Advances in atomic emission, absorption and fluorescence spectrometry and related techniques
E. Hywel Evans*a, John B. Dawsonb, Andrew Fishera, W. John Pricec, Clare M. M. Smithd and Julian F. Tysone
aDepartment of Environmental Sciences, University of Plymouth, Drake Circus, Plymouth, UK PL4 8AA
bDepartment of Instrumentation and Analytical Science, UMIST, PO Box 88, Manchester, UK M60 1QD
cEllenmoor, East Budleigh, Budleigh Salterton, Devon, UK EX9 7DQ
dDepartment of Chemistry, University College Cork, Ireland
eDepartment of Chemistry, University of Massachusetts, 701 Lederle Graduate Research Tower, 710 North Pleasant Street, Amherst, MA 01003-9306, USA
First published on 21st May 2002
Abstract
The major trend evident in this year's review is the continued decline in publications dealing with applications of glow discharges, chemometrics and fundamental studies of all types of plasmas used for atomic spectroscopy. There has been an increase in the number of publications dealing with speciation studies compared with last year, particularly dealing with applications, which possibly reflects the growing maturity and acceptance of the field into the mainstream. There were also a considerable number of reviews in this area. The number of papers dealing with preconcentration and matrix removal, and laser based spectroscopy, stayed at levels comparable with recent years, but there was a decline in applications dealing with solid sampling using glow discharges and laser ablation. Routine applications of ETAAS and FAAS were numerous (and not dealt with in this review) and novel developments were few, though several important reviews of fundamental processes in ETAAS were published.
1 Sample introduction
1.1 Flow injection
Various aspects of the application of FI techniques have been reviewed. Cerda et al. compare1 sequential injection (SI) with FI procedures for the monitoring of environmental parameters, Danet et al.2 examine FI methods for the determination of inorganic species in water, and Koscielniak3 evaluates calibration methods and the reasons for the limited use that is currently made of FI calibration (it is unreliable, complicated and of limited application). He proposes a new FI procedure: the integrated calibration technique. He has also developed4 a FI method for examining interference effects in FAAS. Lopez Garcia et al.5 have developed some further applications of their FT-FAAS procedure: standard additions and on-line dilution. The procedures are based on exploiting the pulsations in the signal produced by the rollers of the peristaltic pumps that deliver the various solutions (samples, standards and diluents) to the spectrometer. Antomia and Allen6 exploited Donnan dialysis for the extraction of Pb from sucrose, corn syrup and honey. The procedure was a little slow: 15 min for about 90% extraction. The LOD was 0.4 mg kg−1. One or two publications reporting analyses in which the hydraulic high pressure nebulizer was used (when the sample solution would typically be injected into the carrier) are discussed later in section 1.3. Many HG procedures are based on FI methodology: these are discussed in section 1.4. Most FI papers published during this review period describe some aspect of preconcentration.1.2 Preconcentration
A review in Japanese by Matsunaga7 covers 195 references dealing with the recognition, separation and concentration of metals by chelating resins or polymer resins impregnated with chelating agents. The review highlights the possibilities of “molecular imprinting” as a technique for the development of highly selective, solid-phase extracts. Rao and Gladis reviewed8 the applications of oxines as reagents for the preconcentration of trace inorganic analytes. It seems obvious to state that the goal of most studies involving preconcentration is improved LOD for the measurement of the analyte(s) in the matrix or matrices of interest. Despite this, very few publications on this topic (if any) discuss the figure of merit to be used for the optimization, which is usually some parameter directly related to sensitivity and never a composite parameter involving signal and noise, or signal, noise and throughput. For all procedures, signal increases with increasing sample volume; under some conditions the increase is directly proportional to the sample volume. However, throughput decreases in direct proportion to sample volume, making the need for an optimization obvious. Curiously, few reports of optimization studies include any systematic investigation of the effect of sample volume on relevant performance parameters. Furthermore, there is a tendency for authors to report the results of their studies under general titles such as “the determination of element x in matrix y by preconcentration procedure z with atomic spectrometry technique w.” Sometimes, when the contents of the paper are examined, it becomes clear that the procedure was not capable of determining the element x in matrix y at the naturally occurring concentrations, but that the researchers had demonstrated that element x spiked into matrix y at a concentration above the LOD of the new procedure could be determined (sometimes with the help of the standard additions method). There have been one or two reports of preconcentration procedures in conjunction with HG. These reports are discussed later in section 1.4 under the appropriate individual element.1.2.1.1 Flame atomic absorption spectrometry. In contrast to last year's review period, when there were several reports of preconcentration procedures based on the retention of a neutral metal chelate on C18, the most popular procedure this year has been retention on a chelating solid-phase extractant. Lead has been the most popular analyte. Zhang10 retained Pb on poly(8-HQ-SO3H) from water samples buffered to pH 9. The analyte was eluted with 0.5 mol l−1 HCl for an enrichment factor of 115 and an LOD of 1 ng ml−1. Yebra-Biurrun et al.11 achieved an LOD of 6 ng ml−1 for retention on a poly(aminophosphonic acid) chelating resin with elution by 2 mol l−1 HCl. Certified reference alloys were accurately analysed. Da Silva et al.12 compared the retention of the lead complex with O,O-diethylphosphate on activated carbon with retention on polyurethane (followed by elution with ethanol). The latter produced an enrichment factor (300) about five times that for the carbon with a correspondingly lower LOD of 0.8 ng ml−1. The procedure was applied to the analysis of water. Lemos and co-workers have investigated complexation with 2-(2-benzothiazolylazo)-2-p-cresol for the retention of lead13,14 and nickel.15 For retention on polyurethane foam,13 the LOD was 1 ng ml−1, and this method was used for the analysis of seafood and some other biological materials. For retention on Amberlite XAD-2,14 the LOD was 4 ng ml−1. This material was also used in the determination of Ni15 with an LOD of 1 ng ml−1. The method was validated by the analysis of four biological reference materials. Several possible extractant materials (Cellex P, Chelex 100 and Dowex 50Wx4) were compared by Pryrzynska and co-workers16 for the determination of Cd. The best performance was obtained with Cellex P (a cellulose based material with phosphonic acid chelating groups) for which an LOD of 0.7 ng ml−1 was obtained. They also evaluated17 a macroporous anion-exchange material, AG MP-1, for the retention of the Cd–Tiron complex with elution by 2 mol l−1 HNO3. The LOD was 0.9 ng ml−1. De Pena et al.18 studied the performance of a new material, poly(octadecyldiitaconate), for the retention of the neutral complex of Cd with APDC. They obtained an LOD of 0.08 ng ml−1 for elution with 0.3 ml of IBMK from a 20-mg column. Garrido et al. devised19 a procedure for the analysis of waters in which the Cd–DDC complex was retained on the inner walls of knotted PTFE tubing. For a 6-ml sample and elution with ethanol, the LOD was 3 ng ml−1. A similar mechanism was used by Anthemidis et al.20 for the retention of Cu as the APDC complex on a column containing PTFE turnings. For elution with IBMK, the LOD was 50 ng l−1. The procedure was found to have better performance characteristics (longer lifetime, higher flow rate and greater flexibility) than methods based on other sorbent materials or knotted tubular reactors. Yin et al.21 showed that collection at elevated temperature (65 °C) improved the performance of a sorbent (C18) extraction procedure for the determination of Co as the complex with 1-nitroso-2-naphthol. Elution, with ethanol, of the analyte retained from a 3.2-ml sample gave an LOD of 3 ng ml−1. Almeida et al.22 determined Cu by retention on a polyamine chelating column. The method was applied to the determination of Cu in urine following a rapid digestion with peroxide; for a 5-ml sample, the LOD was 1 ng ml−1. Chinese workers (Chen and Sun) determined23 Cd, Cu, Mn and Pb by retention on a “double chelating resin column”. Sensitivity increases of up to 50-fold were reported. Several extractants were studied by Godlewska-Zylkiewicz et al.24 for the determination of Pt in water; the best results were obtained for alumina. An enrichment of 400 was obtained with elution by 0.025 ml of 2 mol l−1 ammonia.
Iron species (FeII and FeIII) in waters were determined by Bagheri et al.25 by selective retention of the FeIII complex on a silica gel column loaded with 2-mercaptobenzimidazole. The unretained FeII was oxidized with peroxide and retained on another column. The retained Fe was eluted with thiocyanate solution and detected by either visible absorption spectrometry or by FAAS. Costa and Araujo26 determined FeIII in wine by extraction of the complex with thiocyanate into IBMK and determination by FAAS. Total Fe was determined by injecting a small volume for direct transport to the spectrometer. Liobat-Estelles and Mauri-Aucejo27 determined CrIII and CrVI in waters by selective retention of CrVI as a complex with EDTA on an anion-exchange column.
1.2.1.2 Electrothermal atomic absorption spectrometry. At least three papers describe procedures for the analysis of sea-water. Lin et al.28 retained Co, Cu and Ni on a microcolumn of Muromac A-1, then eluted the metals with 0.050 ml of 20% HNO3 directly into the furnace of a multi-element spectrometer. The LOD were 0.2 ng l−1, 1 ng l−1 and 0.5 ng l−1, respectively. Zih-Perenyi et al.29 determined Co, Pb and V in mineral waters, and Ni in sea-water by solid-phase extraction onto iminodiacetic acid ethyl cellulose. Residual solution was flushed from the column with air after loading, washing and elution. The LOD were 60, 20, 60 and 70 ng l−1 for Co, Ni, Pb and V, respectively. Alonso et al.30 adopted the strategy of mounting the column of extractant (silica gel functionalized with bis(di-2-pyridyl)methylene thiocarbohydrazide) in the autosampler delivery system. A switching valve allowed sample solution to be delivered to the column by a peristaltic pump, but eluted by the autosampler pump. The procedure, for the determination of Cd, was validated by the analysis of two BCR reference waters (403 Sea-water and 505 Estuarine Water). Wang and Hansen developed31 a novel sequential injection procedure in which the analyte-loaded extractant was delivered to the furnace for the final measurement step, rather than an eluent solution. The material was SP Sephadex C-25 cation-exchange resin, which was transported to the furnace in 0.03 ml of carrier buffer solution. Nickel was determined in two reference materials and a urine sample down to an LOD of 9 ng l−1 for a sample volume of 1.8 ml loaded at 0.72 ml min−1 (150 s). The enrichment factor was 72 and the throughput 12 h−1, which was achieved by operating the graphite furnace program in parallel with the preconcentration procedure. In addition to these SPE procedures, several HG procedures have been described during the current review period in which the evolved hydride was trapped on the interior of the furnace prior to atomization. These procedures are discussed in the later sections concerned with HG (see section 1.4).
1.2.1.3 Inductively coupled plasma emission spectrometry. The reagent used by Alonso et al.,30 silica gel functionalized with bis(di-2-pyridyl)methylene thiocarbohydrazide, was also used by Zougagh et al.32 for the preconcentration of Zn from water samples. Doehlert matrix and factorial designs were featured in the optimization study of five variables (sample flow rate, eluent flow rate, eluent concentration, pH and buffer concentration) which produced an LOD of 2 ng ml−1 for a 60-s sample loading period. Wuilloud et al.33 obtained a LOD for Pb of 200 ng l−1 for a 10-ml wine sample passed through a column of Amberlite XAD-16, which retained the Pb complex with quinolin-8-ol. Following elution with HNO3 the solution was introduced to the spectrometer via an ultrasonic nebulizer, which gave a contribution of 15-fold to the overall enhancement (relative to that obtained with direct introduction via a pneumatic nebulizer) of 225-fold. Liang et al.34 demonstrated that nanometer-sized titania particles could be used for the on-line preconcentration of the REE Dy, Eu, La, Y and Yb.
1.2.2.1 Solid phase extraction. Nickson et al.35 determined Cd, Co, Cu, Mn, Ni, Pb and Zn in natural waters by ICP-OES following field sampling/preconcentration on twin columns of Metpac CC-1 (an iminodiacetate functionalised material). The columns were eluted in the laboratory in a FI manifold coupled directly to the spectrometer. Loading times of up to 2 h at a flow rate of 0.5 ml min−1 produced accurate results for the analysis of the CASS-2 (coastal sea-water) CRM. The same chelating ligand, immobilized on filter discs, was used by Oshima et al.36 for the preconcentration of REE with separation from alkali and alkaline earth metals. After removal of the analytes by sonicating in 0.1 mol l−1 nitric acid, a 0.025 ml sub-sample was injected into an air FI system for transport to the spectrometer. Jain et al.37 synthesised a new solid-phase extractant consisting of o-vanillinsemicarbazone bonded to Amberlite XAD-4 (a styrene–divinylbenzene co-polymer material). They applied the material to the determination of some REE, including Ce, La, Th and U by ICP-OES and ETAAS. Singh and co-workers bonded pyrocatechol38 and quinalizarin39 to Amberlite XAD-4. The former material trapped Cd, Co, Cu, FeIII, Ni and Zn from water samples at pH 3–6.5 with elution by 2 mol l−1 HCl or HNO3 giving enrichment factors of up to 200, though the lowest concentrations for which quantitative recoveries were obtained ranged from 5 to 25 ng ml−1. The latter material trapped Cd, Co, Cu, Mn, Pb and Zn from solutions of somewhat higher pH, 5–7, with elution by 4 mol l−1 HNO3. Analytes were quantified by FAAS, and Co, in a vitamin tablet, was also determined by both procedures. The quinalizarin material also preconcentrated U for determination by molecular fluorescence spectrometry. Mondal et al.40 determined Cu and Zn by a procedure which incorporated solid-phase extraction (SPE) onto a poly(styrene–divinylbenzene) material to which had been attached imidylazo groups. Procedures involving covalently bound 8-hydroxyquinoline (oxine) were developed by Liu et al.41 and Sohrin and Japanese colleagues.42 In the first of these, the 5-sulfonic acid derivative was linked to cellulose which was packed into a column for the retention REE for subsequent determination by ICP-OES after elution with dilute HNO3. In the second procedure, 34 elements were retained by the oxine immobilized on fluorinated metal alkoxide glass from a synthetic sea-water adjusted to pH 5 followed by elution into 0.5 mol l−1 HNO3 for determination by ICP-OES or high-resolution ICP-MS. Particular care was taken to control the blank values.
The XAD materials have also featured in a number of sorbent-extraction procedures. In such a procedure, the chelating agent is not covalently bound to the column packing but is adsorbed to the hydrophobic surface. In past years octadecylsilyl silica (C18) has been a popular choice in sorbent extractions: however, over the current review period other materials have featured more prominently. Bermejo-Barrera et al.43 collected Ga and In and 1-(2-pyridylazo)-2-naphthol (PAN) complexes on Amberlite XAD-2 from solutions of ore samples with elution at pH 5 and pH 9 for Ga and In, respectively. A similar procedure (PAN complexes on XAD-2000) was applied by Elci and coworkers44 for the determination of Cr, Cu, Ni and Pb in sea-water. The analytes were eluted with 1 mol l−1 HNO3 in acetone and determined by FAAS. This research group has evaluated several variations on this theme: 4-(2-thiazolylazo)resorcinol (TAR) complexes on XAD-16,45 dithiophosphoric acid–O,O-diethyl ester complexes on activated carbon,46 DDC complexes on XAD-4,47 and pyrocatechol violet complexes on activated carbon.48 The first of the procedures was applied to the determination of Co in urine, the second to the determination of Ag, Au and Pd in some aluminium and manganese salts, the third to the determination of Bi, Cd, Cu, Fe, Ni and Pb in industrial waste waters and the fourth to the determination of Cd, Co, Cr, Cu, Ni and Pb in bottled mineral waters. In each case the eluent was predominantly acetone and the determinations were performed by FAAS. The LOD ranged between 4 and 20 ng ml−1. Kubota et al.49 also devised a procedure involving activated carbon in which the pyrogallol complex of SbIII was retained from natural water samples. As the procedure was selective for SbIII over SbVI, it was adapted for the speciation of these two inorganic oxidation states. An activated carbon slurry was introduced in the atomizer for determination by ETAAS. In a variation of this procedure, Shida and Umeka50 retained the SbIII complex with pyrocatechol violet on a suspension of finely ground anion-exchange resin. Following filtration, a 3-mm diameter portion of the filter was excised and transferred to the furnace for determination by ETAAS. The possible interference from a variety of matrix components was masked by EDTA and, for the determination of Sb in river water, the LOD was 10 ng l−1. Nazi et al.51 determined Pb by FAAS following collection of the complex with anthranilic acid on activated carbon. The procedure was applied to the analysis of vegetables containing Pb at concentrations ranging from 90–520 ng g−1. Calibration was against matrix matched standards. Chakrapani et al.52 showed that Ag, Au and Pd could be retained on activated carbon without the aid of a chelating agent. Following dissolution of the solid phase in nitric and perchloric acids, the analytes were determined by FAAS. The method was validated by the analysis of several Canada Center for Mineral and Energy Technology RM and applied to the analysis of geological materials containing the analytes at concentrations down to 0.1 mg kg−1, for which a 10-g sample was required. Shiowatana et al.53 determined Cd, Co, Hg and Ni in sea-water by ETAAS following retention of the complexes with 8-hydroxyquinoline (oxine) on activated carbon. Slurries of the analyte-loaded carbon in glycerol or boiled tapioca were introduced to the graphite furnace atomizer. Enrichments of up to 20000 times were reported, and the LOD were 0.3, 7, 120 and 7 ng l−1, respectively. Ramesh et al.54 retained Cd, Co, Cr, Co, Mn, Ni , Pb and Zn (from natural waters) on a column containing polyurethane foam loaded with piperidine dithiocarbamate. Following elution with dilute HNO3, the metals were determined by ICP-OES. Polyurethane foam was also used by Ferreira et al.55 in a batch procedure for the retention of the Mo complex with thiocyanate, thereby effecting separation form large amounts of iron. Following release into concentrated HNO3 or acetone, the Mo was determined by ICP-OES
A poly(diethylenetriamine) solid phase was used by Bakircioglu et al.56 to retain Cd, Cu and Zn at pH 3, with dissolution of the analytes in HNO3 for determination by FAAS. Goto and other Japanese workers57 developed a procedure for the ETAAS determination of Be in sea-water, in which the analyte was retained on silica gel modified with xylenol orange and trioctylmethylammonium chloride (capriquat). Sample volumes between 100 and 2300 ml were loaded at 5 ml min−1 (20 min to 7.7 h loading times) and eluted into 5 ml of 0.5 mol l−1 HNO3. Alumina, modified with sodium dodecylsulfate and PAN, was shown by Shemirani and Akhavi58 to retain Cd from water samples. Up to 1000 ml of sample (waste, tap or mineral water) was passed through the column at 5 ml min−1 (a 3.3 h loading time), and the retained Cd was eluted with 10 ml of 2 mol l−1 HNO3 for a LOD, by FAAS, of 20 ng l−1. Shamsipur et al. determined Cu by FAAS, following preconcentration on C18 membrane discs modified with 11-hydroxynaphthacene-5,12-quinone, down to concentrations of 0.2 ng ml−1. This general procedure, retention of analyte derivatives on hydrophobic membranes, has been reviewed (in Japanese) by Taguchi et al.59
The number of research groups exploiting the possibilities of naphthalene as the solid-phase extractant has increased to four for this review period. Taher and co-workers continue to report incremental developments in applications: the determination of Cd60 and Mn61 in alloys and biological materials by FAAS. The metals were retained as their complexes with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol ion-paired with tetraphenylborate on columns of microcrystalline naphthalene, which was then dissolved in 5 ml of dimethylformamide. Xiong et al.62 retained REE La, Sc, Y and Yb as the 1-phenyl-3-methyl-4-benzoyl-5-pyrazone complexes and then dissolved the naphthalene in acetone. The final determination was made by ICP-OES with ETV sample introduction to give LOD of between 10 (Sc) and 200 (La) pg ml−1. Cai et al.63 retained Eu, La and Yb as the complexes with tribromoarsenazocetylpyridinium bromide followed by elution (not dissolution) with 3 mol l−1 HCl and quantification by ICP-OES at LOD of 1–9 ng ml−1. The method was validated by the analysis of a vehicle exhaust particulates RM and a citrus leaves RM. Finally, Gholivand et al.64 determined Cu in drug, river water and alloy samples by the retention on naphthalene loaded with 1,5-diphenylcarbazone with elution into 1 mol l−1 HCl. The LOD, by FAAS, was 7 ng ml−1.
Tokalioglu et al.65 showed that Cd, Cr, Co, Cu and Ni could be retained on Amberlite XAD-16 without the need for a complexing agent. As metals bound to humic substances were not retained , the procedure could be used for speciation. LOD ranged from 2 (Cd) to 50 (Ni) ng ml−1 for both FAAS and ETAAS. Bangroo et al.66 retained Sc from mineral samples on an Amberlite IR 120 cation-exchange resin, followed by FAAS determination. Scaccia et al.67 retained Cd and Cu on a Dionex HPIC-CG5 ion-exchange material from solutions containing 3 mmol−1 pyridine-2,6-dicarboxylic acid. Volumes of up to 200 ml were loaded at rates between 1 and 4 ml min−1 with elution into 0.1–0.2 ml of eluent. For ETAAS, the LOD were 0.02 ng l−1 and 1 pg l−1. The method was applied to the analysis of high-purity waters. Seren et al.68 developed a FAAS procedure in which activated bentonite retained Co, Cu and Mn from water samples. Elution was with 0.1 mol l−1 EDTA or 2 mol l−1 HCl, and the LOD were 2, 1 and 1 ng ml−1, for Co, Cu and Mn, respectively. The retention characteristics of Pb on this clay mineral have also been studied by Naseem and Tahir,69 Trace concentrations of B in river water were determined by Yoshikuni and Japanese co-workers70 with a procedure in which the analyte was retained from 0.2 mol l−1 HF solution on a column of Bio-Rad AGI-X8 anion-exchange material. Following elution with HNO3, evaporation in the presence of H3PO4, and the wet ashing of any organic material, the B was determined by ICP-OES. Choi et al.71 separated Au, Ir, Pd, Pt, Rh and Ru from the chromium matrix of chromite by retention of the analytes on an anion-exchange column. Gold was selectively eluted with acetone–dilute HNO3, Pt with concentrated HNO3 and the remaining elements as a group with concentrated HCl. The final determination was by ICP-OES.
Chang et al.72 showed that a poly(acrylphenylamidrazonephenylhydrazide) chelating fiber was capable of preconcentrating Au and Ru from solutions at pH 4 containing 50–500 ng ml−1 of the analytes with elution into 10 ml of a solution containing 4 mol l−1 HCl and 3% thiourea for determination by ICP-OES. One of the advantages of columns packed with fibers is that the back pressure is lower than column packed with small spherical particles and thus loading flow rates can be higher. For example, flow rates of 10–20 ml min−1 were possible for a thiol modified cotton fiber material, developed by Yu et al.,73 which collected Ag, Bi, Cd, Co, Cu, In, Ni, Pb, Sn, Tl and Zn. The analytes were eluted with 2–3 ml of 0.01–2.0 mol l−1 HCl. Even higher rates are possible: Norisuye et al.74 fabricated manganese dioxide impregnated cellulose fibers by the reaction between permanganate and manganeseII and selectively collected Zr from large volumes (50–1000 l) of sea-water delivered at up to 15 l min−1. After a two-step elution, the Zr was determined by ICP-OES.
Ohta et al.75 preconcentrated Cu from river water by cultivating a yeast in the sample solution with the aid of some ammonium hydrogen phosphate at 40 °C for 2 h. No other pretreatment was necessary. The preconcentration factor was about 7-fold, and for determination by ETAAS, the LOD was 90 ng l−1. Perez-Corona et al.76 employed free and immobilized baker's yeast (Saccharomyces cerevisiae) in a procedure for the speciation of Cr; at pH 13, CrIII is taken up by the yeast, whereas CrVI is not. With the yeast immobilized on alginate a column method was devised in which the CrIII was eluted with 0.3 mol l−1 HNO3. The system also formed the basis for an on-line method of remediation of waste water. Bag et al.77 determined FeII and FeIII in waters after separation with Apergillus niger (a mould) immobilized on sepiolite (the calcareous internal shell of the cuttlefish).
1.2.2.2 Liquid–liquid extraction. Hossain and Honjo78 determined Rh in chloroplatinic acid and palladium chloride by extraction of the ion-association complex between the hexhydrated RhIII cation and trichloroacetate into tri-n-butyl phosphate. After back extraction, Rh was determined by FAAS. Zendelovska and Stafilov79 found that they could not determine Tl in solutions of sulfide minerals by ETAAS because of the interferences from matrix components. To overcome these interferences, they extracted the Tl from 10 mol l−1 HCl into isoamyl alcohol. Iron, which was co-extracted, was removed by back extraction with 4 mol l−1 H2SO4 and the Tl was determined in the organic phase with the help of palladium as a modifier. The LOD was 0.05 mg kg−1. Shimizu et al.80 developed a procedure for the speciation of VIV and VV in waters, based on the selective extraction of VV with butanol and VIV 8-mercaptoquinoline. Following back extraction into HNO3, the V was determined by ETAAS with nickel as a modifier. Malyshev et al.81 determined As in high-purity sulfur by ETAAS following extraction into toluene and back extraction into distilled water. Several analytes (Cd, Cu, Mn, Ni, Pb, and Zn) were determined by Ramesh et al.82 by ICP-OES following extraction of the complexes with piperidine dithiocarbamate into IBMK and back extraction into HNO3. Hossain et al.83 determined In by ICP-OES with a procedure that involved extraction of the ion-association complex of the hexachloroanion and dicyclohexyl-18-crown-6 oxonium ion into chloroform.
1.2.2.3 Cloud point extraction. Teo and co-workers developed procedures for the determination of Mn84 and Co and Ni85 in waters by the extraction of the complexes with 1-(2-thiazolylazo)-2-naphthol into phase enriched with the surfactant Triton X-114 (octylphenoxypolyethoxyethanol). The LOD were 0.3 ng ml−1 for Mn and 0.2 and 0.4 ng ml−1 for Co and Ni, respectively. The same surfactant was used by Giokas et al.86 to preconcentrate Co, Fe and Ni as the APDC complexes. The surfactant-rich phase was diluted with acidified methanol before introduction to the spectrometer via a FI system. The LOD were 5, 20 and 10 ng ml−1, respectively.
1.2.2.4 Flotation. Pavlovska and co-workers have devised procedures for the determination of Co and Ni,87 Ni,88 Tl,89 Cd,90 and Co, Cu, Ni and Pb.91 The analytes were co-precipitated with a metal hexamethylenedithiocarbamate which was separated from the bulk solution by flotation in the foam produced on aeration of the solution following addition of a surfactant. Finally, the analytes were determined by ETAAS. The solution LOD were between 3 ng l−1 (Cd) and 30 ng l−1 (Co). For one of the analyses,91 the method was compared with a liquid–liquid extraction procedure in which the DDTC complexes were extracted into IBMK. The flotation method had LOD that were a factor of 2–3 lower. Kim et al. developed procedures for the determination of Au, Pd and Pt92 and Co, Cu and Ni93 in which a carrier precipitate was not used. In the first method, the analytes were collected as their complexes with 2-mercaptobenzothiazole, and in the second the analytes were collected as the oxinates. In both methods, cetyltrimethylammonium bromide was the surfactant and the floated complexes were dissolved in IBMK with subsequent determination by ETAAS directly in the organic solvent. The whole procedure took about 1 h per determination.
1.2.2.5 Miscellaneous. Okamoto et al.94 adapted an emulsion liquid membrane procedure they developed 10 years ago to the enrichment of Cd, Co, Cr, Cu, Mn and Ni. A small volume of a water-in-oil emulsion was mixed with a large amount of the analyte solution. The analytes are transported through the oil (with the aid of 2-ethylhexyl hydrogen 2-ethylhexylphosphonate) and extracted into the inner aqueous phase (3 mol l−1 HCl). After phase separation, the emulsion was demulsified with ethylhexanol and the analytes in this demulsified phase determined by ETAAS. Concentration factors of between 600 and 1000 were obtained to give LOD ranging from 0.4 (Cr) to 20 (Mn) ng l−1. A similar procedure was developed by Li et al.95 for the selective determination of AsIII by ETAAS. In this case the outer phase was HCl solution, and the inner phase was NaOH solution. Total As was determined after reduction of AsV with potassium iodide. The liquid membrane consisted of kerosene with liquid paraffin as stabilizer and a surfactant (L113A) as emulsifier. The LOD was a few ng ml−1. Kashiwagi and colleagues96 devised speciation procedures for inorganic Se96 and both inorganic and organic As97 based on co-precipitation with lanthanum hydroxide. In the latter method, the organic compounds remaining after precipitation of the inorganic As species were distilled as the chloroarsine compounds by reductive distillation in the presence of tin chloride and sulfuric and hydrochloric acids. The LOD, for determination by ETAAS, were 0.3 ng ml−1 and 2 ng ml−1 for the inorganic and organic species, respectively. The procedure was applied to the analysis of high salt waste water. Kagaya et al.98 showed that lanthanum phosphate could also collect trace elements by coprecipitation. For Fe, the collection efficiency was strongly dependent on oxidation state: FeII was hardly precipitated, whereas FeIII was quantitatively collected. Cadmium, In and Pb were also completely collected for determination by ICP-OES. Sato and Ueda99 co-precipitated several metals with bismuth DDTC for determination by ETAAS. Matusiewicz et al.100 collected Pt by electrodeposition on a flow-through graphite tube electrode, which was then transferred to an electrothermal atomizer for subsequent determination of the collected analyte. For a 30-min collection time, concentrations between 0.1 and 10 ng ml−1 could be determined.
1.3 Nebulization
Lubricating oils were analysed by Mora et al.101 by FAAS with introduction by a single-bore, high-pressure pneumatic nebulizer. A carrier stream of IBMK was pumped to the spectrometer, into which the undiluted oils sample were injected. With a conventional nebulizer, viscosity effects meant that concentrations of oil higher than 70% (w/w) could not be introduced. Neira and Povenda described (in Portuguese) a procedure102 in which hydraulic high-pressure nebulization was used following low-pressure solid-phase extraction of the ion-pairs with tetrabutylammonium ions of the Ca, Mg and Sr complexes with xylenol orange. Todoli and Mermet evaluated103 a direct injection high-efficiency (DIHEN) nebulizer for ICP-OES. In comparison with a high-efficiency nebulizer (HEN) and cyclonic spray chamber, the DIHEN gave sensitivities and LOD of 10–18 times better even though the aerosol generated by the DIHEN was coarser. Despite the higher solvent load produced by the DIHEN, matrix effects were minimal. Minnich et al.104 also evaluated this type of nebulizer in terms of the characteristics of the aerosol produced. They determined the two-dimensional spray structure, the planar mass distribution, the spatial drop size distribution and were able follow processes on the ns timescale with a technique called “optical patternation”. They made three major observations: (a) the primary aerosol was confined to a greater degree when the flow rate was 1.0 l min−1 than when the flow rate was the more commonly adopted 0.25 l min−1, (b) the aerosol rotated around the axis of the nebulizer producing satellite droplets, and (c) the satellite droplets were more prevalent at distances further away from the nebulizer tip. The authors speculate on the implications of these findings on performance characteristics such as noise and sample–plasma interactions. Ding et al.,105 compared the performance of a low power microwave thermospray nebulizer with that of a conventional pneumatic nebulizer and found that, although the LOD were lower with the microwave powered device, the precision was poorer. Seelig and Broekaert106 compared the performance of three nebulizers (Legere, hydraulic high-pressure and ultrasonic) for sample introduction into a capacitively coupled microwave plasma with particular reference to the effect of water loading on the LOD. This particular plasma can be run on air and is stable with respect to large amounts of water and carbon dioxide, which makes it suitable for the determination of metals in the exhaust gases of combustion processes. They found that desolvation was not necessary for either the Legere nebulizer or for the hydraulic high-pressure nebulizer although the LOD did increase as the water loading increased, especially for elements with high excitation energies or elements for which ionic emission was measured. However, for a low-pressure ICP, Sung and Lim found107 that desolvation was necessary. They used a double membrane device. Budic108 studied the effects of HCl, KCl, KBr, KI and Ca in HNO3 on the emission signals and transport rates for ICP-OES with sample introduction by ultrasonic nebulization with desolvation. Considerable signal depressions found were for AsIII and SeIV in a HNO3 solution, but the presence of the salts enhanced the signal considerably. These effects were explained in terms of the adsorption of the analytes on the walls of the desolvating tubes. Todoli and co-workers109,110 evaluated four different spray chambers in conjunction with a pneumatic micro-nebulizer (Atom Mist) for uptake rates in the range 0.010–0.160 ml min−1. They found that a conventional cyclonic chamber gave coarser tertiary aerosols, higher analyte and solvent transport rates, higher sensitivity and lower LOD than the others. Two low-volume, cyclonic chambers and a double-pass chamber had similar performances, though the smaller chambers gave shorter wash-out times. Matrix effects, from inorganic acids and salts, were only significant with the double-pass chamber.1.4 Chemical vapour generation
The comments that were made last year still apply: the majority of publications deal with aspects of the determination of As, Hg and Se, there is interest in the determination of multiple hydride-forming elements in the same sample, there is some interest in electrochemical hydride generation and there are reports of the generation of volatile compounds form elements other than the traditional p-block elements. Most determinations are finished with AAS, but there is a significant number of AFS determinations made of hydride-forming elements. A number of speciation schemes have been described based on control of the reaction conditions to impart selectivity to the HG process.1.4.3.1 Arsenic. Arsenic has been determined in a number of multi-element procedures discussed earlier in section 1.4.1.111,112,115–118,131 Most of these procedures were based on the generation of arsine by reaction with borohydride; one paper131 describes an electrolytic generation procedure. The lowest LOD, 3 pg ml−1, was obtained by Pacquette et al.118 for LIF detection following in-atomizer trapping. Krachler et al.132 determined As and Sb in peat by AAS. They found that HF, which was needed to liberate Sb from silicate compounds, caused As to be lost as AsF5. They also found that closed vessel digestions were less effective than open vessel procedures and concluded that two procedures were needed: one without HF for As and one with HF for Sb. Results in agreement with those obtained by NAA were obtained. Digestion temperature and time for a nitric, perchloric and sulfuric acid mixture were investigated by Sakamoto et al.133 for the determination of total As in waters, sediments and biological materials by FI-AAS. They found that the mixture of acids was suitable when methylated compounds were known to be present. For the speciation of As in sea-water by FI-HG-ETAAS, Cabon and Cabon134 were able, on the basis of different reaction conditions, to determine AsIII, total As, hydride-reactive As and, by difference, non-hydride reactive As. They used ET-AAS with trapping on the iridium-coated atomizer surface. They extended the procedure to determine inorganic As, monomethyl As and dimethyl As (DMA), by cryogenically trapping the borohydride active species on a chromatography stationary phase followed by sequential release and atomization in a heated quartz cell. Carrero et al.135 studied the generation of arsines from inorganic and methylated As species as a function of both the HCl concentration and the presence of sulfur containing reducing agents L-cysteine and mercaptoacetic acid. They suggested two mechanisms for the generation of arsines: (a) at low acid concentrations the thiolate As complexes in the 3+ state react rapidly with borohydride, and (b) at high acid concentrations, arsines are generated by the reaction with nascent hydrogen produced by the decomposition of borohydride. A somewhat similar approach was adopted by Quinaia and Rollember136 to speciate As by HG-AAS. In citrate buffer only AsIII was determined; in acetic acid both AsIII and DMA were measured, and in HCl both AsIII and AsV were determined. The LOD were only around 1 mg L−1.
1.4.3.2 Bismuth. Bismuth was determined as part of a multi-analyte procedure developed by Overduin and Brindle112 for the analysis of high purity copper. Moyano et al.137 developed a procedure for the analysis of human urine, in which the Bi was concentrated on-line by SPE of the 8-hydroxyquinoline complex on a column of Amberlite XAD-7 followed by elution with HNO3. For a 100-ml sample, the ICP-OES LOD was 0.02 ng ml−1.
1.4.3.3 Cadmium. As part of a multi-element analysis Cd was determined by a FI-HG-ETAAS procedure devised by Moreda-Piniero et al.117 In a systematic investigation of the key operating parameters, they found that the most important were the HCl concentration, the flow rates of the acid carrier and the borohydride and the reaction coil length. Chinese workers, Yao et al.,138 preconcentrated Cd on various solid phase extractants prior to determination by cold vapour AAS for an LOD of 2 ng l−1.
1.4.3.4 Germanium. Overduin and Brindle112 included Ge in the suite of elements determined in high purity copper by HG ICP-OES. Good recoveries of Ge, spiked prior to dissolution, were obtained. Abbasi et al.139 evaluated different dissolution procedures for the determination of Ge in geological matrices with high chloride contents by HG-AAS, and Roels and Buchet140 determined Ge in urine by FI-HG-ETAAS after digestion and pH adjustment. The LOD was 0.3 ng ml−1, which was adequate to determine Ge in the urine of exposed workers but not sufficient to detect Ge in the urine of control subjects. Magallanes et al.440 applied an artificial neural network procedure to the optimization of the determination of Ge by HG-ICP-OES. They also compared the results with those obtained by simplex and single-cycle alternating variable search methods.
1.4.3.5 Lead. Chinese workers, Yu and Yu,141 determined Pb (and Hg) by HG-AFS after preconcentration on sulfhydryl cotton.
1.4.3.6 Antimony. A number of multi-element HG procedures include Sb in the list of analytes.111,112,115–118,131,132 De Gregori et al.142 determined Sb in alfalfa and soils by both ETAAS and HG-AFS. They investigated several different dissolution methods and reagents for the prereduction of SbV to SbIII, as well as various modifiers for the ETAAS determination. They found the HG-ETAAS procedure to suffer from interferences and to have a poorer LOD (0.5 ng ml−1 compared with 0.08 ng ml−1 for the HG-AFS method). However, if the standard additions method was used for the ETAAS method, both methods gave accurate analyses of several soil SRM and a tobacco leaf RM. A similar LOD of 0.06 ng ml−1 was obtained by Niedzielski and Siepak143 for a sample volume of 0.85 ml injected into a simple FI manifold for HG-AAS. Interestingly enough, they found that better precision was obtained when the injection valve was located upstream of the pump, compared with that for the more conventional location of downstream of the pump. Peak area measurement gave better LOD than did peak height measurement.
1.4.3.7 Selenium. Several multi-element procedures include Se in the list of analytes111,113,115–118,131 Some of these studies are concerned with the possible benefits of sulfur-containing reagents for the control of interferences.111,113 There is still no report of a rapid procedure for the reduction of SeVI to SeIV that is suitable for use in the presence of As, and thus many of these multi-element procedures only determine SeIV or involve two or more stages in which sub-sets of the analytes are determined after different pretreatments. Stripeikis et al.144 removed the potential interferences from copper and iron by retention of the matrix elements as their chloro complexes on a microcolumn of Dowex 1X-8 anion-exchange resin. The SeVI was reduced on-line by passage through a 1.5 m coil at 100 °C and determined, after HG, by ICP-OES down to an LOD of 0.4 ng ml−1. Accurate analyses of a number of geological and copper RM were obtained. Irizarry et al.145 studied the interferences of various transition metals on the determination of Se by HG-AFS. They also studied the effects of some anions and the acids used in the dissolution procedures for various biological and environmental samples. They suggested that accurate analyses could only be obtained with matrix matching or standard additions methods. Moreno et al.146 developed a procedure for the determination of the methylated Se compounds dimethyl selenide and dimethyl diselenide, in which the analytes were separated from the various matrices without derivatization by pervaporation and then detected by AFS. In sewage sludge, garlic and oyster the concentrations ranged from 0.07 to 1.4 mg kg−1.
1.4.3.8 Tin. There would appear to be no publications during the present review period in which Sn has featured as the only analyte in a HG procedure. There are three reports of the HG determination of Sn as part of a multi-element determination.112,115,131 The first of these deals with the determination of several elements in high purity copper, though losses of Sn during the dissolution were noted for samples containing high Sn concentrations. The second of these is concerned with the analysis of beer and wort by ETAAS, and the third with the electrolytic generation of the hydrides at a variety of electrode surfaces. Glassy carbon was found to be the best for the generation of stannane, though the solution precursor needed to be SnIV.
1.4.3.9 Tellurium. Marcucci et al.113 studied the effect of thiourea and some thiols on the interference effects from various metals in the AFS determination of Te (and Se). They found conditions under which the elements could be determined in various metals and ores with no pretreatment other than dissolution and the on-line addition of thiourea and iodide in a continuous flow method. Korez et al.147 devised a procedure for the determination of TeIV and TeVI based on preconcentration of the analytes on a mercapto-modified silica microcolumn. The AAS LOD in sea-water, after a 50-fold enrichment, was 0.04 ng ml−1.
1.4.3.10 Mercury. Although there is still considerable interest in the determination of Hg, the relative numbers of publications concerned with non-chromatographic speciation appears to have declined during the current review period. Although, as was pointed out in last year's review, there is some confusion over the nature of the reaction between borohydride and organomercury compounds, many researchers do not highlight the chemistry of generation in the abstracts of their papers and often it is not clear (a) what reagent has been used to generate the volatile derivative(s), and (b) what the reaction conditions were in terms of acidity or borohydride concentration, two parameters that appear to govern the nature of the reaction products. As part of a multi-element determination, Matusiewicz and Mikolajczak115 determined Hg in beer and wort by cold vapour generation (CVG) with trapping on the gold-coated interior of a graphite furnace atomizer. The trapping efficiency was 66%, giving (for a 10-ml sample) an LOD of 90 ng l−1. Chinese workers141 determined Hg (and Pb) by AFS after preconcentration on sulfhydryl cotton. Boylan et al.148 developed a new instrument for on-site analyses based on thermal desorption/decomposition, amalgam trapping and AAS. The instrument could handle solid samples directly without any chemical pretreatment and produced accurate values for the analysis of a variety of CRM. The procedure formed the basis of the new US EPA Method 7473. Chen and Chou149 described a simple modification to a commercially available HG system (a T-junction allowed the injection of sample and reductant solutions and the system was configured to allow longer reaction times) to improve the reliability of Hg determinations in biological and sediment materials. The LOD were 0.06 and 0.01 mg kg−1 (dry weight), respectively. Capelo et al.150 employed a factorial design in the optimization of a FI-AAS procedure in which borohydride was used to generate Hg vapour. Not surprisingly, they found that there was interaction between the borohydride concentration and flow rate, but contrary to what might be expected intuitively, they opted for a low concentration and high flow rate. The LOD was 0.4 ng ml−1 for a 0.5 ml sample, and the procedure was applied to the analysis of seafood, which was digested in a HNO3 + H2O2 mixture with the aid of microwave radiation. A similar LOD, 0.5 ng ml−1, was obtained by Doering et al.151 for a sequential injection AAS procedure. In this methodology, a single-line manifold is used in which the sample and reagent zones are stacked next to each other with the aid of a programmable selection valve and bi-directional syringe pump. Hepp et al.152 developed a flow microwave digestion procedure for the pretreatment of certifiable color additives prior to the determination of total Hg by AAS. The LOD was 0.2 mg kg−1, a factor of 5 below the specification concentration of 1 mg kg−1, at which the method uncertainty was ±0.2 mg kg−1. Hafez et al.153 preconcentrated the Hg released into solution after sample digestion on a chemically modified chloromethylated polystyrene–PAN ion-exchanger in a batch procedure. The factors affecting the degree of preconcentration were studied in some detail, as was the nature of the Hg complex formed with the solid reagent. The procedure was applied to the determination of ng ml−1 concentrations of Hg in water, milk and urine. Several papers contain reports of procedures in which the evolved Hg vapour was preconcentrated by amalgam trapping.154–156 Kopsyc et al.154 evaluated the procedure for the determination of both inorganic and methylmercury by reaction with borohydride in the presence of HCl, and used the procedure developed to determine Hg by AAS in lake water. Murillo et al.155 determined Hg down to an LOD of 10 pg ml−1 by MIP-OES following trapping on a gold–platinum collector. The method was validated by the analysis of an atmospheric particulate CRM (NIST 2704). Wurl et al.156 obtained a LOD of 0.1 pg ml−1 by AAS following trapping on gold. They applied the method to the analysis of sea-water in which the mercury species were decomposed by on-line UV digestion. The procedure was applied on board ship to the determination of Hg, over the concentration range 0.2–2 pg ml−1, in the surface of the Baltic Sea. For the speciation of mercury, Monteiro et al.157 retained the dithiophosphoric acid diacyl ester chelates of both inorganic Hg and methylHg on a C18 column. A factorial design procedure was employed for the optimization, which produced an AAS LOD of 10 pg ml−1 for a loading time of 4.5 min at 5.3 ml min−1 (24 ml sample) at which the retention efficiency was about 90% for both species. Baklanov and Chmilenko158 employed ultrasound in the pretreatment of brines and salt solutions for the determination of “total mercury, inorganic, organic and methyl mercury species”. Chinese workers, Liang et al.,159 determined inorganic Hg and total organic Hg in waste waters by an AFS procedure in which the inorganic mercury was first selectively detected, followed by the oxidation of organomercury compounds with persulfate and the determination of total Hg. The total organic Hg was determined by difference. The LOD was 8 pg ml−1. Hladikova et al.160 distinguished between vapor and particulate Hg in a series of analyses of the air at 20 different locations in the Slovak Republic during the period 1996–1997. Vapour was determined by AFS following amalgam trapping, and particulates were analyzed following wet digestion of the material collected on filters. They found (a) that the predominant form of mercury was the vapour, (b) the fraction of particulate Hg was lower in rural areas than in urban areas, though there was no correlation between mercury content and total particulates, and (c) there was no correlation between mercury vapour concentration and temperature.
1.5 Solid sampling
The number of fundamental publications in the area of direct solid sampling in atomic absorption, emission and fluorescence has been particularly low in the period of this review. A significant number of application-based publications, especially on glow-discharge (GD) and laser ablation have appeared and those of particular note are discussed in the appropriate application-based ASU.A systematic investigation of the use of rf-GD-OES for the elemental analysis of bulk solid glass specimens was carried out by Anfone and Marcus.161 Optimization of the major operating parameters was carried out resulting in LODs comparable to those obtained with other optical emission methods. The same group also reported on the use of an atmospheric pressure GD for direct solid sampling of liquids,162 where the liquid sample (an electrolyte containing the analyte) formed one of the electrodes by introduction through a capillary, sourrounded by a coaxial gas flow to improve stability at low liquid flow rates (0.1–0.3 ml min−1). Another systematic study, this time on the application of rf-GD-OES for the direct analysis of bone was described by Martinez et al.163 Comparison of sample preparation strategies was carried out. The preferred method, both analytically and medically, involved the encapsulation of a thin bone sample in poly(methyl methacrylate) and then pasting the sample over a flat support made of Cu. Other publications of note include: a study of the optical interference effect during the depth profile analysis of borophosphosilicate glass films deposited on silicon wafers, using GD-AES;164 and a novel hollow cathode Grimm-type GD has been developed for the analysis of metals and alloys.165
Niemax perhaps summed up laser ablation in the title of his article “reflections on a very complex technique for solid sampling”.166 The complex correlations between experimental conditions in solid sampling using laser ablation were described. Particular emphasis was given to the influence of the laser properties, the surrounding gas, and the matrix on the analytical results that are obtained using laser ablation techniques. The interesting combination of laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence spectroscopy (LIFS) was used by Telle et al. for the remote analysis of metallic samples.167 Al, Cr, Fe and Si were successfully determined at trace level using a pulsed Ti sapphire laser. The possibility of multi-element analysis was also demonstrated with Cr and Fe determined by rapidly changing the tuneable laser wavelength.
The combination of laser ablation sampling with an r.f. helium GD has been reported.168 The approach was used to enhance the sputtering ability of the He GD, and to independently control the sputtering and excitation processes. Excitation was effected between 0.1–7 ms after ablation with a Q-switched Nd∶YAG laser.
Introduction of slurried samples into ETAAS continues to be widely used for a range of sample types. Botelho et al. compared the results of slurry analysis of carbonaceous samples including graphite and coal with results obtained after microwave digestion of the samples.169 No significant difference was observed in the results obtained using either procedure. Slurry sampling was also applied to the analysis of another difficult matrix, namely powdered aluminium oxide powder.170 The sample was mixed with sodium carbonate and boric acid as a mixed flux and in-situ fusion carried out in the graphite furnace. The Si concentration was determined in an SRM with good accuracy by this method.
The combination of direct solid analysis and 3D calibration was used for the analysis of geological samples by Nimmerfall and Schron.171 The performance of the 3D calibration method was optimized by systematic investigation and 2D and 3D calibration techniques were compared. The results of the calibration comparisons and the precision and accuracy of the determination of trace elements in CRMs illustrated that 3D calibration becomes increasingly useful as the refractory character of the analyte increases.
1.6 Electrothermal vaporisation
A comprehensive review of electrothermal vaporization and laser ablation sample introduction into flame and plasma sources was presented by Kantor (180 references).172 Descriptions of the high temperature vaporization and ablation sources are included alongside critical evaluation of the strengths and weaknesses of the various approaches to the analysis of liquid and solid samples.In the second part of a two-part series reviewing vaporization and atomization processes in ETVs, Majidi et al. focussed on surface chemistry173 (89 references). It was noted that although the number of publications dealing with surface chemistry constitutes a tiny part of the ETV-based literature, studies in the area have been carried out for almost three decades. This article includes a review of the field demonstrating the breadth of techniques that have been used in the study of atomizer surfaces. Changing the conventional graphite atomizer surface by using metal liners or by chemical and physical modification of the graphite itself was discussed. From the literature, the general consensus among researchers is that improved analytical signals can be obtained as a result of fundamental improvements in the condensed phase chemistry. The results of experiments utilizing laser desorption mass spectrometry (LDMS) and differential scanning calorimetry (DSC) for the investigation of the vaporization of Mg, Ca, Sr, Ba, and Ni from three different vaporizer substrates, namely graphite, Al, and Ta, were discussed. DSC was used to determine the decomposition and reaction products between the substrate and the analyte salt, while the mass spectra were used to determine the species that exist on the surface. The low temperature (100– 600 °C) surface reactions of the elements studied varied from element to element, as was also observed for gas phase reactions in Part 1 of this work (refs 243 and 305). The behaviour of all the metals studied on a graphite surface was significantly different than from that observed on the other substrates. In some cases, metals on graphite displayed a large energetic process at temperatures above 500 °C. For some metals the process was exothermic, for others endothermic. As a result, these energetic processes on graphite could not be explained by simple graphite oxidation or by lattice rearrangement. In addition, it was noted that the energetic process on graphite did not necessarily coincide with thermal decomposition or with general phase changes within the analyte crystal lattice. The authors postulated, therefore, that the formation of intercalation compounds is the most plausible explanation for the transition near 500 °C. Degradation of the graphite surface is observed as a result of the decomposition of these intercalation compounds and of oxidative damage, ultimately resulting in destruction of the analytical integrity of the graphite tube. This observation also explains the significant increase in lifetimes of graphite tubes coated with Ir or other metal films. The protective layer provided by these metals prevents the formation of intercalates. From the results of this work, and the work of others, it is clear that graphite has an extensive chemistry that is enhanced at high temperature and in an oxidative environment. Although the graphite surface appears to be continually degrading, the rate of material loss due to this degradation is low compared to the total carbon mass in the furnace. This means that the bulk properties do not change throughout the lifetime of the furnace and heating rates are not changed as a result of surface degradation. The authors conclude that the effects of surface damage should not be overlooked and steps should be taken to minimise the rate of degradation. Both papers in this series should be essential reference material for anyone using ETV for sample introduction in atomic absorption, atomic emission or mass spectrometry.
The use of W-coil devices in ETAAS, ETV-ICP-AES and ETA-LEAFS was described by Hou et al.174 For most cases in ETAAS and ETA-LEAFS, limits of detection (LODs) within a factor of ten of those observed with commercial graphite furnace systems were obtained using the W-coil. The compact W-coil device lends itself well to use in portable atomic spectrometry instrumentation. The use of the W-coil as an inexpensive ETV device for a commercial ICP-AES instrument was also demonstrated via the thermal separation of Pb from elemental interferents Al, Mn, and Fe.
The development and characterization of a prototype induction heating (IH)-ETV sample introduction system for ICP-AES was described by Rybak and Salin.177 In IH-ETV, long, undercut graphite probes, similar to those used in direct sample insertion (DSI) for ICP-AES, are used to support samples in a cylindrical quartz chamber axially encircled by an induction coil. Application of an RF current to the coil results in inductive heating of the probe without the need of electrical contact. Heating of the probe consequently dries, pyrolyzes and vaporizes the sample, and the sample plume is swept out of the chamber by a gaseous flow into a plasma excitation source located downstream. Temperature control similar to that possible with ETV can be accomplished by adjusting the RF current or by adjusting the position of the probe in the RF field. In this work, characteristics such as high vaporization surface temperatures, rapid heating response, linear temperature control, reproducible analyte volatilization, high analyte transport efficiency and LODs in the pg–ng range were demonstrated. These attributes certainly suggest that an IH-based ETV arrangement has potential as a thermal sample-introduction technique. The drawback of this particular prototype, however, is the maximum vaporization temperature of 1860 °C, correctly identified as being too low to permit wide acceptance of the technique. This limitation is not a characteristic of IH, but of the arrangement used in this study. The same authors successfully applied IH-ETV-ICP-AES to the direct determination of a range of metals in soils and sediment samples.178 The samples were deposited either directly as a solid or by means of slurry sampling into graphite cups that were then positioned in a radiofrequency (RF) field and vaporized in a carrier flow of 15% (v/v) SF6-Ar. In general, sample delivery was found to be simpler and the observed signal precision better with slurry sampling when compared to the analysis of the solid directly. A number of CRMs were analysed and the experimental results found to be in good agreement with the certified values.
2 Instrumentation
2.1 Spectrometers
A “micro flame emission spectrometer”, which can be fabricated by standard micro-machining procedures, was described by Zimmermann and Muller.181 The microburner uses an oxy/hydrogen mixture generated in a battery-operated electrolysis cell. Liquid samples are injected through a piezo-driven ultrasonic nebulizer. The small weight of all these components, including the miniature spectrometer, makes the complete system very easily portable and, with no separate gas supplies, safe in operation. No performance details were quoted in the abstract other than that the detection limits for the alkali metals are about half a magnitude greater than those associated with more conventional systems. Similarly, a parallel plate CCP operated at atmospheric pressure (5–25 W power and 17–150 ml min−1 gas flow) has been formed in a quartz wafer of dimension 0.25 × 0.25 × 5 mm.182 A dielectric barrier discharge plasma operating at <1 W power has also been developed for use in plasma modulation diode laser absorption spectroscopy for detection of Cl and F in CFC compounds, with LOD of 400 ppt and 2 ppb for Cl (837 nm) and F (685 nm), respectively.183An échelle spectrometer with a wavelength range from 130–800 nm, for use with an ICP source, was developed by Luan et al.184 To access the vacuum UV region, an MgF2 prism was used as the cross disperser and a thin coating of a wavelength conversion phosphor was applied to the surface of the charge injection detector to increase its sensitivity. For Cl at 134.724 nm, a bandwidth of 0.006 nm is achieved with a 3σ detection limit of 100 ng ml−1. In a similar development, an argon-filled CCD spectrometer in the range 125–195 nm has been developed for ICP-OES,185 with LOD in the ppb range for Cl (134.724 nm) and Br (154.065 nm).
For work on ICP-AES, Gillespie and Cranahan186 replaced their spectrometer with a quartz acousto-optic tunable filter. The wavelength range was 206–405 nm and the wavelength calibration is extremely accurate. The measured bandpasses were 0.06 nm at 226 nm and 0.36 nm at 405 nm, comparable with those of a medium resolution monochromator. Performance for a number of trace elements was assessed and for most, the detection limits were between 1–100 ng ml−1 with linear response at concentration levels near the detection limit.
2.2 Sources for AES
Japanese workers used a gas-modulating plasma together with a digital lock-in amplifier to improve detection limits in ICP-AES.187 The lock-in amplifier acts as an extremely narrow bandpass filter and is able to separate the modulated signal from noise, thus improving signal∶noise ratios. Modulation of the ICP was achieved by periodically mixing small amounts of air with the argon of the outer gas using an electromagnetic valve. The emission intensity of ionic lines was then cyclically enhanced by use of the mixed gases. The system was used particularly to measure the Mn II line at 257.61 nm and improvements in detection limits over conventional systems are claimed. A modulation approach was also adopted to eliminate spectral interferences in a microwave boosted GD.188 Modulation of the source was found to have little effect on the molecular bands or lines of the plasma gas, but did affect analyte emission, hence modulation could be used to eliminate interference of OH on Mg, Bi and Sn and Ar II on Al.The pulsing of a helium MIP has caused problems of stability in the past and these have been overcome by Mohamed and Ghatass.189 They constructed a torch in which an auxiliary plasma is maintained in continuous operation while the main plasma within the plasma chamber receives the sample and solvent injection and is subjected to pulsing. Every parameter affecting the performance of the device was carefully investigated, including plasma excitation temperature. Acceptable performance was quoted for eight common elements in the analysis of real biological samples.
A high power (1000 W, continuous wave, 2.45 GHz) annular shaped He MIP was developed by Yamada and Okamoto190 for measuring non-metals in aqueous solutions. The plasma was generated at atmospheric pressure in a modified Okamoto cavity and no external cooling was used to stabilize the plasma. Using an ultrasonic injection system, with desolvation and condensation, the electron density was 1014 cm−3 and the excitation temperature 5000 K. Detection limits for Cl II (479.5 nm) and Br II (470.5 nm) were 100 ppb and 200 ppb respectively.
In order to avoid erosion of the discharge tube by the plasma in MIP spectrometry, Matusiewicz191 designed a simple torch that is internally cooled by a water aerosol. This aerosol was generated and circulated by means of a conventional ultrasonic room humidifier. Erosion, as measured by ablated Si and F, was essentially eliminated and analytical performance for several common elements was shown to be acceptable and comparable with that of Ar-MIP systems.
2.3 Detectors and data acquisition for AES
An authoritative article by Mermet192 discussed parameters which influence the quality of spectra acquisition in ICP-AES. Resolution of the dispersive system is crucial, not only to minimize spectral interferences but also to improve signal∶noise ratios. As a result of the quality of gratings now available, practical resolution is limited mainly by bandpass and optical aberration. Very high resolution is not possible in the visible region because high line numbers cannot be used with conventional gratings, nor high diffraction orders for échelles. Since, however, it is now possible to arrange for the acquisition of entire spectra, or at least of large blocks of spectra, wavelength reproducibility itself may become the most important parameter affecting data processing tasks such as spectrum addition, subtraction, filtering, deconvolution and correlation.Pennebaker and Denton193 evaluated, for precision and noise, an ICP–échelle optical system coupled with a charge injection detector. The simultaneous collection of all analytical wavelengths, with simultaneous background correction, has invalidated assumptions that had to be made in the past for some traditional equipment. The system under discussion eliminated the effects of flicker noise. Most fluctuations were found to be caused by small wavelength shifts along the detector axis rather than by the theoretically predicted shot noise, and it was shown how such fluctuations can be corrected. In this work, theoretical expressions were developed, allowing for major sources of noise, based on the specific readout characteristics of the array detector. These expressions match experimental data very closely for measurement of relative standard deviations in background and of detection limits. The advantages of multichannel detection with charge transfer device detectors were thus demonstrated.
Chinese workers from Changchun194 studied the characteristics of two-dimensional spectra obtained with ICP-AES coupled with charge injection detection. Frequency distribution was established following Fourier transformation. Low frequencies were characteristic of the spectra being measured, and high frequencies were characteristic of background and noise. It is not clear from the abstract (the original paper being in Chinese) whether the attempted smoothing of the high frequency signal successfully reduced the effects of noise.
3 Fundamentals
Few fundamental studies were published during the review period. Of general interest was a paper describing the influence of isotopic composition on the accuracy of lithium determinations.195 For an enriched (40 atom% 6Li) sample, relative errors as much as 50% for AAS and 6% for AES were observed, depending on the differences in isotopicabundances between standards and samples, and in the hollow cathode lamp in the case of AAS. This would be a problem for any element for which the wavelengths of the isotopes were sufficiently far apart, and would also presumably depend on the resolving power of the spectrometer used. Remaining on the subject of isotopes, the isotope shift ΔT (156Gd–160Gd) of the even parity energy levels of 4f(8)6s(2) and 4f(7)6s(2)6p configurations of neutral Gd atoms have been evaluated.196 A series expansion for the Voigt function has been derived which is valid for all values of the ratio between Lorentzian and Gaussian widths, with coefficients which are independent of the wavelength (measured in units of Gaussian width).1973.1 Plasmas
Power interruption experiments have been used to study the excitation balances and transport properties in an Ar MIP.199 The time dependent behaviour of emission lines was studied during power interruption and revealed that electron loss processes due to diffusion, convection and dissociative recombination of molecular ions were much larger compared to the ICP. It was also found that electron-dominated excitation mechanisms were predominant in the ionizing part of the plasma, but electron-independent (thermal) excitation of easily excited elements was predominant downstream in the recombination region of the plasma. Spatial measurements of ne in an atmospheric pressure Ar MIP operated at 100 W, using the Hβ 486.13 nm line, have been made and the influence of K studied.200 The presence of K was observed to considerably lower ne. The influence of the positioning of the cylindrical metal attenae on the performance of the microwave plasma torch (MPT) has been studied,201 resulting in improvements in analytical performance when it was positioned at ¼ λ from the intermediate tube compared with ¾ λ.
Some fundamental studies of electron cyclotron resonance (ECR) MIPs have been made.202,203 Langmuir probe and optical measurement revealed202 that the electron energy dropped sharply as the power was increased from 200 to 400 W, but remained mostly unchanged at the centre of the discharge between 400–1200 W. Ion and electron density increased over the same power range.
Pulsed glow discharges have received some attention.207–211 A double pulse method has been used to enhance atomic emission in a Ne GD, whereby the second pule is applied at a variable delay after the first pulse.208 A microsecond pulsed Ar GD has been modelled to predict the densities of Ar metastables, and sputtered Cu atoms and ions, both spatially and temporally.209 The models were used in conjunction with hybrid Monte Carlo-fluid model for electrons and argon ions/atoms.212 This model has been used to predict: several electrical characteristics of a GD; electron and argon ion densities; and electron, fast argon ion and atom impact ionization rates. Spatial and temporal maps of excited Cu atoms and ions have also been obtained for a millisecond pulsed Ar GD.210 Low energy transitions of Cu atoms (e.g., 324.75 nm emission) were found to predominate at 2.5 mm from the cathode surface during the plateau time regime, whereas high energy transitions (e.g. 368.74 nm emission ) predominated at 5–6 mm from the surface during the afterpeak time regime. Ionic emission was only observed during the plateau time regime. A pulsed d.c. GD (−550 V) combined with an ICP (400–1000 W), operated at 400–800 Pa pressure, has been characterised.211 An increase in the intensity of N2 and N2+ emission was found to correlate with an increase in the nitriding case depth for case hardening of steel, which was also improved when using the ICP in tandem with the GD.
Theoretical Stark-broadened H-β profiles have been modelled using the model microfield method, and used to calculate ne.213 This was accomplished by using least squares to fit the emission profile to the theoretical Stark-broadened profiles. Experimental profiles for a He ICP, a GD, and a high voltage spark (spanning T = 2500–80000 K and ne = 1010–1017 cm−3) were used to test the method and compared to the traditional ‘unified theory’. The density of ground state N2 molecules in a 2.3 Torr, 100 mA, d.c. GD has been measured using cavity ringdown spectroscopy,214 and determined to be ∼1020 m3 (0.2% of the total N2 density). The rotational gas temperature was 500 K.
Modelling of an Ar d.c. GD has been compared with experimental results for the optical emission of Ar and Cu lines.215 Model parameters included the gas temperature, erosion rates and emission intensities (calculated as a function of voltage and pressure) and reasonable agreement was obtained provided that the gas temperature was assumed to be a function of the voltage.
A high pressure (up to 600 Torr), microhollow cathode discharge in He has been studied.216 Atomic emission of He2* excimer emission at 58–65 nm and 65–95 nm was observed, and emission of O, N, H between 95–125 nm was attributed to near resonant energy transfer from the He2* excimers. Formation of the excimers requires electrons with energies well above 20 eV, so this indicates that these discharges are extremely efficient sources of energetic electrons.
A spatial comparison has been made of ICPs operated at 27.12 and 40.68 MHz using a switchable-frequency generator.219 The Mg II/Mg I ratio, Fe I Texc and discharge volume (from N2 ion emission) were studied. After optimisation the authors concluded that slightly better figures of merit were obtained with the 40.68 MHz ICP. Spatial imaging has also been used to elucidate fluctuations in the ICP.220 Using a two-wavelength spectrovideo camera the authors simultaneously monitored an atomic emission line and background emission, then Fourier transformed the background-subtracted data to obtain the frequency-domain spectra. These were used to construct two-dimensional profiles of fluctuation in the ICP which indicated that most fluctuation occurred around the central channel and periphery, attributed to sample vaporization in the central channel and convection of surrounding air, respectively.
Matrix effects in axially and radially viewed ICP-AES have been studied.221–224 Ratios of emission lines, principally the widely studied MgII/MgI ratio, were used as a diagnostic tool. Axial viewing was found to result in matrix effects which were most sensitive for atomic lines223 and detection limits which were lower but more affected by background emission at higher power,221 compared with radial viewing. It was also possible to extend the linear range in axial viewing by suppression of self absorption, using appropriate operating conditions.222
3.2 Furnaces
The Arrhenius equation defining the exponential dependence of evaporation or desorption rate on temperature is widely used in ETAAS for the investigation of the mechanisms and kinetics of analyte release. For several years, however, L'Vov (ASU 1999, 2000, 2001) has proposed an alternative approach. The traditional approach is based on the original Arrhenius hypothesis that the reaction involves only an ‘active part’ of all reactant molecules which, according to the Maxwell–Boltzmann distribution law, is an exponential function of temperature. The second approach is based on the Hertz–Langmuir prediction of the proportional dependence of the evaporation rate on the equilibrium partial pressure of the vapour which, in its turn, depends exponentially on temperature. This second approach, known as the physical approach, has been reviewed by L'Vov.229,230 The first part of the review is a substantiation of the author's theory whilst the second part reports the application of the approach. The results of this application are concluded to support the advantages of the physical approach over those of the more traditional chemical approach. Among the results reported were: a method for theoretical calculation of both Arrhenius parameters (A and E) or, in the authors words, the absolute rates for solid-state decomposition reactions, was developed; an explanation for peculiar kinetic behaviour observed in analyte evaporation from the bulk; and the identification of analyte release based on the “absolute” value of the Tapp/E ratio, and/or of the A parameter. Based on these observations, it was concluded that the retention of analytes by Pd and other platinum group modifiers (PGMs) is associated with the mechanism of chemisorption. The desorption mechanism of release from graphite takes place for some analytes at low (sub-nanogram) masses of samples. The mechanism of low-temperature migration of analyte in graphite tubes at pyrolysis temperatures and, in particular, analyte migration onto a Pd modifier is explained by L'Vov's theory as the congruent dissociative evaporation of reactant in the primary step of decomposition. The Pd modifying mechanism was further developed in two additional papers by the same author.231,232 The physical theory proposed by L'Vov, and in particular to his challenge of the commonly accepted chemical model, is sure to promote further interest and research in the areas of ETAAS mechanisms.A number of other workers have reported on investigations into the mechanisms of Pd based modifiers. The effect of Pd and Pd–Mg modifiers on the determination of Te was studied by Shiue and co-workers through observation of the analyte absorbance profile.233 Positive shifts in appearance temperature and increased peak tailing were observed when the mass of Pd was increased. These effects were eliminated when large amounts of Mg(NO3)2 were introduced. These effects were also observed when ETV-ICP-MS was used to monitor the analyte behaviour through a heating cycle from 120 to 2650 °C. A number of signal pulses were observed between 1500 and 2650 °C and assumed to result from the different migration rate of Te out of the droplets of Pd and the desorption of the vapor condensation for Te–Pd coupling. Volynsky and co-workers used XPS and electron microscopy to study the low-temperature interaction of a Pd modifier with sodium sulfate and sodium selenite on a pyrolytic graphite platform.234 Electron microprobe analysis showed that Pd and S occupied different areas on the platform surface, whereas Se tended to occupy areas of the graphite surface covered with palladium. This was attributed to the chemisorption of SeIV on the Pd surface during the drying stage. No changes in the XPS spectra of metallic Pd and SVI were observed when Na2SO4 and Pd were heated together on the graphite platform in the range 100–550 °C. The reduction of sodium selenite was observed during the drying stage and the reduction was accelerated in the presence of pre-reduced Pd. The transformation of SeIV into Se was complete at 250 °C.
The relatively large amount of Pd modifier used in ETAAS has a number of physical and chemical effects on the analyte. Sadagov and Katskov235 investigated the effect of Pd on analyte transport in a graphite furnace atomizer using a fast heating graphite furnace. Use of a fast heating furnace permits rough estimation of the analyte dispersion rate via observation of the tailing of the absorption profile. Small masses of pre-treated Pd (0.1 µg) were observed to significantly change both the vaporization pattern and residence time of the atomic vapours, but affected the peak area only slightly. In this case, the vapour removal was 1.5–1.8 times slower than that predicted by a purely diffusion-based theory of vapour release. This was suggested as being due to the involvement of vapour adsorption by the wall and molecular diffusion in the vapour transport. An increase in Pd mass (up to 2.5 µg) was observed to result in the gradual reduction of residence time and peak area, due to the increased atomic vapour diffusion rate. The two-line method of temperature measurement showed that this effect was accompanied by a temperature increase of 75–200 K, when 0.5–4 µg Pd are vaporized. An exothermal interaction between the Pd vapour and the graphite substrate was suggested. The observations of variability in the atom residence time as a result of matrix and/or modifier mass are highlighted as practical limitations of the theory of absolute analysis.
The modification mechanism of Pd, Ir and Rh and in particular the deposition of these metals on the graphite surface was investigated by Bulska and co-workers using electron microprobe analysis.441 They found that when the noble metals were deposited on the graphite surface they do not form a compact layer on the surface but penetrate into the pyrolytic graphite structure as early as the deposition step. The degree of penetration of the pyrolytic graphite at high temperature differs for various metals. It was also demonstrated that electrodeposited noble metals remain in sub-surface domains of the graphite for hundreds of atomization cycles, meaning that they can be used as permanent modifiers.
Tsalev and co-workers used XPS to investigate the modifying action of Ir deposited on W and Zr coated graphite furnaces.236 When W was deposited on platforms and thermally pre-treated, W carbides and oxides were observed, whereas the signals obtained from Zr treated platforms resulted from mainly oxygenated species. In both cases the existence of species anchored to the graphite substrate via oxygen and chlorine bridges was hypothesized in order to account for the binding energy values obtained by a curve-fitting process. The deposition of Ir on either pretreated surface was observed to significantly modify the surface chemical composition. In particular, the presence of Ir promoted the formation of higher oxidation state W and Zr species. This was accompanied by a decrease in overall signal intensity, most likely due to the removal of the modifier from the platform surface and/or migration into the platform subsurface region as a consequence of thermal pre-treatment of the modifier. All the modifier systems studied (W, Zr, W–Ir, Zr–Ir) produced at least two eventual forms of the modifier at the end of the preatomization treatment, with a prevalence of oxygen-containing species. It was concluded that the differences in thermal stabilization and in the analyte atomization process could be ascribed to the presence of chemically different active centres that can be characterized by different morphologies.
Studies on the modifying effect of Pt and W on the atomization of Pb were reported in a two part article by Zachariadis et al.237 The crystalline formations left on graphite platforms after the pyrolysis and atomization of Pt and W compounds were studied using SEM and the chemical composition of the residues and the substrate investigated using EDXRF. The crystalline form of each modifier was found to change at temperatures between 500 and 1250 °C. Severe corrosion of the graphite surface was observed in case of H2PtCl6 at temperature above 750 °C, while only minor damage was observed in case of Na2WO4. In the second part of the work, the efficiency of the modifiers was tested for maximum loss-free pyrolysis temperature and atomization of Pb. The optimum results, concerning these parameters, was estimated but the values not given in the abstract.
Thermogravimetric measurements were used to study the atomization of Pb from a W-atomizer in the presence of a number of modifiers by Santos and co-workers.238 XRD was used to elucidate the composition of residues on the W substrate after the pyrolysis step. The decomposition mechanism observed throughout was the formation of PbO by heating, oxide reduction by H2 contained in the purge gas, followed by volatilization of Pb.
The interference effect of nitrates of Na, K, Mg and Ca on the atomisation of Mn, Cd and Fe were studied using multiple linear regression modelling by Grotti et al.239 The models were observed to efficiently predict the effects of the matrix elements over a wide concentration range. This was illustrated through the analysis of sea-water CRMs after preconcentration. The analytical results obtained by applying the multiple linear regression models compared favourably with those obtained using both external calibration and the analyte addition technique.
The effect of the graphite surface composition on the desorption of As and Au in ETAAS was investigated by Imai et al. in two separate publications.240,241 Arrhenius plots obtained for As and Au deposited onto pyrolytic graphite, non-pyrolytic graphite and graphite modified with ascorbic acid were observed and kinetics of the processes used to propose the desorption mechanisms. Based on the differences in slopes of Arrhenius plots for As on modified and non-modifed graphite, the existence of two types of atomization mechanisms was suggested. The first atomization corresponds to the desorption of As binding to active sites and the second atomization corresponding to the desorption of As intercalated within the graphite structure. In the experiments using Au, the roughening of the graphite surface as a result of modification with ascorbic acid resulted in a measured increase in activation energy corresponding to the desorption of Au dispersed at active sites on the rough surface.
In order to avoid all the drawbacks of the models suggested by Holcombe, L’Vov, Sturgeon, Smets, Rojas et al., Aller proposed a new model for atom formation in ETAAS.242 In this version. the activity of surface species was equated to the number of atoms with an energy value above the activation energy of the atom formation process. Dissipation of the analyte atoms was considered to be the result of diffusion, thermal expansion and redeposition processes. A model based on Maxwell's theory was then used to determine the temperature-dependent activation energy and the order of release of the analyte atom formation process from one single absorbance–time profile and under non-isothermal conditions. The advantage of this method is that the activation energy can be evaluated without prior knowledge of the order of release of the analyte atom formation process. A second advantage is that the calculation of the activation energy can be carried out at any point in the atomization process. This means that the method uses the total rising edge of the peak (covering a wide temperature range) and not necessarily at the first moments of the absorbance–time profile. A comparison with the previously developed models using the same earlier published experimental data sets for Cu, Ni and Al was made and satisfactory agreement was found between the results derived from this work and those published previously.
4 Laser-based analytical atomic spectrometry
In analytical atomic spectrometry lasers may be employed either as an intense energy source or as a bright radiation source of precise wavelength. This review will be divided into those two broad categories and will be directed principally at techniques for the determination of elemental composition of samples. The presentation format will be similar to that employed in last year's review.243 The use of lasers for fundamental studies of the properties of atoms, atomic vapours and plasmas and for the production of thin films will not be considered here. Applications where a laser is used solely to vaporize the sample are reviewed in Section 1.5 “Solid Sampling”.4.1 Lasers as energy sources
Lasers may be focused to deliver a high energy pulse onto a small area to vaporize (ablate) a small volume of sample (microsampling). Repeated pulses may be used to obtain a depth profile of the sample. The laser energy may also generate a plasma and excite the emission spectrum of analyte atoms. The laser wavelength is not usually critical (wavelengths from 266 nm to the infrared have been used) but matching wavelength to sample material can be advantageous. Pulse energies are of the order of millijoules with repetition rates optimised to match analytical requirements.4.1.1.1 Fundamental studies. The complexity of the processes occurring in LIBS is graphically illustrated by the diversity of the information generated by studies of those processes. It would appear that, as yet, there are no universal experimental conditions suitable for all analyses and consequently each application of LIBS requires careful evaluation if optimum analytical sensitivity and accuracy are to be achieved.
An international conference on LIBS was held in the year 2000 and has been reported in a special issue of Spectrochimica Acta (Part B).244 Papers from that issue will be referred to in the appropriate sections of this review.
The influence of wavelength on the LIBS signals from glass, polymer, Si and Cu has been investigated using 1064 nm and 266 nm laser pulses.245 The threshold fluence was lower and the emission intensity greater for UV rather than IR irradiation. Simultaneous irradiation with UV and IR further decreased the threshold and increased the emission intensity. The wavelength dependence of the generation of a plasma in a vapour or gas by laser irradiation has been investigated by Capitelli et al.246 They found that resonant irradiation energy was much more rapidly transformed into plasma energy than was off-resonant radiation. This technique, photoresonant plasma (PRP), was evaluated using alkali metal vapours.
A new technique using rainbow refractometry has been used to characterise the shock wave plasma induced by a Q-switched Nd∶YAG laser from various metal samples at reduced gas pressure.247 Initially the blast wave and emission fronts coincided but in the case of 140 mJ pulses the two fronts began to separate 5 mm above the sample with the emission front leading.
A one-dimensional Cartesian Langrangian fluid code has has been used to model the ultra-short laser pulse ablation of solid Al and the subsequent plasma expansion in ambient air.248 Predicted and observed plasma temperatures were in good agreement and consistent with thermodynamic equilibrium. The performance of femtosecond (fs) laser pulses in the LIBS determination of Fe, Cu, Mg, Mn and Si in Al alloys has been evaluated by Drogoff et al.249 The LODs were in the ppm range and were element dependent. As may be expected when using fs pulses there was a faster decay of the continuum and spectral line intensities with a shorter plasma lifetime compared with longer laser pulses. These observations have been confirmed by other workers,250 who noted that emission intensity increased with increasing laser energy. The increased intensity appeared to be mostly attributable to increased amounts of material being ablated. A comparison of 170 fs with 6 ns laser pulses for the LIBS analysis of brass found that much of the difference in the temporal behaviour of spectral line intensities could be explained by the shielding effect of the Ar plasma for ns pulses compared with the free expansion of the plasma of the ablated material in the case of the fs pulses.251 Ablation was effected in an Ar atmosphere at 40 mbar pressure. Internal standardisation of Zn to Cu gave linear calibration curves for both fs and ns pulses. Picosecond pulses, (570 nm, 1.3 ps) used for the LIBS analyses of glass, steel and Cu have been found to decay more quickly and to show significantly lower background emission than plasmas generated by ns pulses (1064 nm, 7 ns).252 The radiance (brightness) of the two plasmas were similar but as the ns plasma was physically larger it emitted more radiation in total. The reduction in background resulting from the use of ultra short pulses (ps or fs) has been noted by other workers.253 Increased LIBS signals were obtained by operating at 1000 Hz. Signal enhancement was also achieved using a dual ns pulse system. A pre-ablation pulse was combined with an ablation pulse for the analysis of Cu, brass and steel.
The distribution of atomic emission intensities throughout the laser induced plasma (LIP) provides valuable information for the understanding of the mechanisms of plasma production. Liquid crystal tunable filters (LCTF) and acousto-optic tunable filters (AOTF) have been used in the imaging of atomic emission by Cu (521.8 nm) and Pb (722.9 nm) in LIPs.254 Additional filters were required to reject wavelengths outside the tuning range of the devices. The temporal and spatial distribution of C2 and C3 molecules produced by laser ablation of graphite has been studied using laser induced fluorescence (LIF) imaging and OES at various pressures in different ambient gases (vacuum, N2, O2, H2, Ar).255 A large yield of C2 was observed in the centre of the plume and near the target surface. Fluorescent C3 was studied in Ar gas; the yield of C3 increased at higher gas pressures and longer delays after ablation. The time evolution of electron number density and of electron, atom and ion temperatures produced by laser ablation of TiO and TiO2 targets has been studied by temporally and spatially resolved OES at several laser fluences and O2 pressures.256 A numerical model for calculating plasma parameters was developed and gave results which agreed with experimental observations. Other workers have developed a one-dimensional time independent fluid dynamic model including chemical reaction in LTE to describe the expansion of a TiO plasma.257 The distribution of ground state Si atoms in a plasma laser-ablated from polished silica has been measured by LIF as a function of time at a distance of 5–7 mm above the surface.258 The angular distribution of atom velocities was highly asymmetric and the LIF signal dropped abruptly at angles approaching the laser direction. This effect was attributed to preferential ionization by multiphoton off-resonance absorption. The velocity of the Si atoms was in the 1–13 km s−1 range. The characteristics of Si plasmas produced by the ablation of silicon in air have also been studied.259 Electron temperatures were in the range 6000–9000 K and ionic temperatures 12000–17000 K with electron number densities of the order of 1018 cm−3. The morphology of the plasma as a function of laser energy and measurement delay time was also examined.
The influence of optical depth on the observed line intensity of an LIP has been investigated using seven self-absorbing lines of Fe I.260 The sample was a Fe–Ni alloy ablated with an IR laser. An observation delay of 5 ps and gate width of 1 ps were employed. Theoretical calibration curves were prepared using a model based on fundamental atomic parameters and a plasma temperature of 8210 ± 100 K. Agreement between the predicted and experimental curves suggested that matrix effects were not present in the ablation of the Fe–Ni alloy. A theoretical model of an inhomogeneous optically thick LIP has been developed by Gornushkin et al.261 The model describes the time evolution of the plasma continuum and specific atomic emission after the laser pulse has terminated and interaction with the target material is complete. The model assumes LTE, which justifies the application of a collision dominated plasma model and standard statistical distributions. The outputs of the model are spatial and temporal distributions of atoms, ion and electron number densities, evolution of atomic line profiles, optical thickness and the resulting absolute intensity of plasma emission in the vicinity of a strong non-resonant atomic transition. Calculations were performed for a 2-component Si/N system.
Another model incorporating correction for self-absorption has been developed to address the calibration problems created by the practical need to carry out analyses on the wide variety of matrices encountered in soils and marine sediments.262 Calibration curves were generated for each element using a variety of certified materials. Using the model, the correlation coefficient between the LIBS measured value and the certified concentration could be derived. These coefficients could then be applied to the direct determination of single elements in any sample. In trials of the procedure, concentration uncertainties of 15–40% were found over concentration ranges of several orders of magnitude.
A study of the plasma chemistry occurring during the laser ablation of Al, graphite and Ti in air observed the formation of oxides and dimers.263 In the ablation plasma, oxides were present in both ground and excited states and were formed by interaction of metal atoms and oxygen. The dominant mechanism for the production of C2 was atomic carbon recombination. The plasma produced by pulsed laser ablation of boron nitride in low pressure N2 or Ar has been found to contain boron nitride radicals.264 The emission of several vibrational bands was observed in the spectral range 340–380 nm and identified by comparison with synthetic spectra obtained by computer simulation. The interaction between mercury and oxygen species in an LIP has been studied by Gleason and Hahn.265 They concluded that the 253.7 nm Hg emission line was selectively quenched by oxygen species, primarily O2 and NO formed during the plasma recombination process.
Nunez and Omenetto266 have investigated the photofragmentation in air of dry aerosols of Na salts (NaCl, NaOH, Na2SO4) by 193 nm photons from an ArF excimer laser. Each salt had a different threshold for the overall process of vaporisation, dissociation and excitation of atomic emission. The LOD for the detection of Na in airborne particles was 0.3 ng Na g−1 air (≡0.017 ppbv).
4.1.1.2 Instrumentation. Modest developments in instrumentation for LIBS have taken place in three areas: portable instruments, improved laboratory systems and fibre optic coupling of sample excitation/emission to the laser source and spectrometer. Reviews of LIBS analysis in the field have been prepared by Bell267 and by Hou and Jones.268 A comparison of LIBS field and laboratory instruments for the determination of environmental Pb in soil and latex paint concluded that the performance of the systems in the 0.05–1% Pb concentration range were similar.269 The Pb 406 nm line was the preferred analytical line. It was found that the lower spectral resolution and absence of detector gating of the portable instrument were no disadvantage and that multiple laser pulsing had no significant positive or negative effect.
As the laser plasma can excite the emission of all the elements within it, the technique has the potential for simultaneous multielement analysis. To achieve such analysis, in addition to an instrumental system capable of collecting the required data, sophisticated data processing is necessary if precise and accurate results are to be obtained. Body and Chadwick270,271 have described a multiple spectrograph with a synchronized multiple CCD spectral acquisition system. For the accurate analysis of heterogeneous material such as mineral ores and black coal, many spectra were collected. The effect of pulse to pulse variation was considerably reduced by normalisation of the elemental emission to the total plasma emission intensity. Data filtering and spectral deconvolution were also required. The absolute accuracy achieved was ±10% with detection limits for a number of elements (Al, C, Ca, Fe, H, K, Mg, Na, Si, Ti) of the order of 100 ppm of moist sample. A valuable comparison of two spectrometric systems for the simultaneous multielement LIBS analysis of Al alloys has been carried out by Detalle et al.272 The systems compared were an Echelle spectrometer coupled with an intensified charge coupled device and a Czerny–Turner spectrograph with an intensified photodiode array. The plasmas were generated in air at atmospheric pressure. Both systems were used to generate calibration curves and to determine LODs for minor elements (e.g., Cu, Mg, Si, etc.). Results showed that there was no significant difference in the analytical figures of merit of the two systems. The limitations and differences of the systems lie in their speed of operation, dynamic range and spectral range. A flexible Echelle spectrometer with interchangeable dispersion and detector systems for use in LIPS has been described.273 The instrument thus can operate in any wavelength region from the vacuum-UV to the near-IR.
A computer controlled auto-sampler has been reported which facilitates calibration and analysis of multiple elements in multiple samples.274 Each analysis for a single element was the average of multiple sampling locations, thereby reducing the effect of sample heterogeneity. Single element measurements were used to optimize experimental conditions. The system was applied to the determination of Al, Ca, Mg, P and Si in phosphate ores. Preparation and analysis time per sample was less than 5 min and the RSD was 2–4%. A 75 m long fibre optic system has been applied to the remote determination of Cu in stainless steel tubing by LIBS.443 The Cu content was in the range 0.04–0.60% and was measured within 3 min with an accuracy of ±25%.
The in-situ LIBS analysis of liquid steel at surface to probe distances of up to 1.5 m was effected using a variable focus lens system attached to a 12 m fibre optic.275 Determination of individual elements (Cr, Cu, Mn, Ni) within the concentration range 0.04–14 wt% was carried out in 7 s.
4.1.1.3 Applications. Table 1 summarises information on specific applications of LIBS. Where the original publication contains information of significant wider interest or is not suitable for tabular presentation it will be presented in this section.
| Matrix | Analyte | Comments/LOD | Reference |
|---|---|---|---|
| Bulk analysis— | |||
| Steel | Cr, Mn, Ni | Measurement in air at atmospheric pressure using internal standardisation. Analysis in the field and for process control | 276 |
| Steel | Multielement | Comparison of LIBS with spark AES using same instrument LODS: Cr, Mn, Mo, Ni, etc., 1–1200 ppm; C, N, P, S, 2–100 ppm. LIBS has potential to be 3× better than spark AES | 277 |
| Steel melts | C, Cr, Ca, P, S | New “lance” developed to give slag-free optical access to melt. LODs: <11–21 ppm | 278 |
| Iron ore slurry | Al, C, Ca, Mg, Si | On-line analysis. For the determination of C, multivariable calibration was necessary to correct for matrix effects | 279 |
| Brass | Cu, Zn | Non-linear calibration curves linearized by normalising Zn line intensities to intensities of Cu | 280 |
| Pu | 281 | ||
| Al–Li alloys | Li | Measurement in an Xe atmosphere at reduced pressure. Limit of determination Li: 300 ppm | 282 |
| Precious metals | Determination of caratage and trace elements down to 1 ppm | 283 | |
| Starch-based flour | Trace elements | Direct analysis in atmosphere of Ar at reduced pressure. Linear calibration curves: 0–160 µg g−1. Precision (RSD): 2–10%. Results for Mg and Mn in NIES standard rice samples agreed with certified values | 284 |
| Wood | Inorganic presevatives | Simultaneous multielement analysis. C used as reference element. LODs in the ppm range. Comparison with ICP-AES. Penetration depth correction factors required | 285 |
| Recycled thermoplasts | Multielement | Echelle spectrograph, time resolved detection. C as internal standard. LODs in ppm range | 286 |
| Soil | Metals | Direct push fibre optic LIBS probe for in-situ analysis | 287 |
| Soil | Heavy metals | Evaluation of in-situ analysis. LODs (µg g−1): LIPS, As, 3.3, Cd, 6, Cr, 2.5, Cu, 3.3, Hg, 84, Ni, 6.8, Pb, 17, Tl, 48, Zn, 98; LIF, Cd, 0.3, Tl, 0.5 | 288 |
| Soils, baby food | Ca | BaCl2 used as matrix, internal standardization using Ba lines. Optimization of experimental conditions | 289 |
| Concrete | Al, Ca, Si | Demonstration of the potential of LIBS | 290 |
| Surface/depth analysis— | |||
| Pottery glaze | Al, Ca, Fe, Mg, Si | LIBS analysis in air at atmospheric pressure compared with XRF | 291 |
| Paper, paper coatings | Fillers, pigments | Studies of depth profiles and lateral splash-over from ablation process | 292 |
| Beta-Ti | Al | Studies of temperature dependence of grain boundary diffusion of Al in β-Ti | 293 |
| Ni | Al | Studies of temperature dependence of diffusion of Al in Ni | 294 |
| Maganin coating on Si | Cu, Mn, Ni | Coatings prepared by laser ablation from solid manganin target. Composition of coatings found to be significantly different from that of the target | 295 |
| Auto catalytic converters | Pt, Pd, Rh | Three-dimension distribution analysis by simultaneous multielement imaging. Lateral resolution, 1.75 nm; RSDs, 3–23% | 296 |
| Auto catalytic converters | Ce, P, Pb, Pd, Pt, Zn | Two-dimension distribution maps of catalytic and poisoning elements | 297 |
| Zn coated steel | Zn | Comparison of LIPS with LI-TOF-MS. Multielement capability and absence of background favoured the latter. Ablation rate 20–30 nm pulse−1 | 298 |
| Screen printed electrodes | Ag, Al, Au, C, Pd, Pt, Ti | Depth analysis at several electrode zones used to characterise miniature multi layer electronic devices | 299 |
| Marble encrustation | Al, Fe, Si, Ti | In-situ depth profiling; Ca internal standard | 300 |
| Liquid analysis— | |||
| Liquid | Cr, Mg, Mn, Re | 301 | |
| Gas/aerosol particle analysis— | |||
| Incinerator gas stream | Be, Cd, Cr, Hg | Method evaluation for multielement continuous emissions monitoring | 302 |
| Aerosol particles | Heavy metals | Collection on band filters: LODs, As, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sb, Sn, Tl, 0.02–2.73 µg m−3 | 303 |
| Aerosol particles | Al, Ca, Mg, Na | Real-time analysis revealed analyte spectral intensities could vary from time to time by 103-fold. Analyses of individual particles indicated particle sizes ranging from 100 nm to 2 µm | 304 |
| Nanoparticles | Chemical characterization | 305 | |
A review of the use of LIBS as a means for monitoring toxic elements in the environment has been prepared by Sneddon and Lee.306 An imaginative use of LIBS proposes that laser ablation could be used as an alternative to the mechanical drilling of teeth and that the plasma generated could be spectrally analysed to monitor the removal of caries.307 The potential of this technique has been demonstrated using in-vitro experiments. The accumulation of toxic elements in calcified tissue (teeth and bone) has been studied as a means of monitoring environmental exposure to those elements for some years. The application of LIBS facilitates the mapping of the distribution of elements in the tissue. Such maps provide information on (a) time/concentration relations, i.e., monitoring exposure, (b) mineralisation of the matrix and migration of elements within it, e.g., metal implants, (c) identification of disease states, e.g., caries. Studies of the analysis of teeth and bone by LIBS used synthetic pellets of CaCO3 containing Al, Sr and Pb in the range 100–10000 ppm relative to the Ca content of the matrix for calibration purposes.308 The results were in agreement with those obtained by AAS and with literature values.
Developments in sensitivity and signal processing have increased the potential applications of LIBS in the steel industry. These applications include analysis of the top gas of the furnace, liquid steel, slag and finished products. The advantage of LIBS lies in the speed with which information on the operation of the furnace, the composition of the steel and the quality of the final manufactured product can be obtained. Noll et al.309 have prepared an overview of R and D activities and of routine industrial applications of LIBS in the steel industry with particular attention to the topics mentioned above. A more detailed study of one of those topics, slag analysis, has been carried out by Doujak et al.310 In this work, a special device was used to take the sample and, by optimizing the filling and cooling conditions, samples with a smooth surface suitable for immediate analysis by LIBS were achieved. The results agreed with those obtained by XRF but the precision was worse.
A significant signal enhancement (10–20 fold) in the LIBS analysis of glass and vitrified waste glass resulted when a pre-ablation laser spark above the sample surface was employed to pre-heat the target area.311 Reproducibility in the measurement of Al and Ti was improved by use of an internal standard element (Fe). An attempt has been made to explain an increase in the spectral response of trace elements (at constant concentration) in sand–soil mixtures as the proportion of sand increased.312 It was concluded that the main matrix effects could be attributed to the depth of the laser induced crater and the proportion of laser energy that penetrates into sand particulates but does not cause direct ablation.
In the analysis of high concentration Au–Ag–Cu alloys by LIBS under reduced air pressure (2 torr) it was observed that the total emission intensity of the analytical shock wave plasma was proportional to the intensity of the primary plasma.313 It was proposed that quantitative analysis can be achieved by normalising the intensity of the analytical emission lines to the total intensity of the primary plasma. The practicality of this procedure was confirmed experimentally. A calibration procedure for the analysis of binary and multicomponent solid samples has been proposed by Galbacs et al.314 The method is based on the concentration dependence of a linear correlation coefficient calculated between spectra of a range of certified standards and the spectrum of a reference sample of the analyte in the form of the pure metal. This coefficient was found to be a function of analyte concentration in a binary sample. The method could be extended to multicomponent samples and was applied to the analysis of Cu in brass and Al, Cu and Si in A1 alloys and was accurate to within 1–5% for major components.
By virtue of its capability to remove controlled micro-amounts of material from a specific site on a sample LIBS is an attractive technique for the investigation of surfaces and layered structures. It has been used for the on-line control of the selective removal, by UV laser surface ablation, of Co binder from tungsten carbide hard metal.315 The ablating pulse also generated the LIPS signal which provided important information about the optimal ablation parameters. The deposition of indium tin oxide (ITO) films by pulsed laser ablation of ITO targets was monitored in situ by OES.316 Measurement of the InO emission provided a means for maintaining optimum deposition conditions by regulation of the O2 pressure. Depth profiling of multilayers of Ag, Cu and TiN–TiAlN on silicon and iron substrates has been effected using femtosecond laser ablation.317 The layers were 280–600 nm thick and ablation rates were 15–30 nm pulse−1 with laser fluences in the range 0.3–1.5 J cm−2 at pressures of 10–1000 mbar. A comprehensive review of the application of LIBS to surface analysis in art and archaeology has been prepared by Angelos.318 In this field of study LIBS has been combined with IR spectroscopy to characterize more fully pigments and binding media used in paintings and manuscripts. Micro-Raman spectroscopy has been combined with LIBS in a study of blue pigments (azurite and lapis-lazuli) in mediaeval manuscripts.319,320 A combination of four spectroscopic techniques FTR, FT-IR, LIBS and LIF was used in a comprehensive study of pigments (vermilion, αHgS; white, 2PbCO3, Pb(OH)2; orpiment, As2S3; red, Pb3O4; black, C) and binders (gypsum, CaSO4·2H2O; aliphatic esters; free acids; carbohydrates; amides).321 Low level laser ablation provided a cleaning effect except in areas containing white pigment.
LIBS is a well established technique for the determination of metals but relatively little attention has been directed at non-metals such as C, H, O, N, F, Cl and Br. Winefordner's group has reported several investigations into the determination of such elements. The major element ratios in solid organic powders were determined with an accuracy of 2–3%.322 An atmosphere of He and Ar was necessary to eliminate interference from air. This work was extended to the rapid identification of plastic by building up a library of LIPS spectra from known plastics with which the unknown may be compared.323 The probability of correct identification was close to unity for most polymers. Halogens in solid organic compounds have also been determined.324 Operation in an atmosphere of He gave greater sensitivity and lower background than in one of air. The advantages of the technique were simple sample preparation and rapid analysis. In a study of the analysis of polluted surfaces by means of time-resolved LIBS, C, Cl, F, P and S were determined on targets of PVC and PTFE.325 It was suggested that the technique, adsorption onto PVC or PTFE, could be used for gas analysis in hazardous or corrosive environments where conventional sampling methods were not suitable.
The analysis of liquids by LIBS is not widely practised and much of current activity is directed at investigating alternative instrumental configurations for such analyses. The simplest procedure involves directing a laser beam at the surface of a solution. This approach was followed by Fichet et al.326 who focused a pulsed laser at a tilted angle to the surface of oil or water samples. Twelve elements were determined with LODs of 0.3–120 µg ml−1 and reproducibilities of 3%. This arrangement was suitable for on-line analysis. A novel approach to the elemental analysis of water focused the laser into a water stream flowing from a slender nozzle.327 Detection limits were of the order of 1 ppm. An alternative approach converted liquids into solids by fast freezing of the sample (water) and thus creating a solid sample for LIBS analysis.328 LODs were in the ppm range and calibration curves for Al and Na in water over the range 0.01–1.0% were prepared. A relatively simple way of measuring metals in water samples combined electrolytic extraction with LIBS determination.329 The electrode on which the metals were deposited served as a sub-target for the relatively soft metal film, thereby aiding the generation of a shock wave plasma. At an atmosphere pressure of 1 torr LODs were of the order of 10 ppb. The importance of the form of the analyte element in the LIBS of solution was clearly demonstrated in two reports on the determination of Eu.330,331 In both cases the laser was focused into the sample. It was found that the signal from suspended particles of 203Eu was two orders of magnitude greater than that from Eu3+ ions. It was also observed that Eu adsorbed onto TiO2 particles in aqueous solution could be detected when Eu ions at the same concentration could not. This differential sensitivity was used to study the formation of Eu(OH)3 colloids.
LIPS is rarely, if ever, applied to gas analysis except when aerosols are present; however, it has been used to probe the gas composition of an H2–air diffusion flame.332 The atomic emission intensities of hydrogen and oxygen were normalised by comparison with that of nitrogen. It was concluded that LIPS has a high potential for the quantitative elemental analysis of flames. A comparison of the predictions of a theoretical model, for the distribution of excited states and electrons in a hydrogen plasma, with experimental observations revealed deviations from both Boltzmann and Maxwell equilibrium distributions.333 The sensitivity of the analysis of sulfuric acid aerosols produced by the oxidation of DMS in air was increased over 300 fold when a NaCl aerosol was added to that of the H2SO4.334 An LOD of 46.5 ppbv in a 10 s measuring time facilitated quasi-real-time observations.
4.2 Lasers as sources of intense monochromatic radiation
A comprehensive review of the instrumentation for and the applications of LEAF has been prepared by Stchur et al.336 They conclude that the technique is well established and that continuing technical developments herald a bright future, particularly if multielement capability can be improved and the complexity of laser systems reduced. Such hopes have been expressed for many years: whether they are any nearer realisation today remains to be seen! A study of background signals from a frequency-doubled wavelength-modulated diode-laser system in the 422 nm region found that 2f detection was plagued by greater background than was detection at higher, even harmonics.337 This effect was demonstrated by the detection of Ca in a C2H2–air flame. It was also found that background signals were more dependent on modulation amplitude than were the analytical signals. Optimum detectability was often found to be at lower modulation amplitudes than those normally used.
Once again the suitability of tungsten coil devices as atomisers for atomic absorption, fluorescence and emission has been investigated.174 It was claimed that the W-coil was an ideal atomiser for portable instruments and can achieve LODS within a factor of 10 of those of commercial graphite furnace systems. As a technique for remote sensing, LIBS atomisation in air at atmospheric pressure has been combined with LIF detection.167 The system was tested using trace levels of Al, Cr, Fe and Si. Excitation of off-resonance fluorescence was by 2nd or 3rd harmonic outputs of a pulsed Ti–sapphire laser combined with collisional energy transfer processes.
A pyrolytically coated commercial graphite furnace with LIF detection was used in the determination of Cr in water, gastrointestinal perfusates and serum.338 The merits of Mg(NO3)2 and Triton X-100 as matrix modifiers were examined. The LOD was 80 fg (≡4 ng 1−1). Continuous flow hydride generated LIF has been applied to the speciation and determination of As, Sb and Se.118 The atomiser was an Ar shielded H2 flame. The performance of this LIF system was compared with that of laser enhanced ionization using the same instrumentation and with an ETA-LIF system using hydride trapping in the graphite furnace. The LIF-flame system was found to be several orders of magnitude more sensitive than either of the other systems. A wide ranging study of Tl in Canadian water was carried out using a highly sensitive LEAF instrument.339 The natural background level of Tl in Arctic snow and ice was found to be between “not detected” and 8.4 ng 1−1, while that in water across the country ranged from “not detected” to 4390 ng 1−1 and typically was 1–50 ng 1−1. A survey of 16 metals in the waters of the Gediz River, Turkey, was carried out using both LEAF and AA.340 Factor analysis identified four factors, industry, sea, agriculture and geology, as accounting for 67% of the total variance in the data.
A novel application of LIF has been in the measurement of gas velocities in a C2H2–air flame.341 The method used two lasers, one to ionize the probe atom, Sr, the other to induce planar ionic fluorescence to give a 2D bright image at a given time delay. A slot burner yielded a velocity of 5.1 m s−1 while that of a circular burner was 9.3 m s−1.
Changes in the polarisation of the radiation exciting the resonant transition of Na atoms have been found to affect the electrical conductivity of a Na–He discharge plasma.349
An optical heterodyne signal has been generated from CW laser radiation by rapidly sweeping the optical cavity to shift the frequency of the radiation stored in the cavity and combining the frequency shifted radiation with incident laser radiation.354 The use of a swept optical cavity simplifies the application of heterodyne detection in CRS by eliminating the need for fast optical switching or wavelength locking of the cavity length. High resolution CRS has been achieved by placing a Fabry–Perot etalon at the exit of a ringdown cavity to disperse spatially the frequency components of a broad band ringdown signal.355 This arrangement permits narrow width spectral measurement based on a single exponential decay signal. Off-axis cavity alignment geometry has been used to eliminate systematically resonances commonly associated with optical cavities while preserving their absorption signal amplifying properties.356 It was claimed that this approach considerably reduced the complexity of the apparatus compared with other high resolution cavity-based absorption methods.
Broad band (20 nm) CRS combined with a two-dimensional clocked detector array has been used to generate time and wavelength resolved information for the detection of nitrogen oxides in air.357 It was suggested that this approach could be used for analysing samples containing multiple absorbers, e.g., atmospheric samples. Nitric oxide concentrations in N2 have been measured at the ppb level with a standard error of 0.7 ppb by CRS using a single frequency CW laser operating at 5.2 µm.358 Laser tuning and interrupt were effected by control of the laser current. The absorption sensitivity for a single ringdown event was 2.2 × 10−8 cm−1. Combination of CRS and LIF imaging methods has been used to study absolute concentration profiles of CN and CH in low pressure premixed flames.359 The distribution of CN and CH above the burner compared favourably with theoretical predictions and served to validate the proposed chemical mechanisms. The high sensitivity of CRS has been used to measure the absolute density of ground state nitrogenmolecules absorbing at 227 nm in a low pressure nitrogen d.c. glow discharge.214 The formation and loss of Si and SiH radicals in an Ar–H2–SiH4 plasma used for the high rate deposition of device quality hydrogenated amorphous silicon has been investigated using CRS.360 The atomic absorption line of Ba at 553.548 nm has been studied using an atomic oven and CRS.361,362 The precision of the AA measurement was 6 × 10−7. The temperature dependence of the absorption line profile and its transition probability were determined.
5 Chemometrics
The application of chemometric procedures to atomic spectrometric analysis continues to attract some limited interest. Methods for interference correction in ICP-AES have been recently reviewed (in Chinese).363A parabolic model of signal change has been used to correct for spectral shifts in ICP-AES with solid-state detection,364 over both narrow (10–20 pixels) and wide (10000+ pixels) spectral ranges. Parallel factor analysis has been used to study long-term drift in ICP-AES365 using a drift diagnosis of 30 emission lines to study the effects of power and nebuliser gas flow rate. The results were used to evaluate the robustness of the technique under different operating conditions. The same group have studied systematic and random errors in the determination of metals in tea-leaf digests, by using principal components analysis (PCR) and analysis of variance (ANOVA), in order to establish the errors associated with each stage of the analysis.366
Staying with the topic of tea, pattern recognition methods have been used by two groups to determine the geographical origin of the said beverage by ICP-AES. The first367 studied the effect of using a half range and central value transformation prior to pattern recognition using PCA, cluster analysis, linear discrimination analysis (LDA) and soft independent modelling of class analogy. They found that this improved the classification compared to using traditional data pre-treatment methods such as autoscaling and normalisation. The second paper368 described the use of PCA, LDA and an artificial neural network (ANN) to discriminate between green and black teas (not possible), and between Asian and African teas (possible), based on their trace metal content. Similar techniques have been used to classify wines using FAAS.367
Other classification methods such as kNN, Bayesian and C4.5 inductive learning have been used to differentiate between types of aluminium and steel alloys based on element composition.369 The Bayseian method gave the best results using simulated data.
Multivariate methods have also been used in quality assurance.370 A combination of Hotelling's T-2 statistic and PCA has been used for error recognition and diagnosis of proable cause. A 25 element dataset acquired for an in-house reference material, over 6 years, by ICP-AES was used for evaluation.
One potentially interesting paper (in Chinese) described the use of simulated annealing–back propagation of error–artificial neural network algorithm for the simultaneous determination of As and Cd in grain using a single wavelength.371
6 Coupled techniques for speciation
There have been several reviews and overviews of chromatography coupled with either AAS, AFS or AES in this review period. The IUPAC recommended guidelines for terms related to chemical speciation and fractionation have been published.372 This overview also described a general outline of fractionation procedures and gave examples of methodological approaches used for speciation analysis. Other reviews have been more specific. Solid phase trapping subsequent to further chromatographic or electrophoretic analysis has been reviewed (with 329 references).373 Solid phase micro-extraction (SPME) as a tool for trace element speciation has also been reviewed (with 79 references) by Mester et al.374 This review gave a short overview of the principles behind SPME and followed this by describing typical applications to elemental speciation. The use of SPME for the study of equilibria in complex systems and the possible future roles were also discussed.Two publications have reviewed speciation using ion chromatography.375,376 The latter of the two reviews concentrated on ion chromatography coupled with ICP-AES and -MS detection. Other review types have concentrated on speciation analysis of specific sample types. An example is the review of the speciation of physiologically active elements in biological fluids.377 The review is in Chinese and contains 53 references. The determination of plant metallothioneins has been reviewed (in Czech), with special attention being paid to the coupling between chromatographic and electrophoretic methods with atomic spectrometric detectors.378 Voltammetric methods and isolation procedures were also discussed. A literature survey of on-line elemental speciation in aqueous solutions has been produced by Das et al.379 The review contained 178 refs and the topics discussed included the speciation of analytes such as Al, As, Cd, Cr, Cu, Fe, Hg, N, Ni, P, Pb, Sb, Se and Sn. The general strategies required and the problems associated with the analyses were discussed. New analytical techniques in food science have been overviewed and this publication also contained a section concerning on-line couplings.380
A number of overviews of the speciation of individual elements have also been published. These include one by Krachler et al.381 who highlighted the current state of the art for Sb speciation. The overview contained 48 references and focused on hyphenated instrumental techniques as well as the problems encountered. The principal limitations including the lack of Sb standard compounds, species preservation and extraction suitability were all discussed and the future directions for improving the reliability were described. An overview of the speciation chemistry of As, containing 84 references, that described the problems associated with sample preservation, stability of species, lack of appropriate CRMs, incomplete recovery of all species and interferences has been published.382 A number of methods were compared in terms of their figures of merit, so that their applicability to real samples could be assessed. Another review of recent advances in analytical methods of As speciation which discussed liquid chromatography, capillary electrophoresis, gas chromatography and spectral analysis has been produced.383 The review is in Chinese, but contains 82 references. The determination of butylated Sn species in sediments has been overviewed.384 Commonly used approaches were evaluated and the potential problems associated with extraction, derivatization, clean-up, separation, standardisation and detection were highlighted. A similar sort of overview concerning Hg speciation in environmental and biological samples (78 references) has also been produced.385 The methods compared were classified into three groups, i.e., those without chromatographic separation, gas chromatographic (GC) methods and liquid chromatographic methods. The characteristics of many of these methods were described and compared. Mercury speciation in gas condensates has been overviewed (33 references).386 As well as presenting the state of the art of the analytical methods, an assessment of the accuracy of the data available was made. This assessment is required because of the limited stability of the Hg species in organic solutions. In addition, the possibility of artefact formation and the lack of suitable CRMs were commented upon.
A review containing 178 references of microwave induced plasma (MIP) AES spectrometry for environmental analysis has been prepared.387 Although not all of the references were appropriate for this section of the ASU review, some methods concerning the speciation of toxic heavy metal compounds were described.
6.1 Gas chromatography
The development of an automated speciation analyser (ASA) of organometallic compound content has been reported.388 The instrument automated the isolation procedures, the enrichment of the species and the time-resolved introduction into a detector. After extraction of the species from the sample matrix, the analytes may be isolated from the extracts by derivatization and then enriched on a capillary cryotrap. Separation of the different analyte species is achieved very rapidly (approximately 30 s) using a multi-capillary GC column. The ASA may be used with an assortment of derivatizing agents (depending on the analytes) and used in conjunction with MIP-AES, ICP-MS or AAS instrumentation as a detector. The only requirement (as is common for all GC analyses) is that the analyte or its derivatives should be volatile.A sample preparation procedure for the speciation of organotin compounds in sediment using GC-AED has had all of the analytical steps optimised.393 The proposed method leached the analytes from the sediment using aqueous acetic acid and then the leachates were extracted into n-hexane–ethyl acetate by diethyldithiocarbamic acid. After evaporation to dryness, the analytes were derivatized using sodium tetraethylborate in an aqueous acetate buffer (pH 5.0, 0.1 M), extracted into hexane, cleaned up over basic alumina and then the ethylated species were determined by GC-AED. The abstract did not provide any figures of merit, but did state that the method was validated by the analysis of CRM BCR 646, Freshwater Sediment. A paper (in Chinese) has described the effect of different kinds of gas used as the plasma scavenging gas during the determination of B, P, Si and Sn by GC-microwave plasma torch-AE.394 The paper reportedly discussed the detection limits, linear ranges and precision of the determinations, but these were not listed in the abstract. As an application, P was determined in an emulsifying oil.
A paper by Orellana-Velado et al.395 has used SPME prior to GC-glow discharge (GD)-AES for the determination of alkyl-Pb and butylated Sn species. After optimisation of both the SPME conditions (adsorption time, desorption time, injector temperature and position of the fibre in the injector of the GC) and of the GD conditions (pressure, radiofrequency power, and helium flow rate used as the plasma gas), LODs of 0.021, 0.026, 0.075 µg l−1 for mono-, di- and tributyl tins, respectively, and 0.03 and 0.15 µg l−1 for triethyl- and trimethyllead were obtained. The accuracy of the proposed method was checked by the analysis of the certified sediment PACS-2.
Another paper used SPME during the determination of methyl- and ethyl-Pb and butyl- and phenyl-Sn compounds in aqueous samples and extracts.396 After in-situ derivatization using sodium tetrapropylborate, the analytes were extracted using SPME and then separated and detected using GC-AED. Under optimum conditions, LODs of approximately 0.2 ng l−1 (as the metal) could be achieved for a 20 ml sample volume. Precision at the low ng l−1 level was found to be between 2 and 9% RSD.
A fast and simple procedure has been reported for the determination of mono-, di- and tributyl and phenyl-Sn compounds in sediment samples.397 The analytes were removed from the sediment using ultrasound assisted leaching with acetic acid, and the analytes in the leachates were then derivatized using sodium tetraethylborate prior to analysis by GC-MIP-AES. Spiking of the phenyl-Sn species followed by the extraction procedure indicated recoveries of between 70 and 90%. The butyl-Sn results were validated by the analysis of the CRM PACS-2. Quantification limits were 5–10 ng g−1 and precision from three replicate sediment samples varied between 3 and 10%. Alkylphenols have been determined by GC-AES in non-polar matrixes after derivatizing them to ferrocenecarboxylic acid esters using ferrocene carboxylic acid.398 Each phenol molecule is therefore labelled with one Fe atom. They could therefore be determined using GC-AED in the Fe selective mode. Detection limits were 0.05 pg s−1 and, because the derivatization took place before the first step of sample preparation, the risk of analyte loss by adsorption or volatilisation was minimised. Recoveries of low ppm levels of analytes were 79–125%. The quantification of 20 alkylphenols was demonstrated from a shale oil (SRM 1580) and a petroleum crude oil (SRM 1582). Complete sample preparation was performed in 45 min.
6.2 Liquid chromatography
Several off-line separations of analytes have been reported. Included in these is a paper that reported the determination of Se in infant formula whey fractions using size exclusion chromatography (SEC)-HPLC-HG-ETAAS.408 After preconcentration of the whey by ultra-filtration, the proteins were separated by SEC and the fractions produced were digested and analysed by HG-ETAAS using an iridium coated graphite tube to collect and preconcentrate the hydrides. The hydride generation and collection parameters were optimised and the method was applied to the analysis of six infant formula milks. It was found that the Se was associated with three fractions, i.e., 40 kDa, >160 kDa and mostly with the <10 kDa fraction. A similar sort of paper produced by the same group has determined Cu fractionation in breast milk and infant formulae.409 The protein fractions were separated using SEC with a TSK-Gel 2000 column and a mobile phase of ammonium nitrate (0.2 M, pH 6.7). The effluent fractions were collected and analysed directly by ETAAS. The LOD was reported as being 0.4 µg l−1, and the LOQ 1.5 µg l−1 in the fraction (7.5–22.5 µg l−1 in the milk sample). Precision was also reported to be good (better than 10% RSD) and recovery from the column was 101.2%. Copper was found to be associated with the 76 and 15 kDa regions for the breast milk, but in the cow's milk it was associated with fractions at 14 and 38 kDa. The determination of organic acids (citric, malic and fumaric) and their role in Ni transport in cucumber plants has been reported.410 Ion pair and reversed-phase chromatography were used for the determination of the acids, but their role in Ni transport was studied using SEC with off-line ETAAS detection. The authors concluded that the method was not suitable for real samples because all of the acids had a similar molecular size and because of the presence of a high concentration of ions such as nitrate. Liver homogenates of the Dab (Limanda limanda) have been analysed for metallothionein isoforms.411 The homogenates were saturated with Cd and the cytosols so obtained were purified by a two stage process (acetone and precipitation) and the metallothionein isoforms separated using ion exchange chromatography. The fractions were then analysed off-line by ETAAS, where the Cd was determined and hence the metallothioneins were determined indirectly. Repeat determination of the metallothioneins yielded a precision of 12% for both isoforms. The LOD was reported to be 2 ng mg−1 of protein. Speciation of both As and Se compounds has been achieved by ion-pair reversed-phase chromatography used in conjunction with off-line determination by ETAAS.412 A chemometrics approach was used to optimise the chromatographic conditions. The abstract reported that numerous As species (AsIII, AsV, MMA and DMA) and Se species (SeIV, SeVI, Se-Cys, Se-Met, selenoethionine (Se-Et) and selenocystamine) could be determined. Unfortunately, the abstract did not give any figures of merit other than to say that the linearity covered the range 25–200 µg l−1 for both As and Se species. Serum from six continuous ambulatory peritoneal dialysis patients was analysed for low molecular weight Al complexes.413 Samples were micro-ultrafiltered through a membrane filter to separate Al–transferrin from the low molecular weight complexes. These were then separated on an anion exchange fast protein liquid chromatography (FPLC) column using ammonium nitrate (4 M) as eluent. Fractions of 0.2 ml were collected, diluted 1∶1 with water and the Al determined off-line by ETAAS. Characterisation of the Al complexes was achieved using electrospray tandem mass spectrometry. Three species were identified. These were Al–phosphate, Al–citrate and a ternary Al–citrate–phosphate complex. A similar distribution was observed in serum from healthy volunteers. The same group has also used FPLC to separate Zn species prior to off-line detection by FAAS.414 The method was applied to aqueous soil extracts and industrial waste-water from a lead and zinc mining area.
A method of matrix separation has been described in Spanish by Yanez and Berndt.415 The method utilised HPLC using a reversed phase C18 column and a high temperature hydraulic high pressure nebuliser (HHPN) as a sample introduction method to flame AAS. The analytes (Cd, Co, Cr, Cu, Mn and Zn) in a concentrated sodium chloride matrix were complexed with trans-1,2-diaminocyclohexane-N,N,N’,N’-tetraacetic acid (CDTA) and whilst these complexes were retained on the column, the salt matrix passed to waste. Matrix elimination and metal determination took less than 1 min. The abstract did not specify the eluent used or any figures of merit, but it did say that the method offered LODs that were a ten-fold improvement on those obtained by pneumatic nebulisation.
An ICP has been used to detect the sugars glucose, fructose and sucrose in fruit juices.418 The analytes were separated using a calcium form ligand exchange column and water as the eluent. The eluting species were detected by ICP-AES using the C line at 193.09 nm. The LODs were found to be 50 ng for sucrose and glucose and 60 ng for fructose.
Size exclusion has been used extensively for AES detection too. The binding pattern of Ca, Cu, Fe, Mg, Mn and Zn in human milk has been studied using SEC-USN-ICP-AES by Bocca et al.419 These authors found that the concentration of Mg was higher in mothers >30, whereas Zn was higher in mothers <30. The only analyte showing a significant difference in concentration between rural and urban areas was Cu. The SEC-HPLC-USN-ICP-AES indicated that there were five major fractions, corresponding to caseins, immunoglobulins, human serum albumin and lactoferrin, alpha-lactalbumin and, finally, the small molecular weight non-protein molecules. Copper and Fe were found to be evenly distributed throughout the fractions, Ca and Mg were bound to the low molecular weight fraction, Mn was found in the first and fifth fractions and Zn was found in the fraction corresponding to alpha-lactalbumin. The trace metal binding patterns of Cd, Cu and Zn metallothioneins in cirrhotic human liver samples have been determined using SEC coupled with ICP-AES.420 A calibration procedure was performed by means of a column by-pass injection of elemental standards. Although experimental details were lacking in the abstract, the authors noted that their ultra-filtration technique kept the metal content of the metallothionein in its initial form as opposed to the often used technique of heat treatment, which changes the Cu distribution significantly. Size exclusion chromatography has also been used on line with both ICP-AES and ICP-MS for the multi-element determination of trace elements in black tea leaves.421 The leaves were placed in boiling water for 5 min and then the extracts were analysed for approximately 40 elements. The extraction efficiency was estimated by comparing the concentration in the extracts with those in the tea leaves. Again, no experimental details were given in the abstract, but the authors concluded that many diverse elements were present as complexes associated with large organic molecules. The molecular weight distributions of carrageenans have been studied by gel permeation chromatography coupled with ICP-AES detection.422 All samples of the food grade carrageenan had major peaks at very high molecular weights (ranging from 193 kDa to 324 kDa), although this data differed from that obtained using refractive index as a method of detection (453–652 kDa).
Some more novel methods have also been reported in this review period. Amongst these is the coupling of supercritical fluid chromatography with MIP-AES detection for the analysis of lubricant additives.423 A discussion of the requirements for the coupling and an evaluation of the conditions critical to detector response for the analytes Cl, Fe, N, P, Si and Zn were also made. It is unfortunate that the abstract did not give any figures of merit for the proposed method. Capillary electrophoresis using a PTFE capillary (50 µm id) coupled with ICP-AES using a modified concentric nebuliser has been reported.424 Baseline separation of Ba2+ and Mg2+ using a simple buffer of 0.014 M sodium acetate was achieved. In addition, the paper also reported the speciation of metallothioneins of rabbit liver. A tubular PTFE membrane desolvator has been used as an interface for HPLC and USN-MIP-AES.425 Analyte transport efficiency was enhanced by the addition of dilute sulfuric acid to the sample. In addition, the sulfuric acid minimized the diffusion of analyte through the membrane and helped linearize the calibration. The membrane desolvator conditions such as temperature, solvent composition and counter-current gas flow were optimized. Using optimum conditions and a 120 W plasma, 2,6 dichlorobenzamide and 4-chlorobenzamide in methanol were separated and determined with a LOD for the former of 2 ppm.
An application of HPLC-ICP-AES has been used in a pharmacokinetic study.426 The drug, a disodium salt of a large Dy containing complex known as Dy-EOB-DTPA, was measured in blood, urine and faeces by HPLC-ICP-AES to determine whether or not the body had processed it into metabolites. Unfortunately, the paper appeared in a medical journal and therefore the experimental detail of the separation was not given in the abstract.
Arsenic species (AsV, AsIII, DMA and MMA) have been separated on an anion exchange AS11 column using a gradient elution with sodium dihydrogen phosphate and water as the eluent.430 The eluents underwent post-column photolysis, catalysed by alkaline persulfate, to ensure digestion of the organic compounds and were then detected on-line by HG-AFS. Limits of detection were in the range 0.11–0.15 ng (for a 20 µl injection), the procedure was validated by analysis of GBW 09103 freeze dried urine (where good agreement with the total As concentration was found) and applied to blood samples. Arsenic species have also been determined in fresh water samples by HPLC-HG-AFS.431 A new gas–liquid separator improved the performance markedly with absolute LODs for AsIII, AsV, MMA and DMA being 5–7 pg for a 100 µl injection. Several As species have been determined in oyster tissue after low power microwave extraction (40 W, 5 min) using methanol–water (1∶1) as the extractant solution.432 The species were separated by HPLC and then detected on-line using UV irradiation and HG-AFS. Three As species were found. These were AsB (87%), DMA (4.7%) and a species that was possibly an arsenosugar (4.9%). The microwave assisted extraction was not found to cause species instability. Orthophosphoric acid has been used to extract As species from a range of environmental CRMs and real samples.433 Extraction of the species (AsIII, AsV, MMA and DMA) was found to be close to quantitative in sludge and sediment samples, but soil yielded only 62% recovery. Detection was by HPLC-HG-AFS. Detection limits were excellent, ranging from 0.02 to 0.04 mg As kg−1 for all the species. The species AsB does not form a hydride and the concentration of total As in samples may sometimes be underestimated if the HG technique is used and the AsB is not completely mineralised. The behaviour of AsB during several acid digestion procedures was monitored in an attempt to determine what species are formed.434 Five acid digestion procedures were compared. These included nitric acid at 200 °C for 10 min and for 30 min, nitric–sulfuric acid at 300 °C for 10 and 30 min and nitric acid–sulfuric acid–hydrogen peroxide at 300 °C for 10 and 30 min. The resulting digests were analysed by HPLC-UV-HG-AFS. It was found that the AsB was converted mainly to inorganic As, trimethylarsine oxide (TMAO) and DMA. The stronger the digestion protocol, the more mineralization occurred.
Reversed phase chromatography coupled with HG-AFS has been used to determine several Se species.435 The chromatographic and HG parameters were all optimised carefully and the efficiency of the HG for each species determined. Recoveries were in the range 90–110% and LODs were 18, 70, 96 and 16 µg dm−3 for Se-Cys, Se-Met, Se-Eth and SeIV, respectively. A comparison of sample preparation methods based on proteolytic enzymatic processes for Se speciation in the edible mushroom Agaricus bisporus has been reported.436 Five different extraction methods were tested, but the most efficient was found to be a three step process involving a water extraction and two proteolytic enzymes (pepsin and trypsin). This had an extraction efficiency of 75%. The extracts were analysed by HPLC-HHPN-AFS. Although complicated background signals and matrix problems were reported, the method was still capable of analysing the sample, which was found to contain a high concentration of Se-Cyst and inorganic Se.
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