, sample uptake rate of 1.5 ml min⁻¹, capillary temperature 320 °C, aerosol spray chamber temperature 200 °C and condenser cooling water temperature 0 °C, LODs were found to be in the range 0.019–426 µg l⁻¹ and precision was 0.4–2% RSD. Inclusion of up to 20% methanol in the analyte solution was found not to disturb the plasma, hence offering the potential for coupling with HPLC.

Very little in terms of novelty has been reported for sample introduction to flame atomic absorption spectrometers (FAAS). Two papers written in Chinese have reported the determination of Zn in milk or milk powder. One²²³ used an emulsifying agent and reported that the results were in agreement with those obtained using a dry-ashing procedure. The other²²⁴ used pulse nebulization with injection volumes of 100 µl. This latter paper reported a LOD of 28 ng ml⁻¹, an average recovery of 99% and precision at 3.3%. No interferences were reported. Another Chinese paper reported the determination of several analytes (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) in air after passing them through a micropore membrane filter at a rate of 80 l min⁻¹.²²⁵ After collection, the membranes were acid digested and the resulting solutions analyzed by FAAS. Digestion was reported as being complete and recoveries using the standard additions method were 91–110%. Total Cr and Cr^VI has been determined in cigarette ash and smoke using FI and HHPN.²²⁶ The FI approach involved a sorption preconcentration system. The system was described in more detail (in Hungarian) in another paper.²²⁷

The majority of novel sample introduction systems described in the literature have concerned ICP-AES detection. A wide variety of different nebulizer types have been described. A sonic spray nebulizer (SSN) has been developed that has an orifice diameter of 250 µm.²²⁸ The orifice was made of a polyamide material to prevent metal contamination and allowed a gas flow rate of 1 l min⁻¹. A comparison with a conventional concentric nebulizer was made and it was found that the LOD were similar, although the sample uptake was very much reduced (1–50 µl min⁻¹ compared with 850 µl min⁻¹ for the conventional nebulizer). Absolute sensitivity was improved by a factor of 13.

Direct sample insertion (DSI) is a technique that is still being used by some authors because it enables both liquid and solid samples to be analysed directly. A review of the technique containing 79 references has been made by Sing.²²⁹ The review provides an overview of the instrumentation, operating parameters, figures of merit and applications. An application of DSI in which several analytes (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn) were determined by ICP-AES in sea-water and ground water samples has also been presented.²³⁰ The presence of several salts and common solvents such as hydrochloric, nitric, sulfuric and phosphoric acids was found to decrease the emission intensity. The extent of the matrix interferences was element dependent.

Direct injection high efficiency nebulizers (DIHEN) have been described in two presentations.^231,232 The device enables a transport efficiency of 100% to be achieved, whilst introducing the sample at a flow rate of 1–100 µl min⁻¹. The analysis of 'difficult' samples e.g., petroleum products, was described in one paper,²³¹ and nebulizer diagnostic techniques based on optical patternation were used to characterize the aerosol in the other.²³²

An assortment of other nebulizers have been used. These include a modified concentric glass nebulizer capable of analysing high salt content samples.²³³ It is a modification of both a standard Meinhard and the LB nebulizer. It is reported as being capable of aspirating a saturated solution of sodium chloride (26.54%) for two hours without clogging and the stability is superior to that of the LB nebulizer. The analytical figures of merit are also favourable when compared with the Meinhard, with higher SBR and lower LOD. A comparison of several different nebulizer types [high efficiency nebulizer (HEN), microconcentric nebulizer (MCN), Micromist and a conventional concentric nebulizer] operating at low sample uptake rates has been made.²³⁴ Parameters investigated included the gas back pressure, the free liquid aspiration rate, primary and tertiary aerosol droplet size and the solvent and analyte transport rate. It was found that the conventional nebulizer produced a coarser aerosol and subsequently had a lower solution transport rate through the spray chamber and a lower sensitivity than the other nebulizers do. Overall, it was found that the HEN provided the lowest limits of detection. A microwave thermospray nebulizer (MWTN) has been developed that operates by breaking the liquid stream by the expansion of the solvent vapour inside a capillary when heated by a microwave field.²³⁵ It was found that a certain level of dissolved ions must be present in the sample for nebulization to occur and that the analysis of pure water was not possible. An interesting characteristic of the nebulizer is that the higher the dissolved salt content, the finer the aerosol produced. For samples containing dissolved ions, the LODs and sensitivities obtained using this nebulizer were improved when compared with a conventional nebulizer. A modified nebulizer has been described that minimizes transition element interferences with selenium hydride generation (HG) ICP-AES.¹⁰⁵ A capillary tube is placed in the sample introduction channel of a conventional Meinhard nebulizer. This carries the acidified solution into the sodium tetrahydroborate solution that is introduced via the normal sample introduction channel. Gas–liquid separation is performed by a conventional Scott double-pass spray chamber. It was found that this small modification enabled Se at the 0.5 mg l⁻¹ level to be determined without interference from 50 g l⁻¹ nickel, 25 g l⁻¹ cobalt and 20 g l⁻¹ copper (all introduced as their 2+ salts). The LOD was 2 µg l⁻¹, precision was typically <2% and the method was validated by the analysis of NIST certified material nickel oxide.

Ultrasonic nebulizers (USN) have again found wide usage for a number of applications. A USN has been used in conjunction with a long torch to improve the sensitivity in an axially viewed horizontal ICP.²³⁶ Sample transport efficiency was increased when compared with a cross flow nebulizer and hence net signal intensity increased and the intensity and fluctuation of the background decreased, thereby yielding improved LODs and dynamic ranges. The device was used during the analysis of a water CRM. Since the sample transport rate using a USN is substantially higher than with conventional nebulizers, desolvation is often necessary to prevent plasma perturbation. A paper that has addressed this has been published by Allen et al.²³⁷ A microporous membrane desolvator (Cetac MDX-100) was used whilst determining the effects of sodium on the analytical signal of several analytes. The authors concluded that the enhancement of the signal caused by the sodium was greater in the presence of the desolvator but that the mechanism by which the sodium interfered was the same. A HG-USN system has been described that is capable of determining both hydride forming analytes and other analytes simultaneously.¹⁰⁷ Using the spray chamber as the gas–liquid separator, it was found that the LODs and SBR of the HG-USN device were the same as, or superior to, HG and USN separately. A USN has been used for the determination of Bi in urine using a FI preconcentration technique.⁵³ After digestion of the urine using nitric acid, the solution was taken to near dryness on a sand bath. After re-extraction with nitric acid, the white ash was taken up in aqua-regia, diluted, mixed with ethanolic 8-hydroxyquinoline adjusted to pH 5 and passed through a column. The Bi–8′-hydroxyquinoline complex was eluted with 2 M nitric acid. The LOD was 0.03 µg l⁻¹, precision at 2 µg l⁻¹ was 2.5% and the recoveries for 1–8 µg l⁻¹ were in the range 93–100.5%. Matrix effects caused by the use of a USN have been discussed by Budic.²³⁸ The effects of calcium, potassium, sodium, nitric acid and other matrices during the determination of several analytes were discussed. It was found that, in the presence of 20% nitric acid, the transport of the sample decreased by 8% when compared with water and that the presence of 1 mg ml⁻¹ matrix elements decreased the sample transport efficiency by 10%. Consequently, this led to a reduction in emission intensity. It was also found that the presence of potassium enhanced both the electron density and excitation temperature, whereas the presence of the other ions had the opposite effect.

Matrix effects have also been discussed for a number of nebulizer–spray chamber designs. An overview of nebulizer diagnostics, including a discussion of tools such as optical imaging (e.g., high speed photography), particle imaging velocimetry, optical patternation, Fraunhofer laser diffraction, phase Doppler particle analysis, rainbow refractometry, absolute intensity of scattered light and the ratiometric technique, has been made.²³⁹ Another overview of matrix effects and how they can be minimized and compensated for has been made by Mermet.²⁴⁰ There have been several papers that addressed the effects of acids on the determination of analytes. In one paper, the transient effects of different acids on the signals of several analytes were discussed.²⁴¹ It was found that the presence of the acid changed the extent of aerosol evaporation in the spray chamber and hence the sample transport rate. The effect was dependent on the type of acid used. A similar paper by the same authors stated that, as the concentration of nitric acid was increased over the range 0–25%, the sample transport rate decreased, but that the nebulizer gas flow rate also had a large effect.²⁴² At low flow rates (0.7–1 l min⁻¹) there was a decrease in transport efficiency up to a nitric acid concentration of 2%, but there was no further decrease up to a concentration of 25%. At gas flows higher than 1.3 l min⁻¹ there was a continual decrease in transport efficiency. It was therefore concluded that the acid dependent changes in aerosol properties occurred mainly during the transport of the aerosol through the spray chamber. Acid effects for a number of low-flow sample introduction methods was discussed by Todoli and Mermet.²⁴³ One method used a MCN with a Scott double-pass spray chamber, another two used desolvation systems comprising two Liebig condensers in series or a porous PTFE membrane, respectively, both using MCNs, and the last system used a DIN. Liquid flow rates ranged from 5 to 120 µl min⁻¹ and a number of acids including 0.9 M nitric, hydrochloric and sulfuric and 3.6 M nitric acid was tested. It was found that for the MCN with the spray chamber, the acid effects increased as the flow rate decreased. The opposite was true for the desolvation systems and the DIN. At a flow rate of <30 µl min⁻¹ no effects were observed if a conventional desolvation system was used at a temperature of 160 °C. In another similar paper, Todoli compared the performances of several nebulizer designs (pneumatic concentric micronebulizer, HEN, Micromist, MCN and DIN) with respect to acid introduction.²⁴⁴ Aerosol characteristics and figures of merit, such as LOD and background equivalent concentration (BEC), were compared. It was concluded that the DIN suffered fewer matrix effects. The analysis of plant material has also led to some matrix effects that were removed if higher plasma power was used, e.g., 1450 W.²⁴⁵ Depressions in emission intensity from the analytes As, Cd, Co, Cr, Cu, Ni, Pb and Se were observed in matrices containing calcium, iron, magnesium, manganese, phosphorus, potassium and sodium and this depression increased with increasing concentration of the concomitant species. A correction for volatility differences between organic sample analytes and standards in organic solutions has been made.²⁴⁶ The method is based upon the measurement of the analyte signal at two different spray chamber temperatures. The technique required no prior knowledge of the chemical structure of the analyte and tests with organosilicon and organochlorine compounds were found to reduce the error by a factor of between 2 and 30.

Spray chambers are the second, but equally important, component of most sample introduction systems. These come in a variety of designs, many of which have been discussed in the literature during this review period. Several designs (a Scott double-pass made of Ryton and three cyclone spray chambers made of glass, polypropylene and PTFE) were compared for their characteristics of the tertiary aerosol, solvent transport rate, analyte transport rate and net emission intensity, LOD and BEC.²⁴⁷ Interferences produced by the presence of widely used acids were also evaluated. In all cases, a glass concentric nebulizer was used to introduce the sample. Results indicated that the glass and polypropylene chambers produced the most coarse tertiary aerosols, but also had the greatest solution transport rates, highest emission intensity, and lower LODs and BECs than the other chambers. The position of the nebulizer was found to be critical for the cyclone spray chambers. Acid effects were found to increase as the tertiary aerosols became more fine and so the Scott double-pass spray chamber exhibited the worst performance. The cyclonic spray chambers were also found to exhibit better short-term stability. A doughnut-shaped spray chamber that had an internal volume of 102 cm³ has been described by Liu et al.²⁴⁸ A Meinhard nebulizer mounted radially at the entrance arm produced a spray that was split into two flows by a central hollow cylinder. These flows were then recombined at the exit of the spray chamber and transported to the plasma. This spray chamber was found to exhibit higher sample transport efficiency, more stable internal pressure, higher signal intensity, lower BEC and improved precision and LODs when compared with a conventional Scott style spray chamber. Another spray chamber design that reputedly had improved washout characteristics (with a four order of magnitude change in 60 s), improved sensitivity by an order of magnitude and lower LODs has also been reported.²⁴⁹ Unfortunately, the design of the chamber was not discussed in the abstract. It was stated, however, that the long and short term stabilities were also improved. The performance of three different spray chambers used in conjunction with a USN has been compared.²⁵⁰ A conventional Fassel type, a Cetac and a spray chamber designed and constructed in-house (details given) were the types compared for the analysis of transient signals. The performance was judged in terms of the washout period required, with shorter times indicating a better performance. The spray chamber designed in-house was found to be the best by far with a washout period 1/5th that of the Fassel and 3/5th that of the Cetac. In addition, it had a similar sensitivity to the Cetac which, in turn, was superior to the Fassel. Consequently, the Cetac and the chamber designed in-house had improved LODs when compared with the Fassel.

Computational and simulation papers that attempt to model the processes within the spray chambers have also been published. Two papers by Berndt et al. discussed the application of such programmes.^251,252 By transferring the physical flow domain (the spray chamber) to the computational domain, a grid consisting typically of 50 000–150 000 volumes was constructed, then mathematical algorithms were used to obtain numerical solutions for the individual volumes, which enabled a distribution of velocity components and pressure to be constructed. In this way it is possible to calculate the equations of motion for each droplet through the spray chamber. The authors have constructed data for both a cyclone and a Scott style spray chamber. The results of the computational model were in good agreement with those obtained experimentally, with a calculated transport efficiency of 2.6% compared with an efficiency of 2.7% obtained experimentally. Despite these encouraging results, the authors stressed that the need to build and test prototype spray chambers still exists. A paper that discussed the simulation of atomic and ionic absorption and emission spectra for thermal plasma diagnostics has also been published.²⁵³ Doppler, van der Waals, Stark and instrumental broadenings were all taken into account and the effect of self absorption was also considered. The input parameters were pressure, temperature, electron density and the emitting species number density. The simulation method was applied to the diagnosis of volatilization phenomena in thermal plasma spraying processes and the selection of diagnostic lines and instrumental and data processing parameters. It was also used to evaluate the accuracy of the temperature and density distributions calculated from spectroscopic measurements.

1.4 Solid sampling

Solids analysis utilizing both AAS and AES continues to be a significant area of research. A number of instrumental configurations are used for the introduction of either slurried or solid samples into flames, furnaces and plasmas. As in previous years, many of the publications in this area are application based and these reports are summarized in Table 1.

Table 1 Solid sampling

Type of atomization	Type of sampling	Elem.	Matrix	Sample treatment/comments	Ref.
CV-AAS	Slurry, prepared using ultrasonic mixing	Hg	Biological, environmental	Total Hg determined in samples using flow injection. Samples suspended in 15% HNO3 (mussel tissue, aquatic plant), or 9 + 1 15% HNO3–15% HCl (river sediment, sewage sludge). Results were comparable with those obtained after microwave digestion	276
ETAAS	Slurry	As, Cd, Cu, Pb	Food	Slurries prepared in 0.15% agar as a stabilizing agent. Phosphate (for Pb and Cd) and Pd–MgNO3 (for As) used as modifiers	277
ETAAS	Slurry	Ba, Cu, Fe, Pb, Zn	Tea leaves	Ba and Pb determined using standard additions, Cu, Fe and Zn determined using aqueous standards. Results from slurry analysis of SRM compared well with those obtained using ICP-AES after microwave digestion. Sensitivity and simplicity found to be the benefits of slurry method; however, for determination of several analytes ICP-AES may be faster	278
ETAAS	Slurry	Be	Rice and flour	LOD 1.4 pg obtained, no details of method given in abstract	279
ETAAS	Slurry	Te	InSb	Palladium nitrate used as modifier. LOD 0.4 µg g^−1. Results agreed well with solution ETAAS and ICPMS	280
ETAAS	Slurry, automatic preparation using Ar bubbling	Co, Cu, Ni	Sediments and soils	Ground samples suspended in HF, samples containing Cu and Ni partially digested using microwave. Fast heating programme used	281
ETAAS	Slurry, automatic preparation using ultrasonic probe	Sb	Geological	Chemometric techiques used for the optimization of procedure	259
ETAAS	Slurry, automatic preparation using ultrasonic probe, manually prepared using hand-held ultrasonic probe or ultrasonic bath	Pb	Mussel tissue and SRMs	Preparation procedures compared for extraction of Pb. Quantitative extraction into liquid only possible using ultrasonic probe. This helps avoid sedimentation and volumetric errors. However, analytical results obtained using automatic preparation are superior	258
ETAAS	Slurry, prepared by ultrasonic mixing	Co, Cu, Ni	Botanical	Coulter particle analyser used to study the effect of grinding and ultrasonic agitation on slurry analysis. 1 h grinding produced similar size distribution for all materials	282
ETAAS	Slurry, prepared by ultrasonic mixing	Cu, Fe	Old manuscripts	6 mm discs punched from paper samples, ground in a mortar, suspended in 5 ml H2O and sonicated for 2 min. 20 µl samples taken for analysis. Calibration by aqueous standards and by standard addition to clean filter paper used. RSDs 0.8–10% for Fe and 0.3–7.6% for Cu. Laser ablation ICP-MS was used for metal distribution studies	283
ETAAS	Slurry, prepared in an ultrasonic bath	Cd	Calcium drugs	A Mo-tube atomizer was employed with thiourea as chemical modifier. LOD 17 pg ml^−1. Results in good agreement with those obtained from acid digested samples	284
ETAAS	Slurry, prepared manually and with ultrasonic mixing	As	Baby food	Samples, 0.2–2 g, suspended in 5 ml solution containing 0.1% m/v Triton X-100, 1% v/v HNO3, 20% v/v H2O2, 0.3% v/v nickel nitrate. 20 µl aliquots taken for analysis during stirring. Fast furnace programme used. Method of standard additions used. LOD 23 pg	285
ETAAS	Slurry, prepared using ultrasonic mixing	Cd	Biological	Sample (10–35 mg) mixed with 1 ml 60% PTFE (as modifier), 0.4 ml conc. HNO3 and 0.2 ml 0.5% plant glue solution. The mixture was agitated ultrasonically for 60 mins. LOD 0.12 pg, RSD (n = 5) 7.8% for 1 ng ml^−1	286
ETAAS	Slurry, prepared using ultrasonic mixing	Co, Cr, Ni	Wheat flour	Sample (150 mg) mixed with 5 ml 15% HNO3–10% H2O2 and agitated ultrasonically for 15 min. 20 µl aliquot taken for analysis. LODs 23–44 ng g^−1, RSDs 4–8% (n value not given)	287
ETAAS	Slurry, prepared using ultrasonic mixing	Pb	Biological	Ammonium phosphate modifier and glycerol used. LOD 6.8 pg	288
ETAAS	Solid	Cd	Equine muscle	Direct analysis used for screening of tissues. Results obtained for direct solid analysis and for analysis of digested samples were closely correlated and were statistically no different	289
ETAAS	Solid	Cu, Fe, Mn	Flour SRM	Sample was vaporized in one graphite furnace and the generated aerosol was transported in Ar and electrostatically deposited on a platform in a second graphite furnace. Multi-element analysis carried out via continuum source coherent forward scattering spectrometer. Results obtained in agreement with certified values	290
ETAAS	Solid	Pb	Muscle tissue	Analysis of samples contaminated with gun-shot residue. Solid sampling suitable for determination of Pb level in non-contaminated samples and for the discrimination between original Pb content and Pb derived from gun-shot	291
ETAAS	Solid	Sb	PVC	Samples (2.5-3.5 mg) were inserted into enlarged furnace injection hole through a paper cone. RSD (n = 14) = 12.6%, which was considered satisfactory for screening purposes.	256
ETAAS	Solid	Various (11)	Tungsten trioxide and tungsten blue oxide	High background due to vaporizing oxides eliminated by use of hydrogen purge gas during pyrolysis. LODs 0.07 (Mg, Na, Zn)–2 (Ni) and 0.01 (Mg, Na, Ni)–1.7 (Fe) ng g^−1 for tungsten trioxide and tungsten blue oxide, respectively	292
ETAAS	Solid	Various (11)	Tungsten powder	Direct analysis of high purity tungsten powders with an ETAAS system equipped with a transversely heated atomizer, deuterium background correction and solid sampling introduction by the 'boat' technique. LODs in range 0.01–4 ng g^−1. RSDs (n = 5) 1.1–30.8%. (See also 293)	294
ETAAS	Solid	Various (15)	Titanium bars	Sample cut from Ti bar and etched with HNO3/HF. Samples (<30 mg) placed onto platform and inserted into furnace. LODs from 0.02 ng g^−1 (Mg) to 30 ng g^−1 (Sn) (See also 294 and 295).	296
ETAAS	Solid	Various (9)	Aluminium oxide powder	Sample (0.06–6 mg) placed onto a pyrolytic graphite coated platform and inserted into furnace. Platform re-coated every 5–6 firings to reduce background absorbance. LODs 25, 3.5, 2, 10, 0.8, 0.25, 1.5, 12 and 0.5 ng g^−1 for Co, Cr, Cu, Fe, K, Mg, Mn, Ni and Zn, respectively. (See also 294 and 296)	295
ETAAS	Solid	Zn	Geological RM	Calibration graphs prepared using different solid RMs had different slopes. To overcome errors, cluster analysis was used to select materials of similar composition and three-dimensional calibrations using intensities, masses and contents were used. The latter was the preferred approach	297
Flame AAS	Slurry	Ca	Maize flour	Solution of flour with 0.15% gelatin, LOD 0.12 mg l^−1	298
Flame AAS	Slurry	Ca, Fe, K, Mg, Na, Zn	Vegetable tissues	Optimized conditions found for slurry analysis. 50 mg lyophilized vegetable material slurried for 5 min using ultrasound and diluted to 50 ml with 0.720 M HNO3. RSDs <3% for all analytes. LODs 2.2, 1.8, 3.4, 8.7, 2.1 and 1.7 mg l^−1 for Ca, Fe, Mg, Na, K and Zn, respectively	299
Flame AAS	Slurry	Cu	Chinese medicinal herbs	Powdered sample (1.5 g) suspended in 25 ml 0.15% agar solution. After shaking, 4 ml aliquot mixed with 1 ml 0.15% agar and 1-5 ml Cu (II) standard solution (10 µg ml^−1) and H2O to 25 ml. LOD 57 ng ml^−1, RSD (n = 6) <2.5%.	300
Flame AAS	Slurry	Mn	Chinese medicinal herbs	Sample treatment identical to that described in 300. LOD 13.6 ng ml^−1	301
Flame AAS	Slurry	Zn	Milk fat	5 surfactants compared for preparation of homogeneous emulsion. Triton X-100 and NaDBS preferred	302
Flame AAS	Slurry, prepared from sample shaken with zirconia beads, particle size <0.8 µm	Ca, Cu, K, Mg, Na, Zn	Hair	Acid predigestion used as a pre-treatment stage. Glycerol and Viscalex HV30 compared as wetting agents. Wetting agents were only beneficial for dilute slurries. LODs 5, 3.5, 3.1 and 20 mg kg^−1 for Ca, Cu, Fe and K, respectively, and 1.7, 0.6, and 3.5 µg kg^−1 for Mg, Na and Zn, respectively	303
ICP-AES	Cup-in-torch	Pb	Fingernails	Re cup inserted 1.5 cm above the carrier gas inlet. Drying and ashing carried out in-situ	304
ICP-AES	Direct insertion	Cu, Fe, Mg, Mn, Zn	Wood pulp	Solid samples were introduced into ICP using pyrolytically coated graphite probe after in-situ treatment with HCl and NaF. Samples were dried and ashed by heating probe prior to plasma ignition. LODs 50–1000 pg. Method suitable for screening of samples, significant time saving over dissolution methods obtained	305
ICP-AES	Direct insertion	Various	-	An automated direct insertion device was described. Solution, powder and slurry sampling were compared and slurry sampling was found to be the most promising. Formation of carbides was eliminated by addition of SF6 to the plasma gas	306
ICP-AES	Direct insertion, sample preconcentrated on activated carbon	Cd, Cu, Pb, Zn	Waters	Metal complexes of oxalic acid, iminodiacetic acid or 8-hydroxyquinoline were collected on activated carbon. Loose particles of carbon were either loaded into direct insertion cup or complexes were adsorbed onto a direct insertion probe with an activated carbon cap. The specially machined cap produced the better results. Recoveries were poor although 8-hydroxyquinoline was the most promising complexing agent	307
MIP-AES	Slurry ETV on W coil	Ag, Cd	Waste water	Samples were vaporized on W-coil. LODs 16 µg l^−1 for Ag, 1 µg l^−1 for Cd	308
ICP-AES	ETV	Various (16)	Graphite, silicon carbide	Solid sample, 20 µm for graphite and <5 µm for SiC, placed on platform and inserted into ETV unit. Freon added to Ar carrier gas	309
ICP-AES	Slurry	Rare earths	Lanthanum oxides	—	—
ICP-AES	Slurry	Various (9)	Milk and infant formula	Simultaneous analysis of slurry samples	310
ICP-AES	Slurry ETV	Al, Ti, Y	Si3Ni4 powders	Samples were analysed with and without PTFE as a fluorinating agent. 90% of a 100 µg sample could be vaporised with no loss of analyte. LODs from 0.11 (Al) to 0.09 µg g^−1 (Ti) with RSD 1.9–4.2%	311
ICP-AES	Slurry ETV	Rare earths (14)	Lanthanum oxide	Samples were analysed with PTFE emulsion as a fluorinating agent. LODs from 2 (Yb) to 130 ng ml^−1 (Ce) with RSD <5%	312
ICP-AES	Laser ablation	Al, Fe, Mg	Environmental samples	Effect of matrix, chemical and physical form of analyte and laser wavelength were studied	264,313
ICP-AES	Laser ablation	Various	Various	Description of a novel bulk solid sampling system	314
ICP-AES	Slurry ETV on W-coil	Various (11)	Al-based ceramic powders	Samples were vapourized on a W-coil. LODs between 0.01 (Mg) and 8.5 µg g^−1 (Co)	315
ICP-AES/ICP-MS	Laser ablation	Various	Amorphous solids	Spatial resolution of 50 µm obtained. RSDs 0.3–0.7% obtained for Al and Si with Ba as internal standard, and 1.5% for B with Si as internal standard	316
Spark AES	Solid	Various	Steel	Steel sample is embedded in an ingot of Sn allowing analysis of small samples. Results in good agreement with those obtained from bulk samples	270

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Nolte et al.²⁵⁴ considered both laser ablation and slurry sampling for ICP-AES in their study of the influence of simultaneous versus sequential measurement of the signal, background and internal standard. The experiments were carried out using an array spectrometer allowing collection of simultaneous spectra; this permitted the use of a single data set to estimate the differences between measurements made in simultaneous and sequential modes. Simultaneous background correction and simultaneous internal standardization gave a mean RSD of 3.6%, sequential background correction and simultaneous internal standardization produced an average RSD of 6.1% and sequential background correction and sequential internal standardization produced an average RSD of 6.4%.

The use of solid sampling for ETAAS as a screening technique can provide a significant time saving over conventional digestion methods. Belarra et al. carried out a theoretical evaluation using 18 000 simulated results.²⁵⁵ The optimum conditions for analysis were determined based on the type and frequency of outliers commonly encountered in solid sampling ETAAS. The median was found to be a more useful result than the mean due to the reduced effect of outliers. The minimum number of samples required for a screening method was determined as 5–20 for a guaranteed recall of 95% (recall = no. of correct identifications ÷ no. of attempted identifications) when the median was used. This sampling rate corresponds to between 15 min and 1 h of work, a significant reduction in time compared with the preparation of liquid samples. The same workers successfully applied solid sampling to the screening of antimony in PVC.²⁵⁶

Slurry sampling for ETAAS has proved to be a popular technique for the analysis of solids with easy automation. Pioneer of the technique, Miller-Ihli, discussed the advantages of slurry analysis with ETAAS and provided evidence that the method can be successfully applied to ETV-ICP-MS.²⁵⁷ Ultrasonic mixing, either manual or automated, is commonly the preparation method of choice for slurry sampling. The maintenance of a homogeneous slurry allows the sampling of both liquid and solid from sample cup to furnace. Amoedo et al. considered the extraction of Pb from solid samples using ultrasound with subsequent analysis of the supernatant alone.²⁵⁸ Probe sonication allowed quantitative recoveries of Pb from several plant and animal SRMs. Several chemometric techniques were used by Cal-Prieto et al. for the optimization of a method for the determination of Sb in geological samples using automated slurry sampling ETAAS.²⁵⁹ Plackett–Burman designs were used to assess the influence of sample mass, volume, HNO₃ concentration, Triton X-100 concentration, ultrasonic probe power and sonication time, and to optimize the variables. A two-way ANOVA was used to evaluate the effects of sample cups and replicates. Control charts were also used to monitor graphite tube performance. In addition, two pipetting options and three quantification methods were studied. The usefulness of the optimized method was demonstrated by the accurate analysis of 5 SRMs.

A review (115 references) of laser ablation (LA) sampling by Russo et al. was published during the period of this review.²⁶⁰ Calibration and optimization, fractionation, sensitivity enhancements, mass loading and particle transportation were discussed. A single calibration graph was successfully used for the determination of major elements in geological materials in LA-ICP-AES by Kanicky et al.²⁶¹ Sc and Y were used as internal standards and limestone and silicate samples were analysed. The same authors^262,263 measured the acoustic signals produced during laser ablation ICP-AES using a Nd:YAG laser at 266 nm. It was found that the acoustic signal could be used to focus the laser on the solid surface by monitoring the maximum in the acoustic signal. The intensity of the acoustic signal could also be used to determine the size of the crater produced during ablation.

A systematic investigation into the effects of the matrix, chemical and physical form of the analyte and laser wavelength was carried out by Motelica-Heino et al.²⁶⁴ Synthetic samples were prepared from different crystalline compounds of Al, Fe and Mg spiked in SiO₂ or CaCO₃ pressed into pellets and analysed using LA-ICP-AES using a Nd∶YAG laser at 1064 and 266 nm. The experiments demonstrated the strong dependence of the LA-ICP-AES response factor on the chemical form of the analyte and on the bulk matrix composition. These effects were also dependent on the laser wavelength, and use of the UV laser did not lead to any improvement in minimization of the effects. In contrast to chemical effects, there was no effect on the response factor from the grain size or binding pressure of the pressed pellets. Matching of mineralogical and chemical composition of the matrix and of the chemical forms of the analytes for calibration standards is recommended to avoid systematic errors in quantitative analysis. The effect of the gas environment on laser ablation characteristics was investigated by two groups of workers. The effect of five noble gases (He, Ne, Ar, Kr and Xe) in the laser ablation sampling chamber was studied by Leung et al.²⁶⁵ An enhancement of ICP emission intensity for laser sampling in He and Ne and a decrease for Kr and Xe relative to Ar was observed. Therefore, the use of He as the sample chamber gas can significantly improve sensitivity. Thareja et al.²⁶⁶ used an intensified CCD to image the plumes produced by the laser ablation of aluminium and PTFE substrates in Ar, He, air and O₂ at various pressures. The effect of ambient atmosphere on the expanding front of the plume was investigated photographically. Stratification of the plasma was observed at moderate laser intensities for both the metal and the polymer materials.

The nature of the aerosols produced from the laser ablation of polymer materials using a UV laser was investigated by Todoli and Mermet.²⁶⁷ Ca, Sn and Ti were determined in PVC and PE samples and the aerosols compared with those obtained from glass samples. For Ca, but not Sn or Ti, the aerosol particle size was dependent on the chemical form of the analyte. A second PVC sample, containing 11 elements, was also studied, and Al, C and Na exhibited different behaviour with particle size with respect to the remaining elements. Carbon was mainly present in gaseous form and in particles of less that 3 µm in diameter; this observation precludes its use as an internal standard to compensate for variations in the ablation process for polymers.

Individual fluid inclusions were analysed using LA-ICP-AES.²⁶⁸ The laser radiation is used to drill the solid sample until the fluid inclusion is reached. Emission from the excited atoms and ions in the fluid was analysed using a spectrometer equipped with a pulsed and gated multichannel detector. Na∶K, Na∶Ca and Na∶Li intensity ratios were measured. Calibrations were established using a range of standards, including glasses and fluid inclusions. LODs achieved were suitable for the determination of ions in inclusions.

A review (79 references) of direct sample insertion for ICP-AES by Sing appeared this year.²²⁹ A useful overview of the technique including instrumentation, operating parameters, system response, analytical figures of merit and applications is provided.

A critical evaluation of dc arc spectrometry for the analysis of solid samples by Florian et al.²⁶⁹ concluded that the modernized dc arc AES method, combining fibre optics and a multi-channel spectrometer, is a ready alternative to existing solid sampling methods for AES and AAS. Grientschnig et al. proposed a procedure for the analysis of small samples of steel using spark discharge AES.²⁷⁰ The procedure involved embedding the sample in an ingot of pure Sn. Results from the analysis of three steel samples prepared by the embedding method were in agreement with those obtained on bulk samples of the same steels. The method is suitable for samples having a diameter of greater than 6 mm.

A review (20 references) in Japanese by Shimizu²⁷¹ considered the use of glow discharge (GD) for multielement surface analysis. Shimizu et al.²⁷² assessed the resolution of GD-AES for the depth profiling of chemical species in anodic alumina films. The films, acting as cathodes, were sputtered in Ar at 3–5 Torr. Emission associated with the sputtered species (Al, B, Cr, H, P, W) was monitored throughout the experiment with a sampling interval of 0.01 s. The depth resolution was reported as being comparable to SIMS profiling of similar films.

The use of plasma etching for the characterization of non-conducting materials by GD-AES was investigated by Barshick et al.²⁷³ Studies were performed in Ar containing 0.01% by weight of CF₄. When a conducting cathode such as Cu was used, the sputtering rate was decreased by a factor of five compared with the use of pure Ar as support gas. When a non-conducting glass was analysed, however, fluoride radicals formed in the discharge reacted with the substrate to form volatile SiF₄, which was spontaneously released into the gas phase, carrying Cu and U with it. As a result, enhancements of 50 and 30% were observed for the determination of Cu and U, respectively, in glass.

Workers in Marcus' group²⁷⁴ conducted studies to determine the practical benefits of mixed discharge gases (Ar and He) in the bulk and depth-resolved analysis of solids using an RF GD-AES source. A number of parameters were assessed for both conductive and non-conductive sample matrices. It was observed that the addition of He to Ar did not improve detection limits in the bulk analysis of conductive materials. However, in the analysis of non-conductive materials, the addition of He was found to enhance analyte emission intensity without significantly influencing the sputtering characteristics. For depth resolution, optimized conditions, especially gas pressure, derived for pure Ar can be improved through the addition of He to the discharge.

Marshall²⁷⁵ evaluated the impact of lamp control parameters on the quality of both bulk and depth profile analysis by GD-AES. The lack of current and voltage stability was shown to have a larger influence on the quality of calibration curves, bulk check standard results and quantitative depth profile analysis than do similar fluctuations in pressure. Therefore, control modes that achieve constant current and voltage at the expense of constant pressure are recommended.

1.5 Electrothermal vaporization

1.5.1 ET-AAS. Modification of the graphite tube surface with metals, or use of metallic atomizers, continues to be of interest in ET-AAS. Rademeyer and de Jager³¹⁷ discussed the use of 'permanent' metallic modifiers in ETAAS. The process of sputtering the metal onto the graphite surface was investigated. Physical characteristics of the graphite surface after sputtering with noble metals were evaluated and reported. Fundamental studies on the stabilizing effect of sputtered modifiers and solution modification were carried out. A permanent iridium modifier deposited on tungsten and zirconium-treated platforms was evaluated by Slaveykova et al.³¹⁸ using XRF, ESCA and SEM. Two atomization processes were observed and explained by the differences between the Ir–W and Ir–Zr interactions and surface distribution. A similar study on the behaviour of As on tubes coated with a range of metals was carried out using XRD and SEM.³¹⁹ The performance of tube treatments in the determination of As with a palladium modifier were compared. Optimum sensitivity and precision were found with a Ta coating. Portable AAS systems based on tungsten-coil atomizers continue to be the subject of several presentations and publications. Temperature measurements of gas and coil surface temperatures have been made³²⁰ and descriptions of systems are given.^321,322

Several papers comparing the performance of chemical modifiers for ETAAS have provided some useful information for users of this technique. For the determination of Sr in biological fluids, La was found to provide optimal performance in an evaluation of 12 modifiers, including Pd.³²³ Pd was found to be the best modifier for the determination of Cd in sewage sludge and Sn in PVC after an evaluation of background effects.³²⁴ Mixed modifiers for particular applications also received some attention.^325–327

1.5.2 ETV-ICP-AES. Few papers have been published during the period of this review reporting fundamental studies with ETV-ICP-AES. A number of publications on the use of the technique for the direct analysis of solid samples have appeared and they are discussed in the previous section of this review. Work from Jones' group on the use of tungsten coils for vaporization was reviewed as conference presentations last year³²⁸ and has subsequently been published.³²⁹ The efficiency of sample introduction into an ICP via electrothermal vaporization was investigated using three methods by Kantor and Gucer.³³⁰ The methods used were: (i) a direct method based on deposition of aerosol particles and analyte vapour by mixing the vaporization product with supersaturated steam and subsequently condensing the mixture followed by the determination of the amount of analyte collected; (ii) indirectly by dissolution and analysis of material deposited on interface components; and (iii) by calculation from line intensities when using ETV and pneumatic nebulization using Hg as a reference element. Using a 200 ml min⁻¹ Ar carrier flow, the transport efficiencies calculated were 67–76% for medium volatility elements such as Cu, Mg and Mn and 32–38% for volatile elements such as Cd and Zn. However, the addition of CCl₄ vapour to the internal Ar flow resulted in transport efficiencies of 67–73% for all five elements studied.

1.5.3 In-torch vaporization ICP-AES. Karanassios et al. evaluated six vaporization chambers for the introduction of micro-samples to an ICP.³³¹ The chambers ranged in shape from conical to oval, had internal volumes ranging from 3.4–18 cm³ and were tested using in-situ generated smoke. The best results were obtained with a 6.5 cm³ internal volume chamber. The sample was deposited onto a Re filament forming a three-coil loop and inserted into the vaporization chamber attached to the ICP torch. Good analytical performance was obtained with the optimized system for both liquid and slurried samples.

2 Instrumentation

The authors of a comprehensive review³³² of recent advances in plasma emission spectroscopy project a promising future for this technique. AE detectors, optics and spectrometers, solid sampling, nebulization plasma sources, computer programming for instrument control, multi-component analysis, intelligent instrumentation and ICP-MS detection are all covered in some detail. The paper is well worth reading by all interested in this field.

Another 'general' article, by Kuss and Bayraktar,³³³ described the development and general principles underlying simultaneous AAS. Some of the advantages of this method are discussed, in particular the simplex optimization of associated heating programmes.

2.1 Spectrometers

The configuration of a new transmission grating spectrograph, composed of a grazing-incidence optical system with a large area transmission grating, was described by Chinese workers.^213,334 High efficiency and relatively high resolution are claimed. From Japan came news of a 'cavity ringdown' spectrometer³³⁵ but no technical details were given in the English abstract.

Compact échelle spectrographs with multi-channel time-resolved recording capabilities, utilizing a gated interline CCD camera, come from Multichannel Instruments, Sweden.³³⁶ The system records 6000 time-resolved channels in the 200–1100 nm region with a shortest gate time of 100 ns. Some applications are described. Instrumental developments at Thermo Jarrell Ash and Baird were the subject of a conference paper.³³⁷

Developments in digital micro-mirror spectrometry are described by Batchelor et al.³³⁸ and de Verse et al.³³⁹ This technique achieves rapid sequential read-out using a single detector. In the former paper, a 2-D array (640 × 480) of individually addressable 16 × 16 µm Al micro-mirrors enables an image of a burner to be projected on to the focal plane of a flat field monochromator. By this means, wavelength, slit width, height and position can be programmed, thus enabling wavelength scanning. In the second paper, emphasis is placed on the application of the instrument to Hadamard transform spectrometry. Applications included use with He MIP-AES.

The use of two German-manufactured mobile AES instruments with a dc arc for semi-quantitative analysis and arc-in-air or spark-in-argon for quantitative analysis of metals, was described by Russian workers.³⁴⁰ U isotope measurement and measurement of U and rare earth elements are among applications described for a high resolution ICP spectrometer³⁴¹ specifically designed for these purposes. A sequential ICP spectrometer with an intelligent wavelength calibrating device has been described.³⁴² This does not require either thermal equilibrium to be attained or the line-profile method to be used to eliminate peak drift. This instrument is also used in a metallurgical context.

For simultaneous measurement of four elements by GFAAS, Wagner et al.³⁴³ positioned the appropriate HCLs on the Rowland circle of a concave holographic grating and measured the signals simultaneously with a CCD detector. Detection limits for Cd, Cr, Cu and Pb were 4, 12, 14 and 12 pg, respectively.

A new concept in AAS is claimed by Unicam GmbH.³⁴⁴ The SOLAAR M instrument is compact but has flame and furnace chambers on either side of a central six-lamp changer compartment. It has an échelle monochromator and facilities for both deuterium and Zeeman background correction. A choice of software is available according to the experience of the user.

2.2 Sources and atom cells

This is the first time that this review has covered all the atomic spectroscopies together and it may be worth while again to clarify the above terms. 'Sources' are the generators of radiation (narrow line or continuum) which is eventually to be measured by the detector. 'Atom cells' (or 'atomizers') are the devices for producing the population of free atoms (or sometimes ions) from an analytical sample. In AAS and AFS these two devices are separate. In AES the source is also the atom cell.

2.2.1 Sources for optical emission spectroscopy.
2.2.1.1 Plasmas. Emphasis this year appears to be again on ICP and glow discharge sources. However, there is one paper from Prudnikov et al., St Petersburg,³⁴⁵ on the arc-in-flame plasma. The cathode was positioned on the burner and the anode about 30–40 mm away, the arc being maintained through the length of the flame. Arc current was 15–20 A at 220 V. The sample solution was introduced with a direct injection nebulizer and an easily ionizable element such as K was added to stabilize the discharge. Any of the usual flame gases (air–C₂H₂, N₂O–C₂H₂, air–H₂–Ar, etc.) may be used. A feature of the discharge is a 'calm' region where temperatures of up to 6000 K are reached. Stability is similar to that of the flame and detection limits are said to be as good as those in ICPs.

Helium plasmas are the subject of a conference paper by Montaser et al.,³⁴⁶ in which a number of advantages over ICPs are claimed, including better selectivity, sensitivity, precision, reliability and ease of operation, lower cost and smaller size of instrumentation. The new sources were described along with appropriate sample introduction systems for both AES and MS.

Greenfield³⁴⁷ has recalled the reasons behind the invention of the annular ICP some 30 years ago. An optical interface was developed by Rutzke³⁴⁸ to reduce the matrix effects observed in an axially viewed ICP and improve recoveries of trace elements in a complex matrix. The components are mounted on a sliding platform in order to focus on different locations along the axis of the plasma. A new, compact, free running solid state generator system for ICP-OES was described by Hensman.³⁴⁹ Discussion of characteristics and capabilities was promised in this conference paper abstract.

Barnes has co-authored two papers concerned with enclosed or sealed ICPs.^350,351 Flow-through ICP systems utilize only 10–50% of atoms present. A sealed system has a much lower gas consumption and also allows the introduction of hazardous auxiliary gases like Cl₂ or more expensive noble gases than Ar. The system is also used to measure hydride-forming elements direct from the aqueous solution in the hydride generation process. The general conference paper³⁵⁰ described all these possibilities and the second paper reported on the measurement of Fe and Ni in electronic grade chlorine, which is made to form a 50% Cl₂–Ar discharge mixture. Calibration was achieved by mixing in known quantities of iron carbonyl, ferrocene or nickelocene.

Problems in the measurement of volatile elements by ICP-OES and ICP-MS have recently been claimed to have been solved by reducing sample aerosol interactions with smaller spray chambers or even with no spray chamber at all. Eastgate³⁵² reported on results obtained with cyclonic and tulip-shaped chambers in conjunction with low-uptake nebulizers. Improvements in accuracy for Hg and I, reduction in matrix effects and accelerated sample washout are claimed. An in-torch vaporization system for introduction of liquid and solid microsamples was described and characterized by Badiel et al.³⁵³ In a clip-on chamber, the sample is dried, charred and vaporized in a rhenium cup and the vapour carried to the plasma in a stream of Ar–H₂.

A 22 mm torch for ICP-AES was characterized by Horner and Hieftje.³⁵⁴ This could be operated at powers and flow rates similar to those for a conventional 18 mm torch. Improvements in analytical performance were not claimed but the plasma in the new torch showed less long-term drift as well as other properties which made it more suitable for use in ICP-MS.

A paper in Chinese described the use of an air-cooled ICP for measuring As, Bi, Ge, P, Pb, Sb, Si and Sn.³⁵⁵ Comparison of results showed that elements with atom lines of long wavelength and low excitation potential have as good or better detection limits than with the argon ICP.

Developments in MIP technology came from Japan and Poland. Oishi et al.³⁵⁶ have designed and constructed a nitrogen MIP which was linked to MS for measuring rare earths, and Jankowski et al.²²⁰ described a vertically positioned axially viewed argon MIP, the design being based upon an improved rectangular cavity and strong coupling between the plasma load and the magnetron generator. Sample introduction was by means of a peristaltic pump. Excitation temperatures and analytical characteristics were determined for a number of elements and spatial distribution within the resonator was also studied. Figures of merit compared well with those generally accepted.

Hieftje et al.³⁵⁷ described the use of an air–Ar MIP torch for the detection of tetraethyl lead. The best detection limit (as Pb) was 0.012 ppb with linearity over three orders of magnitude.

Three papers on the capacitively coupled rf Ar plasma torch have appeared from the team led by Cordos in Cluj, Romania. This was developed³⁵⁸ for analysing non-conductive samples such as oxides and silicates. It operates at 13.56 MHz, rf input power of 20–50 W and Ar flow of <11 min⁻¹. The design is modified³⁵⁹ to have a tubular central electrode and outer ring, this time with double the frequency and with 275 W of power, resulting in improved detection limits. The performance of this plasma was also described³⁶⁰ at atmospheric pressure in 'tip-ring electrode' geometry for both nebulized liquid and sputtered conductive solid samples. Excitation and ionization temperatures were given along with performance figures for a number of elements in liquids and in low and medium alloyed steel.

2.2.1.2 Discharge lamps. A comparison of fundamental characteristics has been made between rf and dc powering of single GD sources.³⁶¹ Although the dc mode gave higher emission intensities, rf powering gave better S/N ratios, became stable more quickly and was more stable over extended periods, thus giving better detection limits.

The effect of bias voltage modulation on the analytical performance of rf powered GDs was investigated by Wagatsuma.³⁶² A 'lock-in' detection method was used for measuring Cu in nickel plates and iron alloys which contributed to an overall improvement in detection limits. An rf powered Grimm-style Ar plasma was controlled by introducing a new dc channel network.³⁶³ This is claimed to improve detection sensitivity in ICP AES and to enhance emission lines from He rf plasmas, which then excite non-metallic elements such as F and so can be applied to a wide range of elements. Wagatsuma also found,³⁶⁴ after a study of singly ionized lines of Cu and V, that ionic lines are usually more sensitive than atomic lines and are almost free from self-absorption in GD-OES. The same author³⁶⁵ made a comparison of emission characteristics of the lanthanide elements in GD-OES and ICP-OES, finding again that ionic lines are more intense in the GD.

A microsecond régime GD has been developed by Yang et al.,³⁶⁶ and its performance compared with that in the continuous mode. Pulse widths varied from 0.1–300 µs and frequency from 1 to 10⁵ Hz. The pulsed lamp has a higher sputtering rate, greater signal intensity by several orders of magnitude and better detection limits. There is a different temporal response for atoms and ions, however, emission from the former maximizing during the period of the pulse, and that for ions a few µs after the pulse termination.

Chlorine has been measured in halogenated hydrocarbon vapours by introducing the samples into a He-operated gas-sampling Grimm-type GD.³⁶⁷ Cl II emission was measured at 479.5 nm with a detection limit under optimum conditions of 20 ng s⁻¹. Spatially resolved measurements showed that the distribution in emission intensities of both Cl and He were dependent on the design of the aperture of the gas inlet capillary. Background intensities, too, were found to vary during the sample introduction period, thus the background requires careful monitoring and correction. Trace elements were measured by Park et al.³⁶⁸ in water flowing through a specially designed GD tube, the sample itself acting as cathode. The anode was a Pt rod, the end of which was positioned about 1.5 mm above the surface of the liquid. The space above the liquid was flushed continuously with Ar. The discharge was stable at 1.5 kV and 80 mA and detection limits of less than 1 µg ml⁻¹ were found for Al, Cd, Cr, Cu, Hg, Mn and Pb: unfortunately, though, this is not good enough for application to drinking water.

A two-cascade GD ion source, suitable for use with a double focus mass spectrometer, described by Sikharulidze,³⁶⁹ also operated with liquid samples. As the solution enters the hollow cathode the solvent evaporates into the vacuum, stimulated by ion bombardment, and supports the glow discharge which continuously sputters a film of analyte from the inside wall of the hollow cathode. Sensitivity is said to be comparable with that of an ICP.

An rf GD-OE spectrometer has been designed by Hubinois et al.,³⁷⁰ especially for confined use in a glove box in order to measure light elements in nuclear materials. Preliminary studies with the system were made on low alloy steels and optimizations were carried out to improve emission intensities and reproducibility.

2.2.1.3 Other emission sources. A previously described electrothermally heated HC emission source (Fresenius' J. Anal. Chem., 1996, 355, 272, Microchem. J., 1996, 54, 296) has been further developed.³⁷¹ The measurement cycle of 10 ms is broken down into 9.6 ms of low current followed by 200 µs of high current up to 125 mA, then 200 µs of high current during which the actual measurement takes place. This cycle plus a background reading cycle is repeated 33 times s⁻¹. The full heating programme consisted of one drying, two ashing, vaporization and cup clean phases, with drying of a 10 µl sample being complete in 3 min at 2200 Pa and room temperature. The operating conditions and emission intensities for Cr, Cu, Mn, Ni and Pb were discussed.

2.2.2 Atom cells for atomic absorption spectrometry.
2.2.2.1 Flame atomizers. Shimadzu have taken out a patent for an AA spectrometer³⁷² in which the inside wall of the burner is mirror-finished to prevent the slot clogging. A burner head claimed by Chinese workers to give higher sensitivity by a factor of about five³⁷³ appeared to have separate gas and sample inlets. Russian workers have proposed the use of a N₂O–methylacetylene flame³⁷⁴ which, they state, gives significantly higher temperatures than N₂O–acetylene and is therefore advantageous in the determination of some 54 elements, particularly B, Ce, Cs, Gd, Sb and Ta. Typical flow rates are: N₂O, 10 1 min⁻¹; methylacetylene, 1.25–6.25 1 min⁻¹.

While testing a new design of source lamp, Willis and Sturman³⁷⁵ found that the sensitivity of Te depends on its oxidation state in both air–C₂H₂ and N₂O–C₂H₂ flames, Te^IV giving a better value than Te^VI by a factor of 1.66. Possible reasons for this anomaly are discussed.

A micro-flame atomizer was developed by Sun et al.³⁷⁶ in order to measure organometallic compounds separated by gas chromatography. The volume of the atomizer was made to be one fifth of that of a conventional flame burner atomizer and H₂ acted both as carrier and as fuel gas. A detection limit for Hg of 5.5 × 10⁻¹⁰ g s⁻¹ is quoted.

2.2.2.2 Electrothermal atomization. A review of graphite atomizers modified with high-melting carbides was presented by Volynski.³⁷⁷ Such atomizers are most effective in the determination of elements such as B, Ga, Ge, In, Si and Sn which form stable oxides. If, however, they are applied to carbide-forming elements, e.g., Cr, Mo, Ti and V, there can be a significant decrease in sensitivity. Chemical modifiers are usually needed for high volatility analytes. But these atomizers show advantages for measuring some organometallics for the trapping of volatile hydrides, for the analysis of organic materials and for samples with high concentrations of mineral acids.

A WC–Rh coating on the platform of a transversely heated graphite atomizer has been used by Lima et al. for measuring Cd³⁷⁸ and Cd, Pb and Se³⁷⁹ by ETA. A standard THGA was pre-heated with W and then with Rh. The permanent W–Rh modifier remains stable for 300–350 firings and increases tube lifetime by 50–100% as compared with pyrolytic platforms. Analytical accuracy was shown to be at least as good as with the more conventional chemical modifiers.

Wagner et al. described³⁸⁰ a simple, low-cost multi-element spectrometer with a tungsten coil atomizer made from a standard 150 W projector lamp. Outputs from four HCLs are aligned by means of three 60° beam combiners. The purge gas is Ar–10%H₂.

2.2.2.3 Other atomizers. Absalam, Chakrabarti and others³⁸¹ discussed the geometry of an rf glow discharge for AAS. The location of the gas inlet port has an important influence on the observed signal. The best location is on the sampling orifice disc so that the mass flow of sputtered atoms is entrained in the flow of discharge gas and efficiently transported into the analysis volume of the atomization chamber. Distribution of sputtered analyte atoms is highly inhomogeneous under all conditions, though near the central axis of the atomization chamber the effect isminimal.

The ICP has generally been believed to be too hot for use as an AAS atomizer. Hensman and Rayson³⁸² developed a new optical configuration in an attempt to improve its performance in this context. An enlarged sample introduction tube was found to give lower Fe and Ar excitation temperatures than predicted but the response is affected by the presence of water vapour and by the analyte molecular bond strengths. Many of the problems have been accurately identified and investigated but no satisfactory overall solution seems yet to have been found.

2.2.3 Sources for atomic absorption spectrometry.
2.2.3.1 Continuum source AAS. Harnly's plenary review lecture to the 9th Biennial National Atomic Spectroscopy Symposium, Bath, UK, in July 1998, referred to in last year's update³²⁸ (para. 1.4.2) has now been published.³⁸³ In CSAAS, he claims, absorbances are more accurate, detection limits are better by an average factor of 2×, calibration ranges are 1000× greater and, of course, multi-element detection is possible—these are among its advantages over line source AAS. Multi-wavelength detection provides the equivalent of infinitely fast wavelength scanning, which is at the heart of CSAAS, and which has been made possible only in recent years by the use of CCD arrays. These and all other aspects of the subject are discussed in great detail in the paper. The characterization of calibration curves, discussed in the above paper, was the subject of a further paper by Wichems, Fields and Harnly.³⁸⁴ Log–log plots of calibration curves, both in theory and in practice, have a slope of unity at low concentrations and 0.5 at high concentrations. This was predicted as long ago as 1961 by Mitchell and Zemansky.³⁸⁵ The same hyperbolic curve, with appropriate X- and Y-axis offsets, can therefore be used to fit every calibration plot, its position for any one plot being characterized by means of just two standards, although, according to this paper, better S/N ratios are obtained if four are used.

Becker-Ross et al.³⁸⁶ reported further on their work with CSAAS using array detectors. These enabled them to make background correction with greater accuracy than that given by the methods normally employed in line source AAS. Two échelle spectrometers with focal lengths of 0.3 and 1.0 m were used in conjunction with linear CCD arrays of pixel size 0.023 × 0.48 mm, giving very small band widths per pixel. Optimum instrumental parameters for CSAAS were established and algorithms for computer aided background correction were tested.

2.2.3.2 Other sources. A German patent³⁸⁷ describes a laser diode which, it is claimed, incorporates structures making it suitable for use as a light source in AAS. A long lifetime gas discharge light source,³⁸⁸ tested with air, N₂ and Ar, has been constructed using automobile sparking plugs as electrodes. Stable light pulses of 0.8–4 ns width were generated though there was appreciable heterogeneity of emission intensity distribution along the analysis gap.

2.2.4 Atomic fluorescence spectroscopy. Several papers describing a system for both atomic and ionic fluorescence measurements originated from Huang's group at the University of Xiamen in China. The excitation source is a high current, microsecond pulsed HCL and the atomizer a short torch ICP. Fluorescence techniques are said to minimize the spectral overlap interference characteristic of OES and provide a broader working range than AAS. The work was done partly on a modified Baird AFS 2000. The development and evaluation of the pulse generator system were described³⁸⁹ and performance figures were quoted for 15 elements. The application of the system to the measurement of Eu using desolvated ultrasonic nebulization was described in another paper³⁹⁰ where the HCL pulsing is at 1000 Hz for 0.8 µs and the current 9 A. Both atomic and ionic lines were investigated and the latter gave better detection limits. Ion lines were used exclusively for the measurement of rare earth and alkaline earth elements^391,392 and it seems that, in this case, a small monochromator was employed. Results were claimed to be 'much better' than those to be expected with the original Baird system.

2.3 Detectors

As detectors in atomic spectroscopy, PMTs have held sway for over half a century, but modern requirements such as multi-element operation, instantaneous wavelength modulation and computer compatibility have led to the adoption and development of solid state arrays in both the research and commercial sectors. A manufacturer's overall view of the present state of the art in the use of such detectors was given by Yates of PerkinElmer,³⁹³ who emphasised the need to use the detector most suited to a particular purpose. Limitations and possible future development, particularly of charge injection devices, were discussed.

2.3.1 Charged-coupled devices (CCDs). De Goy et al.³⁹⁴ described the operational features of CCDs in an attempt to answer the question 'Which CCD is right for you?' Important factors requiring consideration include spectral response, operating temperature, spatial and spectral resolution, dynamic measurement range, camera gain and data acquisition speed. Members of the American Association of Physics Teachers suggest³⁹⁵ that CCD detectors from scanners and computer cameras offer convenient low cost means of converting a commercial monochromator to a spectrograph or of constructing a home-made spectrometer. Pointing out that these CCDs are computer interfaced, they go on to describe experiments which demonstrate their usefulness in teaching laboratories.

In investigating its response through a very wide spectrum range, a CCD has been characterized from 1100 nm through the near IR, visible, UV and vacuum UV regions down to 0.14 nm by workers in Italy.³⁹⁶ Their conclusion was that 'the CCD represents an almost optimal detector of radiation in the extended optical domain'. A conference paper from Bohle et al. of Spectro³⁹⁷ also explored an extended spectral region from 130 to 766 nm with an ICP-OES system using CCDs. This is said to give many more possibilities of simultaneous element measurement with uninterfered lines, especially between 130 and 200 nm.

CCDs have been backed by manufacturers Varian and their choice is supported by several papers this year. Knowles et al.³⁹⁸ described an 'innovative' CCD for AES and compared its performance with that of a circular array of PMs and of charge injection devices in a simultaneous ICP AES instrument. Conference papers^399,400 describe the detector itself and two more^401,402 the complete instrument in more detail. The application of the new 'CCD axially viewed ICP-AE spectrometer' to the determination of major and minor elements in silicate rocks is described by Brenner et al.⁴⁰³ of Varian USA.

2.3.2 Charge injection devices (CIDs). Thermo Jarrell Ash/Baird have used CIDs since 1992 and claim in a conference paper⁴⁰⁴ that these overcome 'several major shortcomings' of CCDs. Their new CID gives a higher yield with reduced fixed pattern noise while dark current and read-out noise are reduced and UV quantum efficiency is improved. Newly introduced ICP instruments with radial-view plasmas are claimed to exhibit a level of sensitivity approaching that of axial-view plasmas without their increased susceptibility to chemical and spectroscopic interferences. There were two conference papers^405,406 describing the application of these CID-échelle ICP instruments to, respectively, the analysis of oils and the measurement of Br, Cl and I.

2.3.3 Other detectors. A conference paper⁴⁰⁷ and a Chinese publication⁴⁰⁸ with identical abstracts herald the 'HDD' detector system from Jobin–Yvon, France. No technical details were given but features claimed are: ultra-fast high resolution scanning (220 000 acquisitions in steps of 0.015 nm), giving a resolution of 0.005 nm, and less than 2 min total recording time of the whole spectrum; high dynamic range by 'exclusive real time gain control'; sensitivity as a result of combining this detector with 'excellent optics'.

2.4 Background correction

Bengtson and Hänström⁴⁰⁹ have attempted to improve accuracy at ppm levels for many elements in GC-OES by improving the accuracy of background measurement. This involves use of a matrix-dependent background term in the calibration rather than assuming interference overlaps from other analytes. Careful assessment of instrumental drift is also essential.

True simultaneous measurements of background in analyte and internal standard have been compared with sequential measurements by Noelte et al.²⁵⁴ using the same data in both methods. Simultaneous readings of BG and IS gave an overall RSD of 3.6% while sequential readings of both gave 6.4% (simultaneous IS and sequential BG gave 6.1%). The results applied to laser ablation and to slurry sampling and these two procedures were also discussed.

Three background correction methods, deuterium lamp, high speed self reversal (SR) and Zeeman, were compared by Oppermann and Hohmann⁴¹⁰ for a range of sample types which characteristically give high background values. The SR method appears to have been preferred though the reasons were not given in this conference abstract.

Zeeman background correction in GF-AAS is evidently still able to cause problems. Over-correction in the determination of Pb in bone has been reduced by Zong, Parsons and Slavin⁴¹¹ by substituting an end-capped transversely heated graphite atomizer for the standard version, though the rationale for this is not given in the abstract.

2.5 Data acquisition and control

Two papers from Sun's group in Baoding, China,^412,413 report the use of derivative absorbance measurements, obtained with a home-made differentiating unit. The output signal is directly proportional to the rate of change of the input signal, but both signals were recorded with a double-pen recorder. The papers describe the method as applied to the measurement of trace levels of Cd in vegetables and Pb in copper. Improvements in detection limit and sensitivity by about three orders of magnitude were claimed.

Lopez Garcia et al.⁴¹⁴ are using a sample introduction system for AAS in which two analyte solutions are being pumped at different rates and merged just before reaching the nebulizer. The selectivity of this set-up has been improved by Fourier transformation of the output signals which allows the two solutions to be entirely distinguished. Another suggested application of the system is that if n different solutions of the same analyte are pumped simultaneously and the concentration of one is known, then the concentrations of the other n-1 can be calculated.

The data acquisition and transfer arrangements for in-situ AES described by Yang et al.⁴¹⁵ are based on the 'intranet' structure and work on the Windows NT operation system. Designed for the critical control of steel making, the system allows remote access and Internet connection so that analysis data can be shared, proceeded with and developed by appropriate staff throughout the organization.

A practical assessment of multivariate calibration for a Thermo Jarrel Ash échelle ICP-AES has been made by Madden et al.⁴¹⁶ who attempted to utilize as much as possible of the large quantity of data provided by such an instrument. Rapid semi-quantitation improves throughput and allows better choice of standards for those samples chosen for more accurate analysis. Use of many atomic lines per element can increase precision, and improve identification and interference correction. An iterative approach to classical least squares fitting improved results for samples which contained high levels of interfering elements.

Varian has evolved a fast automated curve-fit technique⁴¹⁷ to provide automatic and on-line deconvolution of overlapped spectra in ICP-OES. Even strongly overlapped peaks can be evaluated and small peaks can be extracted from complex backgrounds, giving greater accuracy than with conventional background correction methods.

Several strategies were compared by Noelte⁴¹⁸ for correcting analytical signals for overlapping lines in the measurement of Cd by ICP-AES. These included (i) parabolic peak fit calculation, (ii) matrix matching, (iii) Boumans' inter-element correction method, (iv) on-peak measurement and (v) multi-component spectral fitting.⁴¹⁹ Best accuracy, reproducibility and detection limits were all found for method (v), followed by (iv) and (ii).

3 Fundamentals

3.1 Plasmas

3.1.1 Microwave induced plasmas. The number of fundamental studies of the microwave-induced plasma has been small this year. MIP-OES was used to study the influence of molecular gases and analytes on excitation mechanisms of an atmospheric Ar MIP.⁴²⁰ Spectroscopic measurements gave information regarding association and dissociation processes while power interruption experiments provided information on the dominant population mechanisms of radiative levels. Nitrogen-containing molecular emission bands were observed when the MIP was expanded into air. The same group⁴²¹ used MIP-OES for the analysis of N₂, CO₂, SF₆, H₂O and SO₂ in gaseous mixtures such as flue gases. Only N₂ yielded observable emission from the non-dissociated molecule. The spectra of the other molecular gases were dominated by atomic fragments and association products, indicating that the atmospheric like plasmas are likely to be unsuitable for the analysis of gaseous mixtures under the experimental conditions used in the study. A modified plasma torch was also used by the group to determine electron number densities and electron temperatures by means of spatially resolved Thomson scattering measurements and photographic records at different working conditions.⁴²²

3.1.2 Glow discharges. In recent years, research into fundamental studies of glow discharges (GD) has beenparticularly active and this year the trend has continued. Depth profiling using radiofrequency GD-OES has become an analytical method of growing importance for the analysis of bulk materials and for profiling layers too.⁴²³ A rf GD has been studied with the aim to optimize the depth resolution by varying the rf-powered guiding parameters such as the peak-to-peak voltage and the argon flow rate.⁴²⁴ By applying a constant peak-to-peak voltage of 2000 V as well as low argon flow rates, the optimal discharge conditions were obtained. For non-conducting materials, the wavelengths used for emission intensity measurements are often prone to interferences and this problem has been addressed and explained for three non-conducting materials.⁴²⁵ In addition, surface roughness, which approaches the dimensions of the film being analysed, may lead to almost total degradation of depth resolution.⁴²⁶

Helium–argon working gas systems in rf GD-OES have been described.^427,428 The optimal emission and sputtering electrical characteristics were studied with varying He concentrations in a mixed gas system. Further, Langmuir probe studies have been used to study the effects of the rf power and the discharge pressure on the electron and ion number densities, the average electron energy and the electron temperature for aluminium and Macor in a rf glow discharge in He, Ar and He–Ar mixed gases. Measured electron and ion number densities decreased with increased He pressure and the sputter rates also decreased, resulting in enhancement of the Al I and Cu I emission intensities.

A collisional-radiative model was developed for sputtered copper atoms and ions in a dc argon GD under conditions normally employed in glow discharge AES.⁴²⁹ The populations of the various excitation levels were calculated in two dimensions for Cu ions and atoms and the relative contributions of the various populating and depopulating processes were determined for all levels. The same authors also developed a set of three-dimensional models to describe a GD in argon.⁴³⁰ In general, satisfactory agreement with experimental observations, for density profiles of various plasma species, the flux energy distributions of ions bombarding the cathode, the crater profiles and the Ar atomic emission spectrum, was achieved.

The majority of glow discharge systems currently operate in a continuous direct current (dc) or radiofrequency (rf) mode so that a steady state discharge is obtained. The amount of power normally applied to a glow discharge is 1–3 orders of magnitude smaller when compared to an ICP source.⁴³¹ Increasing the power of the GD can easily lead to sample overheating: therefore, a pulsed high current mode is used so that high instantaneous power may be applied to yield an intensified transient signal. Primary studies of a microsecond pulsed glow discharge as an emission source have been performed using hollow cathode lamps operated under different pulse conditions.⁴³¹ Passing large currents of short duration repetitively through the cathode lamp and use of a gated detection system resulted in light intensity enhancement of up to 3–4 orders of magnitude with respect to dc operation. Signal to noise ratios were also improved by up to three orders of magnitude for some ionic lines. The advantages of microsecond pulse operation over millisecond pulsing have also been demonstrated.⁴³²

The pulsed GD plasma using both helium and argon gases has been studied for the analysis of three metals, copper, iron and tantalum, by OES.⁴³³ The reactivity of the metals determined the spectral species formed. During the pulsed operation, the metal surface has more time to react with trace oxygen or water that may be in the discharge chamber resulting in the formation of increased oxides.

The Marcus design chamber was used to compare the excitation ability of both a rf and a dc GD using OES detection.⁴³⁴ Sn, Zn and Cu cathodes were used to investigate the behaviour of atomic and ionic emission lines of these analytes under different operating conditions. Lower sputtering rates were observed for the rf glow discharge, resulting in lower emission intensities.

3.1.3 Inductively coupled plasmas. The study of matrix effects on emission intensities using ICP-OES has again received much attention this year. The effect of acid concentration on emission intensity has been studied using a minitorch sample introduction system.⁴³⁵ The emission intensity, sample uptake, ionic/atomic emission intensity ratio and excitation temperature were found to decrease with increasing acid concentration. The degree of decrease depends upon the type of mineral acid used.

A multi-component quasi-stationary thermodynamic model for ICP thermochemical processes was used to calculate the changes in electron concentration in the analytical zone caused by the presence of matrix elements with different ionization potentials.⁴³⁶ The effects of plasma temperature and Na concentration on the electron concentration, ionization suppression and partial pressures of matrix ions and electrons were also measured. The effects were studied for different nebulization systems.

Matrix effects can be reduced in ICP spectrometry by applying robust operating conditions such as a high rf power level and a low nebuliser gas flow.⁴³⁷ The use of power adjustments as an alternative to matrix matching or standard additions is described. In another study, the variation of the excitation temperature with carrier gas flow rate is described.⁴³⁸

An axially viewed ICP has been used to study calcium and sodium interference effects using low and high aerosol loadings⁴³⁹ achieved by using a conventional cross flow nebulizer and an ultrasonic nebulizer. The use of high aerosol loadings caused detection limit degradation in the presence of large amounts of Ca and Na. The optimization of the instrument, in particular the adjustment of rf power to reduce matrix effects, was described. Similarly, the use of internal standardization to compensate for matrix effects owing to the presence of Na was evaluated under robust and non-robust conditions in both axial and radial viewing modes.⁴⁴⁰ Internal standardization was used to eliminate matrix effects due to Na when measurements were made under robust conditions in the radial viewing mode. Internal standardization, however, was not as efficient when measurements were made in the axial mode.

Again this year, the use of simulations on the basis of droplet desolvation and solute-particle vaporization processes have been used to study the mechanism of matrix interferences in ICP spectrometry.^441–445 The generation of an aerosol by pneumatic nebulization, concurrent with the production of a distribution of droplet sizes, may be studied in order to examine the effects of different matrices.

Spatial time-integrated and space–time resolved profiles of excited atoms of oxygen in the ICP have been measured by optical emission spectroscopy.⁴⁴⁶ The spatial emission profiles of the discharge were required to understand the basic kinetics of the ICP. Optical emission data were supplemented by Langmuir probe measurements of electron densities and plasma potentials. A sample gas mixture of N₂ and O₂ was used to study oxygen/nitrogen spectral line ratios. The influence of the thermal disequilibrium on the composition and the volumic enthalpy of the plasma produced in an ICP torch was studied. It was found that, if the temperature was sufficiently low, the thermal disequilibrium had no influence on the intensity of the spectral line ratios.

The effects of natural moisture and of argon addition on the plasma temperature and detection limits for an air inductively coupled plasma were investigated.⁴⁴⁷ The influence of H₂O and of alkali metal salts were studied in terms of their effect on the discharge parameters such as the local excitation temperature in the analytical zone and on the thermal equilibrium of the plasma. The presence of moisture and alkali metal salts both led to decreases in analytical sensitivities and worse detection limits. Detection limits could, however, be improved by adding Ar (up to 50% v/v) to the air ICP. Work was presented to demonstrate that a stepwise series model can approximate the atomic excitation process in ICP-AES. The non-Boltzmann and non-Saha factors of Ca were calculated for 48 levels corresponding to radiative decay, radiative recombination, Penning ionization and absorption processes, in the axial channel of the ICP.

Two conference proceedings discussed the use of low power reduced pressure ICPs.^448,449 Electron number densities of helium and mixed helium–argon ICPs were measured with the aim to optimize the analytical capabilities and ionization characteristics of the mixed gas source.

3.1.4 Other. Temporally and spatially integrated measurements of the intensities of ionic and atomic lines were used to calculate the degree of ionization in a He, Ar and He–Ar mixed gas plasma in a rf furnace atomization plasma emission spectrometry source.⁴⁵⁰ The emission intensities of both ionic and atomic lines and analyte ionization were enhanced as the Ar composition increased in the He–Ar plasma. The maximum degree of ionization was obtained in a pure Ar plasma.

3.2 Flames

The mechanism of hydride atomization and the fate of free atoms in a miniature diffusion flame have been described.⁴⁵¹ Macroscopic movements and free atom diffusion were studied and spectroscopic temperature measurements were made based on atomic absorption. It was found that the temperature distribution was highly inhomogeneous and ranged from 150 °C to 1300 °C.

A steady state kinetic model has been used to study atomization processes during flame atomic absorption on the effects of Group II elements.⁴⁵² Two forms of interference were identified in the study, the formation of refractory oxides of the Group II elements rather than free atoms and the dissociation of AlCl₃ causing collisional recombination of the Group II metal and Cl atoms. Separately, measures to prevent Zeeman splitting of AlCl, causing background absorption, has been addressed.⁴⁵³

3.3 Furnaces

Many workers continue to be involved in the elucidation of mechanisms taking place in the heated graphite furnace. Alternative methods of calibration in ETAAS have been proposed by several groups of workers. Absolute analyte detection in ETAAS was described by Gilmutdinov and Harnly.⁴⁵⁴ The technique is based on the integration of absorbance over wavelength, height in the furnace and time. Adequate resolution in these three dimensions is possible using a continuum source, a high resolution échelle monochromator and a 2-D solid-state array detector. Radziuk et al.⁴⁵⁵ discussed the use of internal standardization in ETAAS. This use of this technique is based on the supposition that the sensitivity of the analyte and the internal standard depend identically on the sample matrix and any variations of conditions occurring during the pyrolysis and atomization steps. Several clinical samples were analysed for lead content using either Bi or Tl as internal standard. Correlation of r = 0.94 was found between the integrated absorbances for spikes of the analyte and internal standard when Bi was used. Tl was found to be unsuitable as an internal standard for Pb determination. RSDs (%) were found to be 7, 2.2 and 4, for urine, blood and placenta samples, respectively. The mean difference in the Pb concentration found in analysed samples by the method of additions and by using an internal standard was 10%.

The acid corrosion of transversely heated graphite tubes and platforms was studied using SEM by Rohr et al.⁴⁵⁶ The effect on graphite surfaces during the atomization of V in the presence of HNO₃, HF, HCl or HClO₄ was compared. Increased corrosion and reduced furnace lifetimes were observed in the sequence HNO₃ < HF < HCl < HClO₄. Morphological changes as a result of corrosive attack were different in each case. Changes in the electrical resistivity of the tubes during lifetime tests were within the specified range, and so the voltage controlled temperature settings were unaffected.

A rf furnace atomization plasma emission spectrometry source was used to measure the intensities of atomic and ionic emission lines of 7 elements in He, Ar and He–Ar mixtures by Sun et al.⁴⁵⁰ Emission intensities and the degree of ionization increased as the Ar content of the plasma gas was increased from 0–100%. The degree of ionization was >70% in the Ar plasma for elements with an ionization potential of <8 eV and maximum degrees of ionization of 99, 80, 81, 72, 76, 12 and 38% were obtained for Cr, Mn, Mg, Co, Fe, Cd and Zn, respectively, in pure Ar.

A review (40 references) in Chinese described the methods used for studying atomization mechanisms in ET-AAS⁴⁵⁷ included thermodynamic, kinetic and instrumental methods.

The gaseous reduction of metal oxides by carbon (ROC) in graphite furnaces was first proposed by L'vov in 1981 to explain spikes on absorption signals of aluminium. The foundation of this mechanism is the assumption that the oxide is directly reduced by gaseous molecules of metal carbides which, in turn, form by the interaction of metal vapour with carbon. Periodicity in the spikes were observed for a number of elements in addition to aluminium, and so the temporal oscillation in the kinetics of carbothermal reduction of oxides was considered. A number of subsequent studies provided alternative theories to the ROC model for the appearance of spikes in the atomization signals for aluminium, aluminium oxides detected using MS appearing at the same time as spike formation being the most compelling evidence against the ROC theory. However, responses by L'vov kept the ROC theory alive. A recent investigation into the temporal oscillations for elements from several groups of the periodic table has been carried out in two laboratories, one by L'vov's group and the other at the National Research Council of Canada by Sturgeon.⁴⁵⁸ Microgram amounts of 10 elements as their nitrates were evaporated using slow heating rates. Periodicity of spikes was observed for 10 elements; this is interpreted as the reduction of oxides. It is recognised in the paper that the study is purely descriptive but the data suggests that the ROC theory is still open for discussion!

Shadow spectral digital imaging (SSDI) with charge coupled device detection was used to study the spatial and temporal distributions of atomic, molecular and aerosol species formed during the atomization of a range of elements by Panichev et al.⁴⁵⁹ Complex, non-uniform structures of aerosol species located away from the tube axis and walls were prevalent. The observed signals were postulated as being due to the presence of metal clusters in the case of Ag, Au and Pd, and oxide aerosols in the case of elements such as Al, Ca and Mg. Scatter of source radiation observed during atomization of large masses of analyte, or during analysis of samples containing high concentrations of matrix species, is due to this aerosol formation.

Langer and Holcombe used TEM and high-energy electron diffraction (HEED) to analyse the Ag particles generated in a graphite tube-type atomizer.⁴⁶⁰ The Ag particles were collected by thermophoresis with a cooled Cu probe. The HEED patterns obtained matched body centred cubic Ag with microcrystalline domains. These observations are in agreement with previous studies suggesting particle formation in the electrothermal atomizer. The appearance of Ag in the gas phase at temperatures below the vaporization temperature was explained as decomposition of oxyanion salts during thermal pre-treatment when internal gas flows are high.

Bozodogan proposed a method for the determination of the order of release and the activation energy for atom formation process from a single absorbance signal for Au, Cu and Ni under non-isothermal conditions.⁴⁶¹ Proportionally spaced cubic B-spline functions were used for fitting the experimental absorbance signals and least-squares regression was used to obtain mathematical expressions for the experimental temperature signals. From this fit, the absorbance (A), its time derivative (dA/dt), and related parameters were obtained. The proposed method was compared with results obtained from the same experimental data set using the methods proposed by Sturgeon, Smets, McNally and Holcombe, Rojas and Olivares, and Yan et al., and to the results reported by Fonseca et al. using the same methods. Good agreement was found between the activation energies and order of release for the three analytes computed using the proposed method and the existing methods. It is claimed that the proposed method is simpler than the methods used for comparison, since the order of release and the activation energies are calculated under non-isothermal conditions.

The atomization kinetics of Au in the absence and presence of a range of chemical modifiers was investigated by Thomaidis and Piperaki.⁴⁶² In the absence of modifier, a two-precursor atomization mechanism was observed in distinctive temperature regions. When long pyrolysis times and low analyte masses are used, atomization from dispersed particles with low activation energies is observed in the low temperature region. At high masses of Au, a fractional order atomization from Au agglomerates with high, mass dependent activation energies, when approaching the heat of vaporization, is observed in the high temperature region. When ascorbic acid is used, fast atomization from surface particles at the active sites produced by the pyrolysis of ascorbic acid is suggested by a high activation energy in the low temperature region. A low activation energy is obtained in the high temperature region, with first-order kinetics indicating a desorption process through the micropores of the amorphous carbon residue of ascorbic acid. In the presence of Re, a two-precursor mechanism was also observed with a high activation energy in the low temperature region, suggesting vaporization from small clusters, and a low activation energy in the high temperature region, with first order kinetics, indicating vaporization of disperse particles from the graphite surface. In the presence of Pd, again, a two-precursor mechanism was observed. A low activation energy process in the low temperature region indicating vaporization of disperse particles from the available free active sites on the graphite surface. The second process, in the high temperature region, begins at the appearance temperature of Pd and shows a high activation energy value and first order kinetics, suggesting that the release of Au atoms occurs only after the vaporization of Pd has begun. In the presence of Rh, a mass dependent activation energy is observed in the low temperature region, suggesting atomization from Au clusters.

Further results from the investigations into the migration of analytes from the sample deposition site on the furnace wall to cooler regions of the furnace during thermal pre-treatment from Jackson et al. have appeared.⁴⁶³ The method was described in last year's review. The migration temperatures for Cd, Mn, Pb and Tl (300–350 °C, 1000 °C, 400–500 °C and 250 °C, respectively) were found to be significantly lower than conventional platform atomization appearance temperatures. This can lead to appreciable analyte migration to the ends of the furnace, which can lead to enhanced chemical interference and diffusional losses of analyte. The effects were observed to be less severe if the sample is deposited on a platform, if gas flows towards the centre of the furnace are used, and if palladium is used to physically stabilize the analyte.

Lamoureux et al.⁴⁶⁴ used synchrotron X-ray absorption fine structure spectroscopy to investigate the mechanism of the palladium modification of Se. The formation of a Se–Pd compound was proposed, thus verifying the results of previous studies using MS and Rutherford backscattering. The stoichiometry of the compound, either Se_xPd_y or Se_xPd_yO_z, could not be determined.

SEM-EDS was used by Zachariadis et al. to investigate changes in a Pt-based (H₂PtCl₆) and in a W-based (Na₂WO₄) modifier during pre-treatment and atomization.⁴⁶⁵ The crystalline form of the modifiers was studied using SEM at ×2000 magnification. At this magnification, the topochemical composition of the products of 50 µm scanning areas was estimated using EDS, the acquired spectra allowing the estimation of the surface chemical compositional changes. Both types of modifier were observed to change their crystalline form by formation of small, well-defined crystals, at temperatures between 500 °C and 1250 °C. Corrosion of the graphite surface was observed during the use of H₂PtCl₆, which was observed to be due to the liberation of chlorides from the crystal structure at temperatures between 750 °C and 1000 °C.

The third paper from Fischer and Rademeyer on their studies of selenium atomization considered interferences due to sulfate and the use of palladium as chemical modifier.⁴⁶⁶ Kinetic parameters were calculated for Se with varying amounts of sulfate when the modifier was either pre-mixed with the analyte solution or pre-reduced in the furnace prior to addition of analyte. At high levels of interferent, the kinetic parameters approached those for the atomization of Se in the absence of modifier. Higher levels of sulfate could be tolerated when pre-reduced palladium was present. The interference mechanism was postulated as being due to the formation of thermally stable palladium sulfate, thus reducing the number of active palladium sites available for Se stabilization. Sulfate only interfered in the high temperature stabilization of Se and the low temperature stabilization, linked to formation of a [Pd, Se, O] compound, was unaffected.

Several papers from Katskov and co-workers describing their work using atomic and molecular absorption spectrometry to investigate the vapours evolved during electrothermal atomization have been published during the time period of this review. Molecular absorption spectra were collected using a linear array of charge coupled device detectors. Absorption by alkali halides was measured during furnace heating from 500 °C to 2000 °C.⁴⁶⁷ The complexity of the molecular bands, and the magnitude of the absorbance, was seen to increase from fluorides to iodides. The appearance of vapours was observed between 680 °C for RbI and 1220 °C for LiF, while the maximum absorbance was reached between 800 °C for CsI and 1440 °C for LiF. The characteristic temperatures of the vaporization peak were shifted towards lower values going from fluorides to iodides. Magnesium chloride vapours were observed during vaporization from pyrocoated and tantalum lined graphite tubes.⁴⁶⁸ A broad molecular band was observed at 210 nm and attributed to MgCl_2(g). The broad signal was followed by a series of sharper bands characteristic of MgCl_(g). The release of MgCl vapours was accompanied by Mg atomic absorption and by light scattering. MgCl₂·6H₂O partially vaporizes as MgCl_(g) and partially reacts with the water of crystallization, leading to a mixture of magnesium hydroxychloride and hydroxide. Further heating of the condensed phase resulted in formation of MgO_(s) and MgCl vapours. The hydrolysis reaction was favoured by long pyrolysis treatments or by stopping the gas flow during pyrolysis. When He was used as the purge gas, or when tantalum lined tubes were used, the fraction of salt vaporized as MgCl₂ was increased, while scattering was not observed. Using the same methods, the vaporization of sea-water and its constituent salts in a graphite furnace, platform furnace and filter furnace was investigated.⁴⁶⁹ The most intense molecular bands, due to NaCl, were reduced, but not completely eliminated, by the addition of HNO₃. The addition of HNO₃, however, promoted the appearance of O-containing species, the intensities of these bands being lower in the filter furnace due to reactions with carbon as a result of the increased surface area for reaction.

Akman and Tekgul⁴⁷⁰ studied the effect of a mixture of sodium, magnesium, sulfate and chloride on the atomization of manganese. In the presence of the four ions, X-ray diffraction studies confirmed that sodium chloride and magnesium sulfate were found to be the major salts formed after the drying step. Sodium chloride caused a major suppressive effect when low pyrolysis temperatures were used; this is due to both analyte expulsion with matrix and to formation of manganese chloride in the gas phase. Magnesium sulfate, however, caused no suppressive effect. The presence of magnesium sulfate was actually found to decrease the suppressive effect of sodium chloride on manganese due to some protecting effect.

Grinshtein et al. used a two-step atomizer for the reduction of matrix interferences during the atomization of Pb and Cd.⁴⁷¹ Samples were dried in a transversely heated tube and then vaporized using pulse heating and carried in a stream of Ar into a pre-heated atomizer for measurement. Significant reductions in background absorbance were noted in the determination of Cd and Pb in 10-fold diluted sea-water, and in the presence of a range of salts.

4 Laser-based analytical atomic spectrometry

In analytical atomic spectrometry lasers are employed in one or other of two modes, they may be used either (1) primarily as an energy source where wavelength is a secondary consideration or (2) primarily as a bright radiation source of precise wavelength. This review of lasers in atomic spectroscopy will be divided into those two broad categories. Several reviews of the analytical use of lasers have appeared.^135,472,473 A book on the topics covered by this present review, edited by Sneddon, Thiem and Lee, is now available.⁴⁷⁴ At a more fundamental level the theoretical origin of non-linear resonances measurable by tunable lasers has been discussed by Ghosh.⁴⁷⁵

4.1 Lasers as energy sources

In this mode of use, the merit of the laser lies in its ability to deliver a high energy density on to a small area, thereby vaporizing a small volume of the sample to facilitate micro-sampling and, by using repeated pulses, depth profiling. The energy delivered by the laser may also be sufficient to generate a plasma and excite the emission spectrum of the analyte atoms. The wavelength of the laser radiation is not usually a critical factor and wavelengths ranging from 266 nm to the infrared have been used, although matching wavelength to the sample material can be advantageous. Pulse energies are of the order of millijoules with repetition rates optimized to match analytical requirements.

4.1.1 Laser ablation.
4.1.1.1 General studies. In addition to its use in analytical atomic spectrometry, laser ablation (LA) has found applications in a number of fields e.g., cleaning delicate objects, surgery and chemical synthesis. The conditions required for cleaning ancient papers and parchment have been investigated by Kautek et al.⁶⁰⁴ Using a model system, they found that the laser induced plasma (LIP) spectrum was a poor indicator of the end-point of the cleaning process and that laser induced fluorescence (LIF) spectrometry was a more promising monitoring procedure. Pulsed LA was used to generate a plasma of Al into a pulsed electrical discharge in N₂ as a means of synthesising aluminium nitride.⁶⁰⁵ The plasma reaction was studied using time- and space-resolved OES. The delay time between laser and electrical pulses was adjusted to give the maximum N⁺ emission to produce the purest aluminium nitride powders.
4.1.1.2 Atom vapour generators. Laser ablation is used as a means of generating an atomvapour for use in conjunction with a number of different analytical techniques. The most common of these is the ICP in which LA replaces the nebulization–vaporization stages of the conventional ICP system. In one newly patented instrument an O₂-containing gas was used to transport the vaporized sample to the ICP.⁶⁰⁶ A study of LA-ICP-AES analysis of synthetic geological powders found that response factors were dependent on chemical form, composition of the matrix and laser wavelength (266 or 1064 nm), but the effect of grain size and binding pressure of the pressed pellets was insignificant.²⁶⁴ The test elements were Al, Ca, Fe, Mg and Si. To obtain quantitative results, matching of the sample and sample matrix properties was necessary. Another in-depth study of the influence of pellet composition on the LA-ICP-AES/MS analysis of geological reference materials found that trace element fractionating occurred on repeated irradiation of the target and that there were significant differences in sensitivity when analytes were added as oxides, sulfates or in a metallic form.⁶⁰⁷ The use of internal standardization was only partly successful but the use of a suitable binding material improved performance. The sensitivity of conventional ICP-AES was enhanced by preconcentration from solution onto an APDC-polystyrene (PS) film which was then sampled by LA into the ICP.⁶⁰⁸ However, the APDC-PS system was found to have poor precision, selectivity and to suffer from chemical interference therefore other ligand-polymer systems were sought.

Laser ablation has been used to inject bursts of material into a GD.⁶⁰⁹ Copper in steel was the test system and the Cu emissions from the vacuum laser and the GD plasmas were investigated separately and in combination. It was found that the GD signal was enhanced when sampling LA material while the high background of the laser plasma itself was avoided. By LA non-conducting materials could be sampled into the GD.

4.1.2 Laser induced plasmas. A review of predominantly 1997 literature with 43 references has been prepared by Rusak et al.⁶¹⁰
4.1.2.1 Fundamental studies. The energies and number densities of ions emitted from laserinduced plasmas (LIP) were measured by a time-of-flight technique.⁶¹¹ The energy and length of the Ti-sapphire mode-locked laser pulses were 1.0 mJ and 100 fs, respectively, with irradiance at the solid elemental target of 2 × 10¹³ W cm⁻². Approximately 2 × 10¹³ atoms were removed per pulse and ion velocities ranged from 5.0–30.0 km s⁻¹ with average kinetic energies of 20–50 eV. The composition and evolution of the plasma produced by LA at 355 nm of a La₂O₃–CaO–MnO₂ target in an O₂ atmosphere has been studied by means of time and space resolved OES.⁶¹² Emissions of atoms and ions lines and of oxide bands were observed. Time delays between the emissions of components of ablated species were investigated as a function of distance from the target surface, gas pressure and laser fluence and possible mechanisms proposed.

Kurniawan and co-workers have investigated the characteristics of laser-induced shock-wave plasmas (LISP).^613–615 The systems, used under reduced pressure (1–10 Torr), were a TEA CO₂ laser (5–80 mJ, 8 ns) with a copper target and N₂ and XeCl excimer lasers with various solid targets.⁶¹³ Using time-resolved spatial distribution measurement of emitted radiation they observed a primary plasma, which gives off intense continuum emission close to the surface for a short time, with a secondary plasma expanding with time around the primary plasma and emitting sharp spectral lines with low background emission. The secondary plasma was attributed to excitation resulting from the blast wave. It is claimed that the secondary plasma is very suitable for spectrochemical analysis by virtue of its high excitation temperature, low background emission, good precision and linearity of response.

Studies of line profiles in low pressure (1–10 Torr) LIP, generated by Nd:YAG LA of CaCO₃ doped with Rb by atomic absorption using a narrow band CW Ti-Sapphire laser, have been reported by Gornushkin et al.⁶¹⁶ The plasma was in non-equilibrium with a kinetic temperature of 3000 K and an excitation temperature of 8000 K. Line broadening for the trace element (Rb) was Doppler dominated but for the matrix (Ca) resonance broadening was the main contributor to line shape. Number densities of electrons and atoms were deduced. Other studies by the same authors used the 'curve of growth' methodology to determine fundamental parameters of LIPs.⁴⁷⁶ The system studied was 0.007–1.3% Cr in steel using the 425.4 nm Cr line in the plasma generated by a Nd∶YAG laser. The damping constant 'a' (∼Lorentzian/Doppler line width) was estimated to be 0.20 ± 0.05 and the number density of neutral Cr atoms corresponding to the transition between low and high optical densities was 6.5 × 10¹² cm⁻³. This point also represents the upper limit of the linear calibration range.

A computer program for calibration-free quantitative elemental analysis has been developed by Ciucci et al.⁴⁷⁷ The program is based on the following assumptions: (1) the plasma composition is representative of the sample; (2) at the moment of measurement the plasma is in LTE; (3) the radiation source is thin; (4) all the atomic components of the sample are detected by measurement of at least one of their characteristic spectral lines. Satisfactory results were obtained when the program was applied to the analysis of atmospheric air and an aluminium alloy. Aluminium alloys were also the test material for the investigation of the plasma formed in air at atmospheric pressure using sub-ps laser pulses.⁴⁷⁸ The plasmas were characterized in terms of their appearance, spectra, space-integrated temperature and electron density.

4.1.2.2 Instrumentation. Redeposition of ablated material from high purity Fe and Zn foils has been found to make a significant contribution to the measured LIPS signal.⁴⁷⁹ This effect was prevented by operating at reduced pressure (1 mbar) with depth resolution improved in an Ar atmosphere. The non-homogeneous distribution of energy in the laser beam profile produces an uneven crater which has a deleterious effect when depth profile information is sought. To improve depth resolution a 2-lens telescope has been used to generate from an XeCl excimer laser a flat profile beam (5 × 2.5 mm) with an energy density of 107 W cm⁻².^480,481 The beam gave flat ablated profiles in Zn coated steel and a depth resolution of a few nm pulse⁻¹. Plasma emission was collected using a single quartz fibre optic cable. When using fibre optics as collectors of plasma emission care is necessary to avoid laser damage to the fibres. A multi-purpose fibre optic probe incorporating both single strand optical fibres and an image guide has been developed for use in LIPS for delivery of laser radiation and for the simultaneous observation of atomic emission, Raman spectra and Raman images of TiO₂ and Sr(NO₃) particles on a soil substrate and of differing regions of a granite sample.⁴⁸² A linear array of fibres (1.5 mm spacing) in an 8-ended fused-silica optic fibre bundle was used to observe the time-resolved emission profile along the axis of the plasma plume.⁴⁸³ The optimum location in the plasma for spectral analysis was found to depend upon the matrix and analyte concentration but a clear global optimum was observed for the analyte signal as a function of location and time.

For spectral analysis of emission from the laser plasma, Czerny–Turner spectrometers are widely used. The spectral resolution of these instruments is not very great nor is their spectral coverage extensive. These limitations are not serious in the case of single element analysis but they are limiting for multi-element analysis. In traditional multi-element emission spectrometry Rowland polychromators and echelle spectrometers are long established as the preferred instrumentation. The combination of echelle spectrometers with time-gated intensified charge coupled devices (ICCD) have been demonstrated to be suitable for multi-element analysis using LIP sources.^617–618 The spectral range covered is determined by the dimensions of the ICCD. The Rowland polychromator with PMT detection does not suffer the same spectral range limitation but is less flexible in the selection of spectral lines and background measurements. The performance of an echelle-ICCD system was demonstrated by the LIPS analysis of an Al alloy using a Nd:YAG laser.⁶¹⁸ Calibration graphs of the intensity ratios (analyte/Al) were prepared for Cr, Cu, Fe, Mg, Ni and Ti; LODs were in the range 1–10 µg g⁻¹. A computer program for processing data from a LIPS multi-channel analysis system has been described and applied to the determination of Mn in steel using a XeCl excimer laser at 308 nm with a pulse energy of 120 mJ.⁶¹⁹

4.1.2.3 Applications. Table 2 presents a summary of recent applications of LIPS. The development of a LIPS analytical method is a demanding process requiring optimization of the instrumental configuration, its operating conditions, and minimizing matrix effects and plasma fluctuations. One such exercise has been described by Panne et al.⁶²⁰ for the analysis of glass and glass melts during the vitrification of fly and bottom ashes in a process designed to immobilize heavy metals. Spectra were averaged over 100 laser shots of 75 mJ each. Considerable pulse-to-pulse variation in the excitation temperature and electron density was observed owing to changes in the laser-material interaction. A normalization procedure incorporating the Saha-Boltzmann equilibrium relationship reduced the variation in line intensity ratios and led to a linear calibration of LIPS intensity ratios against concentration.

In an attempt to speed up the analysis of phosphate rock single laser shot LIPS has been examined as an alternative to ICP-AES.⁴⁸⁴ When only a single laser shot is used to generate analytical data correction procedures are essential. Two sources of fluctuations were identified: firstly, the heterogeneity of the sample; and secondly, plasma interactions. A correction procedure was devised which gave linear calibration curves with a precision of 5–10%.

4.2 Lasers as sources of intense monochromatic radiation

In this mode of utilization the laser radiation interacts directly with the analyte atom vapour which has been generated by an independent process. The wavelength of the laser emission is tuned to meet the requirements of the analyte species.

4.2.1 Laser excited atomic fluorescence. In addition to its use in elemental analysis, LEAF makes a significant contribution to the study of a variety of systems involving atom vapour generation. A general review of LEAF has been prepared by Hou et al.⁴⁸⁵
4.2.1.1 Fundamental studies. The axial and radial distribution of atoms in the cathode hole of an HCL has been studied by LEAF. A model of the spatial distribution of sputtered atoms and fluorescence signal was proposed. The ion beam inside the skimmer cone of an ICP-MS instrument has been probed by LEAF.⁴⁸⁷ Reference measurements were made outside the tip of the cone with Ba and Sc ions and Pb atoms as the test analytes. Scandium ion densities dropped the most rapidly while the transmission efficiencies of both Sc and Ba ions were suppressed by the addition of either Mg or Pb. The lower limit for the transmission efficiency of the Ba ion from the plasma to the second vacuum stage was estimated to be 0.3%. Laser-excited atomic fluorescence of Na has been used to measure temperature, pressure, axial velocity and species concentrations in wind tunnels, rocket engine exhaust and the upper atmosphere.⁶²¹ These measurements were complicated by optical pumping leading to enhanced fluorescence signals. An extension of rate equations originally developed to account for features of the pumped spectrum was shown to facilitate accurate measurements of temperature. When the 2-photon laser induced fluorecence method is applied to the measurement of small CO concentrations in a high-temperature gas mixture containing CO₂ has been shown that consideration should be given to the photodissociation at 230.08 nm of the CO₂ under the action of the probing radiation.⁶²²

Time resolved LEAF has been used to study the interaction between Eu^III and a sugar based diblock surfactant-cage molecule (cyclam).⁶²³ The excitation wavelength was 337 nm. The separate constituents, cyclam and surfactant, had no effect on Eu fluorescence but together there was a significant effect. The complexation constant was calculated from the intensity ratio of the 592 and 618 nm lines.

As an alternative to laser excitation the potential of pulsed HCLs continues to be investigated.⁶²⁴ Using microsecond high current pulses through an HCL intense radiation was generated and used to excite atomic and ionic fluorescence in the tail flame of an extended-sleeve torch. Detection limits for Ba, Ca, Eu, Sr and Yb were improved 1–2 orders of magnitude compared with those obtained using conventional pulsed HCL operation.

4.2.1.2 Applications. Where LEAF is used as an established analytical method papers on such topics will be presented in the ASU review devoted to the specific sample type. In this section developmental topics in LEAF will be presented grouped according to mode of atomization.

Laser ablation combined with LEAF has been investigated by Winefordner's group for the determination of Li and U isotopes, respectively, in solids.^625–626 In the former case, an excimer laser was used for ablation and to trigger a second excimer laser which pumped a tunable dye laser. The sample was mounted in a quartz-windowed stainless steel low-pressure chamber which translated horizontally across the laser beams at 0.5 mm min⁻¹. The output of the dye laser scanned at the rate of 25 pm min⁻¹ across the Li 670 nm doublet. The fluorescence signal was detected using a grating monochromator and PMT. Using ∼2000 laser shots the ⁷Li to ⁶Li ratio was calculated from the average of 10 scans to be 12.1 ± 0.5. For the measurement of U isotope ratios, ablation was by Nd∶YAG laser and fluorescence excitation by diode laser. The latter was used in two modes: either the laser was scanned across the spectral interval including the 2 isotope peaks during each ablation pulse or the wavelength was fixed at the peak of either isotope line on alternate pulses. In the analysis of UO₂ the LOD for both ²³⁵U and ²³⁸U was ∼0.6 mg g⁻¹.

Electrothermal atomization has been used since the early days of AFS and continues to offer the lowest reported LODs. This sensitivity is largely attributable to its efficient sample utilization and relatively low background emission. A graphite rod atomizer and excitation of atomic fluorescence by an excimer-pumped tunable dye laser has been used for the analysis of geochemical samples by LEAF.^627–629 The repetition rate of the excimer laser was set at 20 Hz and the tunable output of the dye laser ranged from the infrared to the UV with pulse powers of 0.01–10 mJ and line widths = 0.2 cm⁻¹. The fluorescence radiation was observed at 90° to the exciting beam and measured by a monochromator–PMT–time-gated detection system. The LODs for Au, Ga, In, Pb and Pd were in the range 10⁻⁴–10⁻¹ ng ml⁻¹ in the digested sample; absolute LODs for Au and In were 0.1 pg and 0.2 pg, respectively.

Graphite tube furnace atomization usually involves the collection of the fluorescence radiation at 180° to the excitation beam. This configuration was used in the determination of Os inaqueous solutions.⁶³⁰ Excitation was at 263.7 nm with a 20 ns pulse of energy of 40 mJ and measurement at 290.9 nm with an intensified diode array. The LOD was 50 pg with a linear calibration from 5 ng ml⁻¹ to 1 µg ml⁻¹. Interaction of Os with the graphite furnace produced memory effects. Palladium in marine deposits has been determined by graphite furnace LEAF using excitation at 247.6 nm and detection at 348.9 nm.⁶³¹ An LOD of 50 pg ml⁻¹ was reported. A L'vov platform and a chemical modifier were incorporated into a procedure for determining Se in serum.⁶³² Excitation was with 196.026 nm radiation with measurement of fluorescence at 203.989 nm. Results for Se in SRMs were in good agreement with certificate values. For the determination of Ge in water and blood again the L'vov platform with chemical modifier has been found to be a satisfactory approach.⁶³³ With excitation at 269.13 nm and detection at 275.45 or 326.95 nm LOD were 900 fg and 670 fg, respectively. A comparison has been made between atomization from the wall of non- and pyrolytically-coated furnace tubes and from L'vov platform with various chemical modifiers for the ETA-LEAF determination of In.⁶³⁴ Excitation was by a dye laser pumped by a high repetition rate Cu vapour laser. Best performance was obtained by platform atomization of In in a solution of 0.02 M HNO₃ and gave an LOD of 1 fg. An alternative to either wall or platform atomization is the use of filter furnace systems wherein the atomic and background vapours diffuse at different rates through a porous graphite filter into the observation zone. For the determination of Pb in whole blood by ETA-LEAF a Katkov filter allowed direct comparison between blood and water standards.⁶³⁵ Using a copper vapour laser for excitation and detection at 405.7 nm an LOD of 0.1 fg was achieved.

4.2.2 Lasers in atomic absorption. Diode lasers are the most frequently used lasers in AA arising from their relatively low cost, compactness, small line width, tunability and ease of operation and where the high power available from other types of lasers is not required. The use of diode lasers in AAS has been reviewed by Zybin et al.⁶³⁶
4.2.2.1 Fundamental studies. The tunability of the diode laser has been utilized as a means of isotope analysis by AA. The output wavelength of the laser is modulated to scan the spectralregion encompassing the atomic lines of interest. The atom vapour may be contained in a low pressure cell or generated in a graphite furnace. The transmitted intensity is detected by photodiodes and the signal processed by an amplifier locked to twice the diode modulation frequency, thereby minimizing the effect of background continua. Deconvolution and interpretation of the output signal is complex and requires knowledge of the structure and profiles of the spectral lines. A detailed study of the theoretical and experimental aspects of the measurement of the ⁸⁵Rb∶⁸⁷Rb isotopes has been carried out by Gustafsson and Axner.^637,638 They concluded that from the theoretical simulations they were able to predict accurately the experimental signal from Rb atom under both low pressure, room temperature conditions and atmospheric pressure, high temperature (furnace) conditions. The ⁷Li∶⁶Li and ⁸⁵Rb∶⁸⁷Rb isotope ratios have been measured by Wizemann^639,640 using the diode laser modulation with a low pressure graphite furnace approach. Computer simulation predicted that, compared with direct absorption measurements, the relative absorption sensitivity for ⁶Li is considerably reduced if wavelength modulation is performed around the centre of the ⁶Li D1 fine structure but will be enhanced if centred on the maximum of the red wing of the line profile. Lithium isotope ratios as large as ∼2000 were measured.

Tunable diode lasers have been used to study the hyperfine structure of the 6s²¹S_o–6s6p ³P₁ transitions of ^135–137Ba isotopes.⁶⁴¹ All hyperfine peaks were seen and a resolution of 50 MHz was achieved. Spectroscopic and atomic parameters were calculated from the observations. As a step toward developing an optical isotope measurement technique the profile of Ba lines in a low pressure graphite furnace have been studied using a scanning laser diode.⁶⁴² Spectral line widths were reduced from 2.8 GHz at atmospheric pressure to 57 MHz at low pressure. There was, however, a 60-fold increase in the LOD owing to the rapid diffusion of the atoms.

A study of background correction and calibration curve linearization in the determinationof Rb by modulated diode laser ETA-AAS has been published.⁶⁴³ The laser radiation was detected by an Si photodiode and converted into voltage by a logarithmic transducer. The absorption signal was observed at the 2nd harmonic of the laser diode modulation frequency with a lock-in amplifier and recorded on a storage oscilloscope. The calibration curve was linear up to 400 times the characteristic concentration. Background absorbances of up to 1.4 did not influence the value of the analytical signal in the graphite furnace. An absorbance of 4 × 10⁻⁵ could be measured against a background absorbance of 1.2.

The distribution of Rb atoms in a graphite furnace with a L'vov platform has been studied using longitudinal and transverse laser beams.⁶⁴⁴ It was concluded that 6% of the total AA signal was generated by atoms outside the tube, that the transverse distribution of atoms in the tube was homogeneous apart from a 25% higher density near the platform and that gas entered the tube convectively at its ends and exited through the injection port through which most of the analyte atoms were also lost.

4.2.2.2 Instrumentation. A significant factor limiting the use of lasers in AA is the absence of a range of diode lasers to cover all wavelengths between 190–900 nm. To produce emission in the UV region complicated schemes of frequency multiplication are necessary, thereby losing some of the benefits of the diode laser. One such scheme to generate radiation of 283 nm for the determination of Pb required the frequency doubling of 850 nm emission from a laser diode to give 425 nm radiation followed by sum frequency generation of the harmonic radiation with a second 850 nm laser diode.⁶⁴⁵ High resolution AAS measurements at 283.3 nm were achieved by scanning the diode laser injection current.

A small simple AAS instrument incorporating a W coil atomizer and a laser diode light source has been described.^646,647 For the determination of Al and Cr, laser modules generated wavelengths of 396.15 and 427.48 nm, respectively. Inherent background correction and high detection power were claimed. Detection limits for Al and Cr, respectively, were reported to be 0.9 and 0.03 ng ml⁻¹ in aqueous solution, 2.5 and 0.3 ng ml⁻¹ in serum and in slurries of graphite and TiO₂ between 0.02 and 0.6 ng g⁻¹.

4.2.2.3 Applications. Atomic absorption spectrometry with tunable diode lasers has been used for the monitoring and study of physical vapour deposition processes.⁶⁴⁸ Wavelength modulation and balanced detection gave high sensitivity and reliability. Direct measurement of the atom flux in electron-beam-evaporated Ba and Y was demonstrated. It was concluded that for deposition rate control was necessary that atom flux rather than density measurements should be made. The vapour properties of electron-beam evaporated U were measured by laser AAS using an argon ion pumped ring dye laser tuned to an absorption transition of ground states of U atom vapour.⁶⁴⁹ Wavelength scanning was used to generate data on angular distributions, density, velocity and translational temperature of the vapour flow.

Wavelength modulated diode laser AAS was combined with HPLC for the speciation of Cr in water.⁶⁵⁰ A double beam optical system and high pressure nebulization into an air–C₂H₂ flame were employed. The outputs of photodiodes were ratioed, amplified logarithmically and the absorbance signal extracted by a lock-in amplifier driven at twice the laser modulation frequency. The LOD for Cr^VI in tap water was 30 pg ml⁻¹.

4.2.3 Miscellaneous uses of lasers.
4.2.3.1 Laser enhanced ionization. The creation of an ionic vapour may be used either directly to generate an analytical signal as an electric current (optogalvanic) or indirectly to provide an input to a mass spectrometer. By correct choice of exciting wavelengths the technique is both element and quantum state selective. The latter property has been used to investigate the continuum structure of CoI and NiI near the first ionization limit using one-colour and two-colour two-step photoionization spectroscopy.⁶⁵¹ A large number of autoionizing features were identified as members of different Rydberg series. By virtue of its selectivity LEI has been used to measure isotope ratios optogalvanically. Uranium isotopes were vaporized in a GDL and excited with diode laser radiation in the wavelength range 775–835 nm.⁶⁵² Using the 831.84 nm U line the ratio of ²³⁵U to ²³⁵U + ²³⁸U was measured to be 0.0026.

Lead in sea-water was determined by LEI in an air–C₂H₂ flame after FI preconcentration.⁶⁵³ Two-step excitation was used and the LOD was 0.51 ppb Pb. Two-step one-colour excitation was used for the determination of Cr in a separation column effluent atomized in an air–C₂H₂ flame.⁶⁵⁴ Based on an injection volume of 100 µl the LOD was 4 ng ml⁻¹. Optogalvanic signals in Ne and Xe dc discharges were generated by an amplitude modulated CW laser.⁶⁵⁵ Oscillations occurring in the voltage drop between the electrodes were used to enhance the optogalvanic signal.

4.2.3.2 Cavity ring-down spectroscopy. In CRDS a laser pulse is injected into a reflective cell containing the absorbing species and the decay of the pulse intensity measured. This provides data from which the absorbance of the cell contents may be calculated. At the present time very few examples of use of CRDS in analytical atomic spectroscopy have been reported. The theoretical and practical aspects of CRDS have been presented by Provencal et al.⁶⁵⁶ The technique was used to monitor trace atmospheric particulates, radical species in flames and plasmas and to characterize surface processes. The coupling of a CW laser to a ring-down cavity has been investigated.⁶⁵⁷ A piezoelectric transducer was used to switch the laser beam entering the cavity. The effects of scanning rate, laser line width and mirror reflectivity were examined. The noise equivalentabsorption was found to be ∼3 × 10⁻⁹ cm⁻¹. In a different CRDS system the noise-equivalent absorption for a single shot from a pulsed, frequency-stabilized optical parametric oscillator light source was 5 × 10⁻¹⁰ cm⁻¹.⁶⁵⁸ When a transversely heated graphite furnace was placed in a 55 cm long ring-down cavity and used for the determination of Pb, using the 283 nm absorption line without chemical modifier, an LOD of 1 pg was obtained.⁶⁵⁹ The laser system used was a frequency doubled tunable dye laser pumped by a Nd∶YAG laser; the UV line width was ∼0.06 cm⁻¹. The laser pulse duration was ∼8 ns with a repetition rate of 20 Hz. It was predicted that an LOD of 40 fg should be achievable.
4.2.3.3 Coherent forward scattering. To date no reports of the use of lasers in CFS have been received though no doubt is but a matter of time before the potential of lasers in this field will be explored. A single publication has reported on the use of a Xe short-arc continuum source lamp as a means of simultaneous multi-element determination by CFS. The sample was atomized in a graphite furnace and the spectrum analysed by an in-house developed polychromator with 20 PMTs and small range wavelength scanning over the absorption peaks.⁶⁶⁰ Inevitably, furnace conditions were a compromise of those used for individual elements and two regimes were employed: one for volatile elements (Ag, Cd, K, Na, Pb, Tl and Zn) and the other for less volatile elements (Al, Ca, Co, Cu, Fe, Mn, Ni and Sr). The LODs were mostly in the µg l⁻¹ range (∼20 pg)

5 Chemometrics

The role of chemometrics is to turn raw data into useful information. Many chemometric methods are available to achieve this so a major difficulty is to decide which is the most appropriate. Sharp et al.⁴⁸⁸ have provided some excellent case studies with tutorial style discussion on the application of chemometric procedures. The use of Baysian methods, described for multi-isotopic data produced by ICP-MS, could equally be applied to modern ICP-AES instruments with simultaneous array detectors. Of specific interest was the potential improvements to precision and detection limits offered by applying correlation processing to simultaneous background correction of peaks in ICP-AE spectra. Improvements in precision of an order of magnitude were predicted for cases where the measured signal was less than 20 times the detection limit. However, the improvement would be less significant above this level, due to noise on the peak itself.

A new algorithm has been produced for investigating and calibrating AAS data.⁴⁸⁹ Four attributes of this methodology were tested with EDTA chelates of Cu, Pb, Ni and Cd. These were maximum permissible range of calibration, maximum permissible spacing between calibration points, accuracy of standard additions extrapolation, and speed of application of the algorithm. This method allowed accurate calibration, i.e., within 1 s, for all elements above 100× the characteristic concentration (99% absorbance). To achieve this accuracy, the concentration range between calibration points should not exceed 20 times the characteristic concentration. Importantly, the speed of the algorithm was fast enough to be implemented in practical analytical situations. The chemometric procedure known as common analyte internal standardization (CAIS) has been modified and used to provide a universal calibration technique for the analysis of trace elements in organic solutions of medium and low volatility by ICP-AES.^490,491 The methodology focused on the problem that, in petrochemical product analyses, an organic matrix is frequently tainted with small but variable amounts of highly volatile solvents that render accurate matrix matching of calibration solutions and samples impossible. The method depended on there being a linear change in the ratio of atom to ion line intensities of the same element as plasma conditions were perturbed by mixed solvents. This dependence allowed the calculation of factors that could be used to extrapolate line intensities for a single pure solvent. For elements without a pair of suitable atom/ion lines, the procedure was extended to utilize factors derived from an appropriate pair of lines of different elements. The effectiveness of the method was demonstrated for 7 elements, by either the addition of increasing concentrations of a second solvent or the same concentration of several different solvents with varying volatilities. Neural networks have also been used to develop a system for semi-quantitative analysis using an ICP-AE spectrometer equipped with an array detector.⁴⁹² Qualitative analysis was achieved using 10 'index lines' to calculate the probability of element occurrence, and semi-quantitative analysis was performed by ratioing the line intensities to reference intensities.

Spectral interference correction is a major application for chemometrics in ICP-AES. Li and Fan⁴⁹³ have reviewed appropriate methods for ICP-AES, including Kalman filtering (19 references). Cross-correlation of sample spectra from ICP-AES using an array detector, with reference spectra of reference elements, has been used to identify peaks.⁴⁹⁴ The problem of interferences was addressed by deleting mutually interfering lines using analogue software masks. The cross correlation was carried out using a shift-multiply-add algorithm, avoiding the use of FFT. Improved methods of peak fitting and spectrum stripping for correction of interferences in ICP-AES are of continuing interest and readers should consider several publications in this area.^495–498 A tutorial guide on the use of wavelet transforms to determine peak shape parameters as a method of interference detection in GFAAS has been published.⁴⁹⁹ The absorbance versus time peak-shape of the GFAAS was recorded and transformed, using wavelets, into a set of coefficients called Lipschitz regularities. The number of these, and their values with respect to time, accurately characterized the peak profile so it was possible to demonstrate changes to this profile resulting from chemical interference. The ability of Lipschitz regularities to detect interferences was then compared with previous measures of peak shape such as time parameter and the phase angle array procedure and found to be more sensitive.⁵⁰⁰ This methodology could equally well be applied to peak shape data resulting from other spectroscopic techniques.

The search for an ideal chemometric method to correct matrix effects and sensitivity changes in atomic spectrometry appears never ending. Simple internal standardization has been regularly shown to provide significant improvements in data quality, but researchers continue to look for something better. The PRISM (parameter related internal standard method)⁵⁰¹ has been resurrected for the determination of Ni in high salt and acid matrices by sequential ICP-AES.⁵⁰² The method relied on there being two underlying factors causing variations in sensitivity, namely depression of the excitation temperature of the plasma and changes in the efficiency of sample introduction. Two internal standards were then chosen that represented the variations caused by these factors, with variation in all other elements being accounted for by convolution. The method was found to provide significant improvements in accuracy, precision and a decrease in 'drift'.

Many figures of merit have been developed for ICP-AES systems, each tending to focus on a particular need or diagnosis. Many of these have now been brought together,⁵⁰³ and it is suggested that: repeatability should be measured by the RSD of the Mg I 285 nm line; resolution by the line profile of Ba II 233 nm; robustness by the Mg II 280 nm∶Mg I 285 nm line intensity ratio; long term stability by the intensities of the Ar I 404 nm, Ba II 455 nm, and Zn II 206 nm lines; and limit of detection using the Ni II 231 nm line. To enable these figures of merit to be applied over the maximum range of elements, spectral coverage should be 120–770 nm.

The preponderance of new ICP-AE spectrometers with multi-element array detectors has allowed the evolution of new approaches to quality control. It has been suggested⁵⁰⁴ that variations in instrument-reported concentrations for multiple lines of a given element be used as a measure of the confidence interval for the determined concentration. It will be interesting to see whether this proves to be a robust concept because of the large variations in line sensitivity and background signals.

A major use of chemometrics is for classification of samples into groups. Principal components analysis is a particularly powerful tool which Klemenc et al.⁵⁰⁵ have used for a detailed evaluation of multi-element data produced by ICP-AES for the simultaneous determination of many major, minor and trace elements in archaeological copper ingots. When coupled with data processing by PCA, this allowed a full archaeological interpretation of the data in terms of a multi-stage smelting process. This was an excellent case study for those interested in interpreting a large amount of data. The use of multivariate techniques, such as cluster analysis, PCA and discriminant analysis, has also been popular in categorizing wines by their trace element content.^506,507 In particular, PCA was used to determine the geographical origin of tea with some success.⁵⁰⁵

Correspondence analysis has been used to study the correlation between sex/age and a range of 16 trace elements determined in hair by ICP-AES.⁵⁰⁸ It was found, for example, that essential elements Ca, Cu, Mg and Zn were higher in women, and Fe, Hg and Pb accumulate with age. A sophisticated approach using neural networks was used to classify normal patients and those with lung cancer by determining the metal content of serum and hair by ICP-AES.⁵⁰⁹ In a similar study, AAS was used to determine Ca, Cr, Cu, Fe, Mg, Mn, Sr and Zn in serum and associate their concentrations with coronary disease.⁵¹⁰

6 Coupled techniques for speciation

6.1 Capillary electrophoresis

There were no publications in this area during the review period; however, two interesting conference proceedings described a new approach to interfacing CE with ICP-AES.^511,512 This was accomplished by incorporating the CE capillary into a modified nebulizer and by, at least in one case,⁵¹² inserting a fine platinum wire into the end of the capillary to complete the circuit. In this way the uptake of the capillary was matched to the electro-osmotic flow, thereby avoiding dilution of the sample; however, no mention was made of the problems that might be encountered when the electro-osmotic flow is negative.

6.2 Gas chromatography

6.2.1 GC-AES. Gas chromatography coupled with AES was a popular technique for speciation studies, due to the excellent element selectivity of the detection system. The predominant technique was GC-MIP-AES, so the discussion below refers to this technique unless otherwise stated.

A reasonable degree of activity has been maintained in the speciation of organometallic compounds, though there were fewer publications than in previous review periods. There were publications on the speciation of organotin,^513–516 organolead,⁵¹⁷ organomercury,^518,519 organoplatinum,⁵²⁰ and organoselenium⁵²¹ compounds. Of particular note were two related papers on the speciation of organotin compounds in environmental samples and reference materials.^515,516 The former paper summarized the outcomes of a feasibility study for the certification of butyl- and phenyltin compounds in freshwater sediment, using a variety of techniques. The major findings were that the long-term stability varied quite considerably for different compounds and a storage temperature of –70 °C may be necessary to ensure long-term stability for some reference materials. The latter paper described the results of a similar study, specifically for tributyltin (TBT) and triphenyltin (TPhT) in water, sediment, oysters and cockles. The compounds were stable in unfiltered, non-acidified sea-water in polycarbonate bottles, stored at 4 °C in the dark, for between 3–7 months, which allows sufficient time for analysis. Comparable preservation time was achieved by extraction onto C₁₈ cartridges. Both TPhT and TBT were stable for longer than 18 months in sediments stored at –20 °C; however, losses were observed due to air drying or pasteurization. Organolead compounds have been determined⁵²² by MIP-AES after GC separation of the propyl derivatives on reaction with tetrapropylborate. The derivatives were simultaneously extracted into hexane. Detection limits were around 0.1 ng kg⁻¹.

Multicapillary GC has been used to speciate organotins in sediment after microwave extraction in acetic acid and derivatization with sodium tetraethylborate.⁵¹⁴ A multicapillary column with 919 capillaries (1 m × 0.04 mm id, 0.2 µm SE-30) was used to achieve separation in 2 min compared with 12 min using a single capillary column (30 m × 0.32 mm id, 0.25 µm BP-5). Detection limits of 0.2 ng ml⁻¹ as Sn at the 303.42 nm line were achieved. A further paper in a similar vein described a purge and trap multicapillary GC accessory for speciation of organomercury by MIP-AES.⁵¹⁸ Mercury species were derivatized and cryogenically trapped on a fused silica capillary prior to flash isothermal separation in less than 30 s. Detection limits of 0.01 pg ml⁻¹ for methylmercury and Hg²⁺were achieved. These compounds were determined in urine and biological at 5 and 100 pg g⁻¹, respectively.

Impressive detection limits of 10, 0.3, and 0.7 pg for dioctyltin, tetrabutyltin and tributyltin, respectively, were achieved using GC coupled with flame photometric detection,⁵¹³ after derivatization and preconcentration of water and beverages by 200–400 fold, though, somewhat worryingly, up to 9.5 ng ml⁻¹ of dioctyltin was found in some of the beverages.

Other papers worth mentioning describe the determination of a volatile Pt^II complex formed with 1,2-bis(5,5-dimethyl-4-oxohexane-2-imino)ethylene and its determination by GC-MIP-AES,⁵²⁰ and the derivatization of selenoamino acids with ethylchloroformate to make them amenable to GC separation and detection by MIP-AES for the speciation of these compounds in selenium enriched yeast-based nutritional supplements. Finally, one paper described the determination of the gasoline additive methylcyclopentadienylmanganese in highway runoff and sewage.⁵²³ The analyte was extracted by headspace solid phase micro-extraction, with detection limits of between 0.3 and 0.5 pg l⁻¹ using the 259 nm Mn line.

The use of GC-MIP-AES for element selective detection of compounds containing hetero-atoms also attracted some attention, with papers describing applications for the determination of organohalide,^524–527 arsenic,^528,529 sulfur^530–533 phosphorus,⁵³⁴ and nitrogen⁵³⁵ compounds. Applications of note include: the monitoring of diazepam and chlorpromazine in human plasma⁵³³ by element selective detection for Cl (837.6 nm), S (921.3 nm) and C (940.5 nm); the determination of lewsite in soil⁵²⁹ after derivatization with 1,3-dimercaptopropane and detection for Cl (480.192 nm), S (181.275 nm) and As (189.042 nm); the verification of lewsite exposure by determination of 2-chlorovinyl arsenous acid, which is a metabolic product of lewsite, in the urine of lewsite-injected guinea pigs, after solid-phase extraction with C₁₈ and derivatization with ethanedithiol;⁵²⁸ determination of impurities in phosphine, arsine and ammonia gases used for semiconductor manufacture.⁵³⁶

Two papers addressed the issue of compound-independent calibration.^535,537 In particular, one study found that the relative responses for a range of organic compounds depended on retention time.⁵³⁷ The responses, relative to nonane, for compounds containing equimolar amounts of carbon, at 247.9 nm line, were <1 for earlier eluting compounds and >1 for later eluting compounds. In a study of empirical formulae,⁵²⁶ it was found that Cl∶C and H∶C ratios were close to expected values whereas F∶C ratios were not (C at 495 nm, H at 486 nm, Cl at 479 nm, F at 690 nm), and the fluorine response was compound dependent. This was partly ascribed to the interaction of F with the silica discharge tube.

An interesting application of simultaneous C and H determination has been described,⁵³⁸ where the C-selective and H-selective chromatograms for a gasoline sample were combined to yield a third chromatogram of relative hydrogen deficiency versus boiling point. These so called aromatograms were used to identify aromatic and other hydrogen deficient compounds in the sample.

Bromination, using 1% Br₂, has been employed for the selective determination of alkenes in gasoline samples.⁵²⁷ This enabled discrimination of the alkenes from aromatic and saturated compounds (which did not react) by Br-selective detection at 478 nm.

On-line supercritical fluid extraction coupled with ICP-AES has been used for the determination of total petroleum hydrocarbons in soil,⁵³⁹ with element selective detection for C, S, P and Si. A low power ICP-AES source operated at 15 W power has been coupled with a portable spectrometer via a fibre optic probe for the determination of organohalide compounds.⁵²⁴ Detection limits of 240, 49, 117, and 52 pg s⁻¹ were obtained for fluorobenzene, carbon tetrachloride, 1,2-dibromoethane and ethyl iodide.

6.2.2 GC-AFS. Two papers were published during the review period. In the first,⁵⁴⁰ a comparison of AFS and ICP-MS detection, coupled with GC, for the determination of methylmercury chloride in marine samples, was undertaken, with detection limits of 0.25 and 0.9 pg, respectively. Similar performance was obtained for the analysis of IAEA 142 and NIST 8044 mussel homogenate reference materials. In the second,⁵⁴¹ three extraction methods for methyl- and ethylmercury in soil, sediment and fish were compared, namely acidic KBr/CuSO₄ isolation–CH₂Cl₂ extraction with and without alkaline pre-digestion and a milder extraction with citrate buffer and dithizone in CHCl₃. The first two methods gave best results.

6.2.3 GC-AAS. Very few publications appeared in this area, with the emphasis placed firmly on element-selective detection. Organotin compounds were determined by GC coupled with quartz furnace AAS.^542,543 In the former case the hydrides were generated with NaBH₄ subsequent to preconcentration on a cation exchange column and prior to extraction into hexane, and in the latter the compounds were ethylated with NaBEt₄. GC-QF-AAS has also been used for the determination of methylmercury in biological samples⁵⁴⁴ and a variant of this, which utilized a stainless steel pyrolyser tube maintained at 700 °C, was used for the determination of methyl-, ethyl- and phenylmercury chlorides in the skin and fur of mink.⁵⁴⁵ One notable development was the immobilization of dithizone on polymeric microbeads, by the suspension polymerization of ethylene glycol dimethacrylate and hydroxyethyl methacrylate, and their use for the preconcentration of methyl- and ethylmercury chloride.⁵⁴⁶

6.3 Liquid chromatography

6.3.1 LC-AAS. Several reviews have been written in this review period, but many of these are on specific topics. An example includes a review containing 12 references of the speciation of metallothionein and metal-binding proteins⁵⁴⁷ that covered early HPLC-AAS determinations through to HPLC-ICP-MS. Future applications and improvements to the technique were also hypothesised. Reviews of speciation of Sb in waters containing 98 references⁵⁴⁸ and in environmental samples (in Czech) containing 113 references⁵⁴⁹ have also been published. The former did not concentrate solely on atomic spectroscopic determination and covered topics such as detection using spectrophotometry with cationic dyes through to ICP-MS. The latter review was more specific, concentrating on detection by ETAAS and HG AAS. In addition, separation techniques, e.g., HPLC, GC and FI, used in conjunction with atomic spectrometry, were also discussed. Inorganic and methyl-Hg speciation in environmental samples has been covered in a review that described methods for sample preservation, extraction, preconcentration and derivatization.⁵⁵⁰ This review contained 162 references. Other topics reviewed include both chromatographic and non-chromatographic separations and different methods of detection. The use of organised surfactant assemblies in analytical atomic spectrometry has been reviewed.¹⁰⁰ Topics covered included the improved nebulization efficiency derived from the introduction of surfactants, improved particle dispersion when nebulizing slurries and the introduction of vesicles to aqueous mobile phases for reversed phase HPLC speciation determinations.

Several analytes have been speciated using liquid chromatography coupled with AAS. The determination of Cr^III and Cr^VI has proved to be a popular analysis in this review period. These species have been determined in polyethylene plastics used as mineral water containers.⁵⁵¹ The Cr^III was separated by coprecipitation on an alumina carrier. 'Satisfactory' results were obtained although it was noted that the speciation analysis of the plastic was impossible because of speciation changes during mineralization of the plastic. Total Cr and Cr^VI has been determined in UHT milk.⁵⁵² For total Cr determination, the milk was mixed with Triton X-100 and diluted with water. For Cr^VI determination, the milk was mixed with acetate buffer (pH 3.5) to precipitate proteins and the supernatant solution passed through a column of Chromabond NH₂ that had previously been conditioned with 1 M nitric acid and water. Elution of the Cr^VI was with 2 M nitric acid. Both total Cr and Cr^VI were determined by ETAAS at the 357.9 nm line and with a modifier of palladium nitrate–magnesium nitrate. Calibrations were linear from 0.15 µg l⁻¹ to 50 µg l⁻¹ for both species and recoveries were >93%. On-line preconcentration and determination of Cr^III and Cr^VI has been reported in two papers. In one,⁵⁵³ Cr^III was complexed with Chromazurol S and Cr^VI with NaDDC and the complexes adsorbed onto a C₁₈ column. Elution was by methanol and detection was at 357.9 nm. The LODs were 2.5 µg l⁻¹ and 0.2 µg l⁻¹ for Cr^VI and Cr^III, respectively. Tap water and soil extracts were the samples analysed. In the other,⁵⁵⁴ on line preconcentration of Cr^VI and Cr^III from sea-water was performed at pH 1.5 and at pH 7 respectively, in the presence of manganese(II). Using a C₁₈ microcolumn, the diethylammonium salt of diethyldithiocarbamic acid as the preconcentration reagent and methanol as the eluent, a preconcentration factor of 500 was achieved. Calibrations were linear between 0.2 and 200 µg l⁻¹, recoveries were 95–105% and the LOD was 0.02 µg l⁻¹.

Various natural waters and a liquid waste have been analysed to determine Te^IV and Te^VI.⁵⁵⁵ Sample (100 ml) was adjusted to pH 4 and then mixed with 2 ml of 0.1% 5-mercapto-3-phenyl-1,3,4-thiadazole-2(3H)-thione (bismuthiol II) and 30 mg of cobalt(III) oxide. After sonication and filtration through an 8 µm nitrocellulose filter, the residue was washed with water and then placed with the filter in water, thus forming a slurry. Aliquots of the slurry were then introduced into a tungsten atomizer, dried, ashed and then atomized in the presence of hydrogen to determine total Te. A second 100 ml aliquot of sample was prepared at pH 1, enabling the collection and determination of Te^IV only. Hexavalent Te was determined by difference. The LOD was 0.1 ng.

Antimony species in tap water, snow and urine have been determined.⁵⁵⁶ Selective sorption of the Sb^III chelate of APDC on a C₁₈-bonded silica gel microcolumn was used to determine Sb^III and total Sb after the Sb^V had been reduced to Sb^III by L-cysteine. Elution of the Sb was by ethanol. Preconcentration enabled a LOD of 0.007 µg l⁻¹ to be obtained, which was substantially superior to 1.7 µg l⁻¹ obtained by direct injection into ETAAS. The method was used to study the stability of Sb species in the matrices of interest. Trivalent Sb was found to be unstable in tap water.

Magnesium speciation in serum has been achieved by liquid chromatography with an anion exchange column and ETAAS detection.⁵⁵⁷ Serum was diluted 2-fold with Tris buffer and 0.5 mol l⁻¹ sodium acetate was used as the mobile phase. It was found that the Mg was associated mainly with albumin and globulin fractions, but not with transferrin.

Mercury speciation from water samples has been reported in two papers.^558,559 In one,⁵⁵⁸ water was filtered, acidified to pH 2 and passed through a column of chelating resin–thionalide Bio-beads. After rinsing with water, total Hg was desorbed with 10 ml of 5% thiourea solution in 0.05 M hydrochloric acid. A 5 ml aliquot was then analysed for inorganic Hg by reacting it with tin(II) chloride and potassium hydroxide (7 ml, 30%). The organic-bound mercury was reacted with cadmium(II) chloride, the cadmium replacing the mercury that was then determined in the normal way. Studies were made on the stability of the mercury species in samples and during the processing. Another paper that determined mercury speciation in waters has also been published.⁵⁵⁹ These authors passed 1 l of water through a column of activated charcoal at a rate of 4 ml min⁻¹. Transferring the charcoal to a vessel and reacting it with sulfuric acid (5 ml), 50 mg tin(II) chloride and then 5 ml of 45% sodium hydroxide produced Hg vapour that was determined by AAS. The organic mercury was determined by mixing the charcoal with 1 ml of 5% cysteine, sulfuric acid (16 N, 5 ml), tin(II) chloride solution (1 ml) and then sodium hydroxide (5 ml, 45%). Mercury speciation in fish and urine has been reported by Yin et al.⁵⁶⁰ On-line solid phase extraction (SPE), HPLC using a C₁₈ column and a mobile phase of 1.5 mM APDC in methanol–water–acetonitrile (19 + 16 + 15) combined with a derivatization mixture of 2 mM APDC in 10 mM ammonium acetate and cold vapour generation using 0.264 M sodium tetrahydroborate in 0.1 M sodium hydroxide enabled mercury species to be detected by AAS at the 254 nm line. Linear ranges extended to 5000 ng l⁻¹ and the LODs for methyl-, ethyl-, phenyl- and inorganic Hg were 9, 6, 10 and 5 ng l⁻¹ respectively. Inorganic and organic mercury has been determined in fish eggs oil using Tween 20 (0.008% v/v) to avoid the formation of microemulsions.²⁰⁸ Inorganic Hg was determined by reaction with sodium tetrahydroborate whilst total Hg was determined after persulfate oxidation. Calibrations were linear to 20 µg l⁻¹, LODs were 0.11 and 0.12 µg l⁻¹ for total and inorganic Hg and recoveries were in the range 93–94.8% and 100–106% for inorganic and organic Hg.

Aluminium speciation has also received some attention, especially in clinical samples. Sanz-Medel has reviewed his own work (10 references) and has covered topics such as off-line ultrafiltration and AAS detection to HPLC-ICP-MS.⁵⁶¹ Fast protein liquid chromatography (FPLC)–ETAAS has been used to speciate Al in serum in 2 papers by Bantan et al.^562,563 In the former paper, Al citrate was spiked into serum and this was analysed on a Mono Q HR 5/5 anion exchange column using a gradient elution from aqueous to 4 M ammonium nitrate at a flow rate of 1 ml min⁻¹ in 10 min. Fractions (0.5 ml) were collected and analysed by ETAAS at the 309 nm line. In the second paper,⁵⁶³ the same system was used to determine several low molecular weight Al complexes. Detection was by ETAAS and by electrospray-MS-MS. It was found that the main Al constituents were Al citrate, Al phosphate and ternary Al citrate–phosphate complexes.

Selenium compounds (trimethylselenonium iodide, selenomethionine, selenious acid and selenic acid) were separated in 8 min using an ESA anion III exchange column and a 5.5 mM ammonium citrate eluent that had been adjusted to pH 5.5.^564,565 Detection was by either AAS or ICP-MS. The AAS LODs were approximately 1 mg l⁻¹ for an injection volume of 100 µl. A comparison of extraction methods for Se species from biological samples has been published.⁵⁶⁶ Enzymes were used to digest CRM 186 pig kidney, CRM 402 white clover and a Se enriched yeast. Determination was by HPLC-ETAAS and selenomethionine and selenocystine were found to be the major constituents. Clinical samples have again proved to be a popular matrix for Se speciation. After cleaning urine through a C₁₈ cartridge, 100 µl of urine was passed through a Hamilton PRP-X100 column using a phosphate buffer.⁵⁶⁷ The column effluent was mixed with 3% potassium persulfate and sodium hydroxide and was then irradiated in a microwave at 90 W. After cooling the hydride was formed and this was detected by quartz tube AAS at 196 nm. Detection limits were 3, 5, 4, 8 and 3 µg l⁻¹ for Se^IV, Se^VI, selenocystine, selenomethionine and the trimethylselenonium ion, respectively. Selenocysteine and selenomethionine have been identified in protein hydrolysates by HPLC-AAS.⁵⁶⁸ The detection limit was 0.3 pmol. A comparison was made between AAS detection and fluorescence detection of the o-phthaldialdehyde derivatives (LOD 30 pmol and 170 pmol for selenomethionine and selenocysteine). An interesting method of microbore HPLC and ultra-low volume sample fraction collection followed by ETAAS detection has been used to separate and detect 5 Se species in 6 min.⁵⁶⁹ A flow rate of 80 µl min⁻¹ through an anion exchange column enabled collection of 20 µl aliquots in sample cups and these were then analysed using ETAAS. Relative LODs of 2.8–4.1 ng ml⁻¹ were obtained for trimethylselenonium, selenomethionine, selenite, selenate and selenocystine and the method was validated by analysis of extracts from CRM 402 white clover. Four selenium species have been separated in 14 min on a Hamilton PRP X-100 anion exchange column using a phosphate buffer and the eluate was then passed through a PTFE coil wound around a water cooled high pressure mercury lamp.⁵⁷⁰ The photolysed Se compounds were then mixed with concentrated hydrochloric acid (1 ml min⁻¹) and 1% tetrahydroborate in sodium hydroxide. The Se hydrides were then flushed to detection in a quartz furnace AAS by a stream of argon. Calibration was linear to at least 800 µg l⁻¹ for selenite, selenate, selenomethionine and selenocystine and LODs were 2.5–46.5 µg l⁻¹. A paper has reported the use of a boiling water bath to accelerate the reduction of Se^VI to Se^IV by 6 M hydrochloric acid prior to HG-AAS.⁵⁷¹ The procedure was reportedly less expensive than microwave assisted reduction and took only 2 min.

Arsenic has been by far the most popular element that has undergone speciation analysis using AAS as the detection system. Matrices analysed have included sediments.^572,573 In the former paper, arsenic species associated with iron oxide were determined after the sediment had been extracted with hydroxyammonium chloride. The extracts were separated using an anion exchange column and a gradient elution using pH 5.8 and pH 10 phosphate (100 mM). Detection was by HGAAS using a quartz T-piece heated in an air–acetylene flame. In the latter paper,⁵⁷³ marine sediments were extracted and the relationships between As species and redox potential and iron and manganese load were studied. It was concluded that most samples had higher As^V than As^III and that the speciation depended on reducing conditions and manganese concentration. Natural waters were analysed for As species by hydride generation followed by trapping of the hydride on zirconium coated tubes at 1000 °C and ETAAS detection.⁵⁷⁴ The hydrides of different species were generated using different reducing media. Total As was reduced using 1% tetrahydroborate in 0.02 M thioglycollic acid, As^III and As^V were reduced by 2% tetrahydroborate and 4 M hydrochloric acid, 0.2% tetrahydroborate and citric acid–sodium hydroxide buffer at pH 5 for As^III and 0.2% tetrahydroborate and 0.14 M acetic acid for As^III and monomethylarsonic acid (MMAA). The As^V and MMAA were calculated by difference. Detection limits were typically 50–80 ng l⁻¹. Water soluble As species in the brown alga Fucus distichus were determined using gel permeation and ion exchange chromatography and ETAAS detection.⁵⁷⁵ Further work coupling HPLC and ICP-MS was also performed. Four major species were identified but one minor species remained unidentified. Inorganic arsenic species were determined in seafood samples using a 9 M hydrochloric acid extraction followed by reduction using hydrobromic acid and hydrazine sulfate, extraction into chloroform, back extraction into 1 M hydrochloric acid, dry ashing and finally detection using HG-AAS.⁵⁷⁶ Recoveries of As^III and As^V were 99% and 96%, respectively, and the LOD was 3.07 ng g⁻¹. Under optimum conditions other As species were reportedly not extracted. Analysis of the CRM DORM-1 yielded results in close agreement with those obtained by other workers using HPLC-ICP-MS. Clinical samples have also received attention. Arsenic speciation in serum has been reported in two papers.^577,578 In the first example low molecular weight As compounds were determined directly using HPLC coupled with HGAAS. Higher molecular weight species were first separated by FPLC (either size exclusion, ion exchange or affinity chromatography) and then digested before measuring off-line by HGAAS. The main species present in uremic patients was found to be dimethylarsinic acid (DMA) and arsenobetaine. The second paper used a similar approach and found that only inorganic As bound to serum proteins. Arsenic speciation in urine has been reported in which 8 species (As^III, As^V, DMA, MMAA, arsenobetaine, arsenocholine trimethylarsine oxide and tetramethylarsonium) were separated using anion exchange resins followed by on-line photo-oxidation and digestion with persulfate and HGAAS detection.⁵⁷⁹ Between 32 and 36 determinations could be performed in 8 h and LODs were reported to be between 2 and 6 µg l⁻¹ depending on the species. The analysis of 3 urine candidate reference materials has been reported.⁵⁸⁰ Cation exchange chromatography was coupled on-line with alkaline persulfate digestion and HGAAS to determine levels of DMA and arsenobetaine, although other species (MMAA and arsenocholine) were also monitored. It was concluded that arsenobetaine concentrations in urine depend solely on food intake and that the species were stable at room temperature in a matrix of dilute perchloric acid. A membrane gas–liquid separator has been used during the FI-HGAAS determination of As.⁵⁸¹ Arsenic(III) was determined by injection (100 µl) into 0.5 M hydrochloric acid and total As was determined by injection into 15% potassium iodide and 6 M hydrochloric acid. For both determinations, the flow was merged with 1% tetrahydroborate and the arsine generated passed through the membrane separator and swept to detection using a flow of nitrogen. Detection limits were 150 pg for As^III and 80 pg for total As. Arsenic(V) was calculated by difference. The sampling rate was 25 h⁻¹ and the results of the analyses of surface waters and soil extracts were compared with those obtained by ETAAS.

The only other publication concerning chromatography coupled with AAS in this review period has been one that separated interfering ions such as chloride during the determination of Cd and Pb.⁵⁸² An anion exchange column separated the chloride ions whilst fractions of eluate were collected for off-line determination of the analytes by ETAAS.

One of the more interesting methods of speciation has been reported by Aller.^583,584 In these papers, living bacterial cells (Pseudomonas putida) were used to adsorb Se species (selenomethionine and selenourea) and then the biomass was analysed directly by ETAAS. It was possible to discriminate between the different chemical species by combining the optimization of both the growth conditions and the relative rates of the retention from the sample solution. Detection limits of approximately 1 ng ml⁻¹ were achieved with a precision of typically 2–5.6% RSD.

6.3.2 LC-AES. A review of the determination of metallothioneins containing 88 references covering separation techniques such as capillary zone electrophoresis (CZE), microbore and capillary HPLC and their interfacing with AAS, AES and ICP-MS has been published.⁵⁸⁵ Applications of the hyphenated techniques to 'real world' samples were also reviewed.

A number of analytes (Cu, Fe, Mn and Zn) in blood fractions were determined by size exclusion chromatography (SEC)-ICP-AES.⁵⁸⁶ The column used was a Superformance Fractogel EMD BioSEC 650(S) column that was calibrated using 5 standard proteins. The optimum mobile phase was 0.3 M sodium chloride in 0.02 M phosphate buffer at pH 6.8. Detection limits were 30, 26, 0.8 and 90 ng ml⁻¹ for Cu, Fe, Mn and Zn, respectively. The system could be used for protein identification, but not for quantification. Three analytical techniques for the speciation of Al in environmental samples have been used.⁵⁸⁷ The methods used were FPLC-ICP-AES, a microcolumn of chelating resin–ETAAS and 8-hydroxyquinoline–spectrophotometry. The FPLC-ICP-AES method successfully separated Al³⁺, Al(OH)²⁺ and Al(OH)₂⁺, whilst AlF²⁺ co-eluted with Al(OH)²⁺ and Al(SO₄)⁺, AlF₂⁺ and negatively charged Al organocomplexes co-eluted with Al(OH)₂⁺. The ETAAS technique separated most of the labile organic complexes, the sulfato and the hydroxy species. A combination of all three techniques could be used to speciate Al in soil extracts and percolating waters.

Silicon speciation is also beginning to receive some attention. Silicate, 1,3-tetramethylsilanediol and hexamethyldisiloxane have been separated on an end-capped C₁₈ column using 30% methanol as mobile phase in 32 min.⁵⁸⁸ The ICP utilized was used in both axial and radial modes at the 251.6 nm line, but the LODs were found to be similar (0.1–0.5 µg ml⁻¹) for both modes. This was attributed to the noise arising from the organic solvent. In another paper⁵⁸⁹ a C₁₈ column was used with acetonitrile (20%) to separate dimethylsilanediol, tetramethyldisiloxane-1,3-diol and trimethylsilanol. A change to 5% acetonitrile was necessary to separate the dimethyldisilanediol from silicate. Detection was at 288.158 nm and LODs were at the 3–50 ng range.

Selenium speciation has been achieved by a number of workers. A number of seleno-amino acids have been separated after esterification using 3% sulfuric acid in methanol or ethanol followed by separation on a C₈ column using a mobile phase containing water–methanol (49 + 1) and 0.1% trifluoroacetic acid (TFA).⁵²¹ Alternatively, the selenoamino acids were derivatized using water–ethanol–pyridine (15 + 8 + 2), extracted into chloroform and then separated using gas chromatography with a column of HP-1 (0.17 µm, 25 × 0.32 m) using a thermal gradient of 100 °C to 200 °C at a ramp rate of 5 °C min⁻¹. Detection was by both ICP-AES and ICP-MS. The method was applied to a number of nutritional supplements and vegetables. Numerous Se species have been separated using microbore HPLC anion exchange columns followed by on-line coupling with an ICP-AES instrument via a direct injection nebulizer (DIN).⁵⁹⁰ The flow rate was 80 µl min⁻¹ and the Se species selenomethionine, selenite, selenate and selenocystine were separated in less than 5 min. Detection limits were between 20 and 38 ng ml⁻¹, precision was typically 2–6 % for peak area and the method was applied to the analysis of CRM 402. Four extraction methods were compared and some evidence of selenocystine breakdown during extraction was also presented.

Antimony speciation has been reported in two papers by one set of authors. In one paper⁵⁹¹ Sb^III and Sb^V were determined by a HGAES method that relied on the reduction rate of Sb^V to Sb^III using cysteine. Calibration was achieved at two different times (t = 2 and 8 min). The LODs were 1.2 ng ml⁻¹ for Sb^III and 4.5 ng ml⁻¹ for Sb^V. The method was applied to spiked water samples. The second paper was not dissimilar but used potassium iodide as reductant, and relied on the preferential determination of Sb^III in the presence of citric acid.⁵⁹²

The use of a membrane desolvation device that enabled organic solvents and liquid chromatographic samples to be introduced at a flow rate of 1.5 l min⁻¹ to a low power (120 W) helium microwave plasma has been reported.⁵⁹³ By increasing the counter-flow of gas to 8 l min⁻¹, the aerosol could be desolvated almost completely (the plasma returned to pink and showed no sign of green carbon bands). Interfacing reversed phase HPLC and the He MIP enabled 2,6-dichlorobenzene and 5,7-dichlorhydroxyquinoline to be separated and determined by monitoring the Cl signal.

Direct elemental and molecular species detection for liquid chromatography by glow discharge atomic emission and mass spectrometries has been discussed.⁵⁹⁴ The system was reported as being capable of determining trace metals at the ppb level whilst C and H (from the ligands) were detectable at low ppm levels.

6.3.3 LC-AFS. An overview of the possibilities of AFS for speciation has been presented by Fodor.⁵⁹⁵ It described the relatively cheap cost of the instrumentation as well as systems used to speciate As and Se. A comparison between AFS and ICP-MS as detectors for speciation analysis of As in environmental samples has also been made.⁵⁹⁶ In this work, HPLC using an anion exchange column was used to separate arsenite, arsenate, MMAA and DMA prior to HG-AFS. An on-line photo-oxidation step was also described that was used for determining arsenobetaine. A similar paper by the same authors quoted LODs of 0.17, 0.45, 0.3 and 0.38 µg l⁻¹ using a 20 µl sample loop for As^III, DMA, MMAA and As^V, respectively.⁵⁹⁷ The technique was used to analyse a urine reference material and a volunteer's sample. The As species As^III, DMA, MMAA and As^V have been determined in sea-water.⁵⁹⁸ The hydrides generated were cooled in liquid nitrogen and then a Variac was used to heat the tube, releasing the species at their respective boiling points. Precision was usually better than 3.5%, and LODs were in the range 0.9–3.7 ng l⁻¹. Solid-phase extraction cartridges have been used as low pressure chromatographic columns to separate As^III and As^V prior to HG-AFS detection.⁵⁹⁹ Separation of the species occurred within 1.5 min, yielding LODs of 0.2 and 0.4 ng ml⁻¹ for the tri- and pentavalent species, respectively. The method was applied to the analysis of water samples, with results obtained from the analysis of NIST CRM 1643d being in good agreement with the certified value. Eight As species (plus several unidentified As species) have been separated and determined using HPLC-UV-HG-AFS.⁶⁰⁰ A total of six reference materials of biological origin were analysed for As^III, As^V, DMA, MMAA, arsenobetaine, arsenocholine, trimethylarsine oxide and tetramethylarsonium. As well as photolysis, the authors also utilized a chemical oxidation using alkaline persulfate to ensure that non-hydride forming species were converted into a reducible form. Both cation and anion chromatographic separations were developed.

Selenium in sea-water has been determined using a FI-HG-AFS system.⁶⁰¹ After filtration and acidification, Se^IV was determined by mixing with hydrochloric acid (1.5 M), potassium dihydrogenphosphate (5%) and tetrahydroborate (1.5% in 0.5% sodium hydroxide). For total Se determination, the sample was mixed with 30% sodium bromide and then irradiated in a microwave oven set at 210 W for 60 s and then passed through a cooling coil. After this reduction stage, the hydride was formed in the same way as described above. Selenium(VI) was calculated by subtraction. Calibrations were linear from 15 to 100 ng l⁻¹ for Se^IV and 15–150 ng l⁻¹ for total Se and LODs were 5 and 4 ng l⁻¹, respectively. Recoveries from spiked sea-water were 93–104%. Selenomethionine, selenoethionine and selenocystine have been separated on a C₁₈ column using 0.1% TFA–2% methanol as mobile phase and then fed directly to a hydraulic high pressure nebulizer.⁶⁰² The aerosol thus generated was then passed through a desolvation system before the dry gas was passed into a H₂–Ar diffusion flame where the Se was determined at 196 nm. The separation was complete within 7 min and LODs were 42, 71 and 50 ng ml⁻¹ for selenomethionine, selenoethionine and selenocystine, respectively. The method was applied to the analysis of garlic. One other Se speciation application using AFS detection has been reported.⁶⁰³ Few details were given, but the species separated were selenomethionine, selenoethionine, selenocystine and the trimethylselenonium ion.

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294. M. Hornung and V. Krivan, Anal. Chem., 1998, 70(16), 3444.
295. A. Ishida, K. Matsuda, S. Chu, F. Tanoue, S. Sakakibara, K. Ishino, S. Fuke and H. Fujiyasu, Mater. Sci. Eng., B, 1999, 59(1–3), 230.
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300. L. H. Liu, F. Yan and L. X. Wu, Fenxi Shiyanshi, 1999, 18(3), 65.
301. L. H. Liu, Q. K. Zhang, M. Y. Wang and J. L. Lei, Yaowu Fenxi Zazhi, 1999, 19(3), 181.
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313. M. Motelica-Heino, J.-M. Mermet and O. F. X. Donard, (Lab. Chim. Bio-inorganique Environ., EP CNRS 132, 64000 Pau, France). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999..
314. T. Howe, P. Krause, A. Cox and R. C. Hutton, (CETAC Technologies Inc., Crewe, Cheshire, UK CW1 6UZ). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999..
315. M. Lucic and V. Krivan, Fresenius' J. Anal. Chem., 1999, 363(1), 64.
316. S. Rings, R. Sievers and M. Jansen, Fresenius' J. Anal. Chem., 1999, 363(2), 165.
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319. S. Q. Lin, S. Y. Chen and M. H. Bi, Guangpuxue Yu Guangpu Fenxi, 1999, 19(1), 81.
320. P. V. Oliveira, Z. F. Queiroz, F. J. Krug, E. C. Lima and J. A. Nobrega, (Dept. Quim., Univ. Federal Sao Carlos, Sao Carlos 13.560-970, Brazil). Presented at 5th Rio Symposium on Atomic Spectrometry, Cancun, Mexico, October 4–10, 1998..
321. T. Narukawa, W. Yoshimura and A. Uzawa, Bull. Chem. Soc. Jpn., 1999, 72(4), 701.
322. K. P. Kureichik, Zh. Prikl. Spektrosk., 1998, 65(4), 604.
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324. M. A. Belarra, M. Resano, S. Rodriguez, J. Urchaga and J. R. Castillo, Spectrochim. Acta, Part B, 1999, 54, 787.
325. Z. Kowalewska, E. Bulska and A. Hulanicki, Spectrochim. Acta, Part B, 1999, 54, 835.
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332. F. M. Pennebaker, D. A. Jones, C. A. Gresham, R. H. Williams, R. E. Simon, M. F. Schappert and M. B. Denton, J. Anal. At. Spectrom., 1998, 13(9), 821.
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336. P. Lindblom, Anal. Chim. Acta, 1999, 380(2–3), 353.
337. G. N. Coleman, R. W. Foster, R. J. Krupa, S. Luan, M. J. Pilon and R. L. Stux, (Thermo Jarrell Ash/Baird, Franklin, MA 02038, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999..
338. J. D. Batchelor and B. T. Jones, Anal. Chem., 1998, 70(23), 4907.
339. R. A. De Verse, R. M. Hammaker, W. G. Fately, J. A. Graham and J. D. Tate, Am. Lab. (Shelton, Conn.), 1998, 20(21), 112S.
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344. U. Jerono, LaborPraxis, 1999, 23(5), 76.
345. E. D. Prudnikov and Y. S. Shapkina, (Earth's Crust Inst., State Univ., St Petersburg 199034, Russia). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999..
346. A. Montaser, J. A. McLean, M. G. Minnich, L. A. Lacone, Q. Jin, S.-A. E. O'Brien and A. Okino, (George Washington Univ., Washington, DC 20052, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7–12, 1999..
347. S. Greenfield, (Albright and Wilson Mfg. Ltd., UK). Presented at Pittcon. '99, Orlando, FL, USA, March 7–12, 1999..
348. M. A. Rutzke, (USDA Plant Soil Nutr. Lab., Cornell Univ., Ithaca, NY 14853, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7–12, 1999..
349. C. Hensman and J. W. Olesik, (Lab. Plasma Spectrochem., Laser Spectrosc. and Mass Spectrom., Dept. Geol. Sci., Ohio State Univ., Columbus, OH 43210, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7–12, 1999..
350. C. Breer, W. Buscher, R. M. Barnes and K. Cammann, (Inst. Chemo- und Biosensorik e.V., 49149 Muenster, Germany). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999..
351. D. S. Bollinger and A. J. Schleisman, At. Spectrosc., 1999, 20(2), 60.
352. A. Eastgate and M. Haldimann, (Glass Expansion, 1323 Romainmotier, Switzerland). Presented at Pittcon.'99, Orlando, FL, USA, March 7–12, 1999..
353. H. R. Badiel, V. Grishko and V. Karanassios, (Guelph-Waterloo Center Graduate Work Chem., Dept. Chem., Univ. Waterloo, Waterloo, ON, Canada N2L 3G1). Presented at Pittcon.'99, Orlando, FL, USA, March 7–12, 1999..
354. J. A. Horner and G. M. Hieftje, Appl. Spectrosc., 1999, 53(6), 713.
355. Y. J. Li, Y. F. Liu, X. P. Zeng, S. L. Wang and Y. M. Ni, Lihua Jianyan, Huaxue Fence, 1998, 34(10), 440.
356. K. Oishi, T. Okumoto and T. Shirasaki, Bunseki, 1999,(1), 81.
357. B. W. Pack, G. M. Hieftje and Q. Jin, Anal. Chim. Acta, 1999, 383(3), 231.
358. S. D. Anghel, T. Frentiu, E. A. Cordos, A. Simon and A. Popescu, J. Anal. At. Spectrom., 1999, 14(4), 541.
359. T. Frentiu, A.-M. Rusu, S. D. Anghel, S. Negoescu, A. Popescu, A. Simon and E. M. Cordos, ACH – Models Chem., 1999, 137(1–2), 119.
360. T. Frentiu, S. D. Anghel, A. Simon, A. Popescu, A.-M. Rusu and E. A. Cordos, ACH – Models Chem., 1999, 136(1–2), 131.
361. X. Pan, B. Hu, Y. Ye and R. K. Marcus, J. Anal. At. Spectrom., 1998, 13(10), 1159.
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364. K. Wagatsuma, Bunseki Kagaku, 1999, 48(4), 457.
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366. C. Yang, K. Ingeneri and W. W. Harrison, J. Anal. At. Spectrom., 1999, 14(4), 693.
367. J. A. C. Broekaert, T. K. Starn, L. J. Wright and G. M. Hieftje, Spectrochim. Acta, Part B, 1998, 53, 1723.
368. Y. S. Park, S. H. Ku, S. H. Hong, H. J. Kim and E. H. Piepmeier, Spectrochim. Acta, Part B, 1998, 53, 1167.
369. G. G. Sikharulidze and A. E. Lezhnev, J. Anal. At. Spectrom., 1999, 14(1), 45.
370. J.-C. Hubinois, A. Morin, P. Marty, J.-P. Larpin and M. Perdereau, J. Anal. At. Spectrom., 1999, 14(9), 1405.
371. O. Banhidi and L. Papp, ACH—Models Chem., 1999, 136(1–2), 75.
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373. X. W. Feng and Y. Q. Yang, Guangpuxue Yu Guangpu Fenxi, 1998, 18(6), 731.
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376. J.-M. Sun, Z. Yan, L.-P. Liu, D.-Q. Zhang, L.-L. Yang and H.-W. Sun, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 6.
377. A. B. Volynsky, Spectrochim. Acta, Part B, 1998, 53, 1607.
378. E. C. Lima, F. J. Kurg, A. T. Ferreira and F. Barbosa Jr., J. Anal. At. Spectrom., 1999, 14(2), 269.
379. E. C. Lima, F. J. Krug and K. W. Jackson, Spectrochim. Acta, Part B, 1998, 53, 1791.
380. K. A. Wagner, K. E. Levine and B. T. Jones, Spectrochim. Acta, Part B, 1998, 53B, 1507.
381. G. Absalan, C. L. Chakrabarti, K. L. Headrick, M. Parker and R. K. Marcus, Anal. Chem., 1998, 70(16), 3434.
382. C. E. Hensman and G. D. Rayson, J. Anal. At. Spectrom., 1999, 14(7), 1025.
383. J. M. Harnly, J. Anal. At. Spectrom., 1999, 14(2), 137.
384. D. N. Wichems, R. E. Fields and J. M. Harnly, J. Anal. At. Spectrom., 1998, 13(11), 1277.
385. Mitchell and Zemansky, Resonance Radiation and Excited Atoms, Cambridge University Press, Cambridge, UK, 1961..
386. H. Becker-Ross, S. Florek, U. Heitmann and M. Schuetz, (ISAS, Inst. Berlin, 12489 Berlin, Germany). Presented at 5th Rio Symposium on Atomic Spectrometry, Cancun, Mexico, October 4–10, 1998..
387. Ger. Offen. DE 19,725,809 (Cl. H01S3/19), 24 Dec 1998, appl. 19,725,809, 18 Jun 1997; 4 pp..
388. T. Komatsu, T. Araki and A. Scheeline, Appl. Opt., 1999, 53(1), 108.
389. L. Da, S. Y. Zhang, B. L. Huang, Z. B. Gong and P. Y. Yang, Guangpuxue Yu Guangpu Fenxi, 1999, 19(3), 352.
390. Z. B. Gong, F. Liang, P. Y. Yang, Q. H. Jin and B. L. Huang, Guangpuxue Yu Guangpu Fenxi, 1999, 19(3), 356.
391. S.-Y. Zhang, B.-L. Huang, Z.-B. Gong and P.-Y. Yang, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 75.
392. S.-Y. Zhang, B.-L. Huang, Z.-B. Gong and P.-Y. Yang, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 76.
393. D. Yates, C. Schneider, I. Shuttler and K. Fredeen, (Perkin-Elmer Corp., Norwalk, CT, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999..
394. S. De Goy, D. Lanzisera, K. Lopez, J. Noonan and J. Rebello, Spectroscopy (Eugene, Oreg.), 1998, 13(10), 36.
395. C. Lengacher, S. Macklin, D. Hite and M. F. Masters, Am. J. Phys., 1998, 66(11), 1025.
396. L. Poletto, A. Boscolo and G. Tondello, Appl. Opt., 1999, 38(1), 29.
397. W. Bohle, T. Brandt, H. F. Falk, H. J. Graf, U. Heynen, P. Heitland, K. Krengel-Rothensee, U. Richter and R. Zeyen, (SPECTRO Analytical Instruments GmbH, 47533 Kleve, Germany). Presented at Pittcon.'99, Orlando, FL, USA, March 7–12, 1999..
398. M. B. Knowles, T. T. Nham and S. J. Carter, Int. Lab., 1998, 28(6), 25S.
399. C. Webb, A. T. Zander, H. Visse, P. V. Wilson, G. Russell and M. Knowles, (Ginzton Res. Center, Varian Assoc., Palo Alto, CA 94304, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999..
400. A. T. Zander, C. B. Copper, R.-L. Chien, M. Knowles, G. Russell and A. Wiseman, (Varian Res. Center, Varian Assoc. Inc., Palo Alto, CA, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7–12, 1999..
401. M. B. Knowles, T. Nham, G. Russell and H. Visser, (Varian Australia Pty Ltd, Mulgrave, 3170 Victoria, Australia). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999..
402. D. Shrader and C. Rivera, (Varian Assoc., Wood Dale, IL 60191, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7–12, 1999..
403. I. B. Brenner, S. Vats and A. T. Zander, J. Anal. At. Spectrom., 1999, 14(8), 1231.
404. R. W. Foster, M. J. Pilon and R. L. Stux, (Thermo Jarrell Ash/Baird, Franklin, MA 02038, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999..
405. G. N. Coleman, R. W. Foster, R. J. Pascucci and R. L. Stux, (Thermo Jarrell Ash, Franklin, MA 02038, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7–12, 1999..
406. G. N. Coleman, G. R. Dulude, R. W. Starek and R. L. Stux, (Thermo Jarrell Ash, Franklin, MA 02038, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7–12, 1999..
407. P. Am Ayasse and A. Le Marchand, (Jobin Yvon Emission, ). Presented at 5th Rio Symposium on Atomic Spectrometry, Cancun, Mexico, October 4–10, 1998..
408. P. A. Ayasse and A. Le Marchand, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 64.
409. A. Bengtson and S. Hanstrom, (Swedish Inst. Metals Res., 1114 28 Stockholm, Sweden). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999..
410. U. Oppermann and H. Hohmann, (Shimadzu Deutschland GmbH, 47269 Duisburg, Germany). Presented at Instrumental Methods of Analysis. Modern Trends and Applications (IMA '99), 19–22 September, 1999, Chalkidiki, Greece..
411. Y. Y. Zong, P. J. Parsons and W. Slavin, Spectrochim. Acta, Part B, 1998, 53, 1031.
412. D.-q. Zhang, C.-m. Li, L.-l. Yang and H.-w. Sun, J. Anal. At. Spectrom., 1998, 13(10), 1155.
413. H. W. Sun, L. J. Chen and J. M. Sun, Guangpuxue Yu Guangpu Fenxi, 1998, 18(5), 597.
414. I. Lopez Garcia, M. Sanchez-Merlos and M. Hernandez Cordoba, J. Anal. At. Spectrom., 1998, 13(10), 1151.
415. Z.-J. Yang, Z.-Y. Zhang, Z.-M. Li, Y.-H. Jia and H.-Z. Wang, Gaodeng Xuexiao Huaxue Xuebao, 1999, 20(5), 25.
416. S. P. Madden, M. B. Denton and M. J. Pilon, (Dept. Chem., Univ. Arizona, Tucson, AZ 85721, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7–12, 1999..
417. C. Webb, A. T. Zander, P. V. Wilson and G. Perlis, (Ginzton Res. Center, Varian Assoc., Palo Alto, CA 94304, USA). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999..
418. J. Noelte, At. Spectrosc., 1999, 20(3), 103.
419. C. Ivaldi and T. W. J. Barnard, Spectrochim. Acta, Part B, 1993, 48, 1265.
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422. C. Prokisch, A. M. Bilgic, E. Voges, J. A. C. Broekaert, J. Jonkers, M. van Sande and J. A. M. van der Mullen, Spectrochim. Acta, Part B, 1999, 54, 1253.
423. C. Yang, K. Ingeneri, M. Mohill and W. W. Harrison, (Dept. Chem., Univ. Florida, Gainesville, FL 32611, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7–12, 1999..
424. V.-D. Hodoroaba and T. Wirth, J. Anal. At. Spectrom., 1999, 14(9), 1533.
425. V. Hoffmann, R. Kurt, K. Kammer, R. Thielsch, Th. Wirth and U. Beck, Appl. Spectrosc., 1999, 53(8), 987.
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466. J. L. Fischer and C. J. Rademeyer, Spectrochim. Acta, Part B, 1999, 54, 975.
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492. M. R. Anderson, A. T. Zander, P. V. Wilson and S. G. Hamilton, (Varian Assoc., Palo Alto, CA 94304, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7–12, 1999..
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496. J. Noelte, D. Yates, P. Krampitz and C. Schneider, (Environ. and Applied Inorg. Anal., Perkin Elmer Corp., Norwalk, CT 06859, USA). Presented at Pittcon. '99, Orlando, FL, USA, March 7–12, 1999..
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544. M. R. Brunetto, J. R. Luna, A. Zambrano, M. Gallignani, M. Burguera, J. L. Burguera and Y. Petit de Pena, Analyst (Cambridge, U.K.), 1999, 124(10), 1493.
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583. L. C. Robles, B. de Celis, J. M. Lumbreras and A. J. Aller, Anal. Commun., 1997, 34(12), 409.
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596. J. L. Gomez-Ariza, E. Morales, D. Sanchez-Rodas and I. Giraldez, (Dept. Quim. Ciencia Materiales, Escuela Politecnica Superior, Univ. Huelva, 21819 Palos de la Frontera, Spain). Presented at European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999..
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Footnote

† *Review co-ordinator, to whom correspondence should be addressed and from whom reprints may be obtained.

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