Atomic spectrometry update. Advances in atomic spectrometry and related techniques

Contents

• 1. Sample introduction
  • 1.1 Liquid
  • 1.2 Chemical vapour generation
  • 1.3 Solids

• 2. Instrumentation and fundamentals
  • 2.1 Instrumentation
  • 2.2 Fundamentals

• 3. Chemometrics

• 4. Isotope analysis
  • 4.1 Reviews
  • 4.2 Isotope dilution analysis (IDA)
  • 4.3 Isotope ratio analysis (IRA)

• 5. Glossary of abbreviations

• References

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Abstract

This is the second iteration of this review covering developments in ‘Atomic Spectrometry’. It covers atomic emission, absorption, fluorescence and mass spectrometry, but excludes material on speciation and coupled techniques which is included in a separate review. It should be read in conjunction with the other related reviews in the series.^1-5 A critical approach to the selection of material has been adopted, with only novel developments in instrumentation, techniques and methodology being included. The major growth areas in evidence were the use of MC-ICP-MS as the method of choice for isotope ratio analyses, and new applications of AMS. The decline in the number of fundamental studies and developments in chemometrics continued. Some novel instrumental methods, such as the portable liquid electrode plasma, were reported and there were many new applications of solid phase extractants for on-line sample pretreatment, particularly using carbon nanotubes.

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1. Sample introduction

This section covers developments in sample introduction for all instrumental methods.

1.1 Liquid

Liquid sample introduction relates to methods wherein the sample is introduced into the instrument in the form of a liquid, such as through nebulisation or into a thermal vapouriser; whereas in vapour generation the sample is initially in the form of a liquid but is converted to a vapour prior to introduction into the instrument.

1.1.1 On-line sample pre-treatment. A review by Kara⁶ highlighted the use of chelating solid phase materials in flow-injection analysis. The review focussed on chelating solid phase materials, and includes information on the type of chelating groups, types of stationary phases, and their efficiency at pre-concentrating metal ions. Discussions of the improvement in sensitivity, improvements in detection limit, as well as the efficiency with which concomitant ions are removed for the FI methods using chelating solid phase materials, were also made.

A critical discussion of selected reports dealing with pretreatment methods of oily samples and the determination of their organic and inorganic constituents using flow systems and spectrometric methods has been presented,⁷ including the empirical preparation of certain organised assemblies like micelles, and emulsions.

1.1.1.1 Solid phase extraction. The following section will discuss recent developments concerning solid phase extraction procedures. To aid the reader, this section is organised in the alphabetical order of the chemical symbol of the target analyte element. Procedures involving the determination of more than one analyte will be grouped at the end of the section.

Tian et al.⁸ used a MnO₂ mini-column to preconcentrate the As species As^III, As^V, MMA, and DMA, during which process As^III was converted to As^Vvia oxidation by the MnO₂, while the other species remained unchanged. Following hydride generation, detection was made by quartz flame-AAS. The recovery of As^V (i.e., the total amount of arsenate and arsenite in the original sample), DMA and MMA from the MnO₂ mini-column is facilitated by TMAH. The procedure was applied for the determination of As in snow water and Hijiki (seaweed) samples. An on-line sequential injection system has been developed⁹ based on SPE coupled with FI-HG-AAS with a quartz tube atomiser for the determination of As^III in groundwater, removing the need for sample pretreatment. The method was based on the selective retention of inorganic As^V, carried out by passing the original sample through a cartridge containing a chloride-form strong anion exchanger. Since the uptake of As^V could be interfered by several anions in waters, the effect of Cl⁻, SO₄²⁻, NO₃⁻, HPO₄²⁻ and HCO³⁻ on the retention was evaluated and discussed. Rahman et al.¹⁰ employed a combination of SPE columns for the selective separation of water-soluble As species; arsenite, arsenate, MMA and DMA. The SPE columns, namely AnaLig TE-01, AnaLig AN-01, and AnaLig As-01 PA, contain immobilised macrocyclic material as the sorbent, commonly known as molecular recognition technology (MRT) gel. Quantitative separation of As^III, As^V, MMA, and DMA was achieved based on the differences in extraction and recovery behaviour of the MRT gel SPE columns for different As species. Novel mercapto- and amine-bifunctionalised silica compounds have been synthesised¹¹ for the speciation/sorption of inorganic As prior to ICP-MS. The bifunctional (NH₂ + SH) silica retained As^III at a wide pH range, from 1.0 to at least 9.0 with the exception at pH 2.0, and also exhibited quantitative sorption to As^V at pH 3.0. A field method for the speciation of As in water samples has been reported¹² that was simple, rapid, cost effective, and safe to use outside laboratory environments. The method used SPE in series for selective retention of As species, followed by elution and measurement of eluted fractions by ICP-MS. The method was suitable for on-site separation and preservation of As species from water. The authors used their system at test sites in Argentina and the UK; in Argentinian groundwater, 4–20% of speciated As was present as MMA, and 20–73% as As^III, in UK surface waters, speciated As was measured as 7–49% MMA and 12–42% DMA.

A procedure for the separation and preconcentration of Bi in a sequential injection system employing bamboo carbon as sorbent has been developed.¹³ For a 1 mL sample volume, the HG-AFS detection limit was 13 ng L⁻¹. The system was validated against a river sediment CRM (CRM 320), along with spiked recoveries of Bi in human whole blood.

The capabilities of carbon nanotubes (CNTs) and modified CNTs have been evaluated¹⁴ to serve as sorbents for preconcentrating Cd with on-line ultrasonic nebulisation (USN) and ICP-OES. Three different CNT substrates, namely CNTs, oxidised-carbon nanotubes (ox-CNTs) and L-alanine-carbon nanotubes (ala-CNTs) were studied systematically. They showed dissimilar adsorption behaviours leading to increased preconcentration factors when used in the proposed on-line SPE system as follows: CNT < ala-CNT < ox-CNT. Under optimal conditions, the adsorption capacity on ox-CNTs was found to be 130 μmol g⁻¹, and the LOD was 1.03 μg L⁻¹ Cd. Zeng et al.¹⁵ used hollow-fibre-supported liquid-membrane extraction coupled with thermospray-flame-furnace-AAS for the determination of Cd in water. 1-Octanol was immobilised in the pores of the polypropylene hollow fibre as liquid membrane and used as the acceptor solution. Under optimised conditions, an enrichment factor of 146 could be obtained, leading to a detection limit of 5 ng L⁻¹ Cd. Tang and Hu¹⁶ used a cone-shaped pipette tip packed with human hair as a solid phase for the preconcentration of Cd based on its complex formation with APDC. The enhancement factor was 19.

Karve and Gholave¹⁷ preconcentrated Co^II using a C₁₈ bonded silica membrane disc impregnated with Cyanex 272. Co^II was quantitatively sorbed at pH 6.0 from sample solution and eluted using 10.0 mL 1.0 M HNO₃ prior to FAAS determination. The LOD was 1.4 μg L⁻¹, and the preconcentration factor was >200. The method was applied for the determination of Co in urine specimens, and industrial sludge samples.

The determination of Cr species in water and human serum samples by FAAS after magnetic SPE was reported.¹⁸ Silica-coated Fe₃O₄ magnetic nanoparticles modified with N-(2-aminoethyl)-3-amino-propyltrimethoxy-silane were prepared and employed for differentiation between Cr^III and Cr^VI; this was selective towards Cr^III but not towards Cr^VI.

There were numerous papers reporting the use of solid-phase extraction for the preconcentration of mercury species. Chen et al.¹⁹ found that the functionalisation of cellulose fibre by grafting L-cysteine on the surface significantly improved the sorption capacity towards Hg and methyl-Hg. Trace Hg^II has been extracted using silica gel modified with DPC.²⁰ At pH 4, the SG-DPC was shown to have rapid adsorption kinetics towards Hg^II. The same authors described the use of a second SPE²¹ C₁₈ bonded silica phase loaded with 4,4′-dimethoxybenzil bisthiosemicarbazone. Oxidised and non-oxidised multi-wall carbon nanotubes (MWCNTs) of different geometrical dimensions have been used for Hg^II adsorption.²² The L-MWCNT-4060-NA solid phase (nitric acid-oxidised MWCNT of external diameter 40–60 nm and length 5–15 μm) had the lowest sorption energy of the MWCNTs tested, and therefore the most favourable uptake of Hg^II. Rofouei et al.²³ employed an C₁₈ silica membrane disc modified by the recently synthesised triazene ligand, 1,3-bis(2-ethyoxyphenyl)triazene to preconcentrate Hg^II, followed by CV-AAS determination. Li et al.²⁴ studied the adsorption ability of duckweed (Lemna minor) powders for removing Hg^II, MeHg, and EtHg with CV-AAS detection. The significant adsorption sites were C–O–P and phosphate groups by surface electrostatic interactions with aqueous inorganic and organic Hg cations, plus selective adsorption which resulted from the strong chelating interaction between amine groups and Hg^II on the surface of the L Minor cells. An octadecyl silica cartridge has been employed as sorbent and 4-bpdb (1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) as a ligand for Hg^II preconcentration,²⁵ with concentration factor and detection limit of 128 and 1.87 ng L⁻¹, respectively. Preconcentration and speciation of Hg^II and MeHg on a solid-phase of poly(acrylamide) grafted onto cross-linked poly(4-vinyl pyridine) has been achieved,²⁶ with excellent selectivity for Hg^II in the presence of Pb^II, Zn^II, Cu^II, Cd^II, and Fe^III ions. Chai et al.²⁷ used a new 2-(2-oxoethyl)hydrazine carbothioamide modified silica gel for Hg determination by ICP-AES; 1-(2-thiazolylazo)-2-naphthol impregnated onto Amberlite XAD-4 resin beads for sorption of Hg^II yielded maximum capacity (q_max) in terms of monolayer sorption of 450 mg g⁻¹.²⁸ Romero et al.²⁹ employed different Pd substrates (i.e. Pd wire, Pd-coated stainless steel wire and Pd-coated SiO₂) for the microextraction of Hg prior to its release in a modified quartz cell with AAS detection. This new design allowed direct sample injection from the SPME device into the quartz cell, thus avoiding analyte dilution. Although Pd wire provided the best performance for preconcentration, Pd-coated SiO₂ fibres could be more easily adapted to the sampling device. Tsoi et al.³⁰ also used SPME but with LC-ICP-MS, to preconcentrate MeHg and EtHg from the headspace of complex urine matrices. The fibre was exposed to a sample headspace for a 45 min extraction, followed by a 5 min desorption.

A Mn^II imprinted 3-mercaptopropyltrimethoxysilane (MPTS)-silica coated capillary has been synthesised and used to preconcentrate Mn^II from biological samples prior to ICP-MS determination,³¹ with a sample throughput of 9 h⁻¹ and an enrichment factor of 16.7. The proposed method was successfully applied for the determination of trace Mn^II in serum samples. Chen et al.³² used single-walled carbon nanotubes as a new adsorbent for the simultaneous preconcentration and determination of Mn^II and Mn^VII by ICP-MS with LODs for Mn^II and Mn^VII of 0.031 and 0.054 ng mL⁻¹, respectively.

A selective solid-phase for the extraction of Ni^II from biological samples has been developed³³ by synthesis of a non-imprinted polymer material by copolymerisation of 2-vinylpyridine as monomer, ethyleneglycoldimethacrylate as crosslinking agent and azobisisobutyronitrile as initiator in the presence of Ni-5-(4-carboxylphenylazo)-8-hydroxyquinoline complex.

Bakircioglu et al.³⁴ physically immobilised coliform bacteria on titanium dioxide nanoparticles as a biosorbent for the SPE of Pb^II using FIA-FAAS. The LOD for water samples was 0.90 μg L⁻¹. The procedure was applied to the determination of Pb^II in river water, wine, and baby food. Pb^II ions were extracted³⁵ using an ion-imprinted functionalised sodium trititanate whisker adsorbent prepared by a surface-imprinting technique. The Pb^II-imprinted Na₂Ti₃O₇ materials were capable of selectively adsorbing Pb^II ions and were used for the preconcentration of Pb in traditional Chinese medicine samples.

An automated low pressure flow analysis method with online columns has been developed for the HG-ICP-MS determination of Sb^III and Sb^V in aqueous environmental samples.³⁶ A chelating resin (1,5-bis(2-pyridyl)-3-sulfophenyl methylene) thiocarbonohydrazide immobilised on aminopropyl-controlled pore glass (PSTH-cpg), and an anion exchanger (Amberlite IRA-910) were used for the columns. After removal of Sb^III by PSTH-cpg, Sb^V was collected on the Amberlite. Zeng et al.³⁷ employed a new method of hollow fibre supported liquid membrane extraction coupled with thermospray flame-furnace-AAS for the speciation of Sb^III and Sb^V in environmental and biological samples. The method was based on the complexing of Sb^III with sodium DDTC. The hydrophobic complex formed was subsequently extracted into the lumen of the hollow fibre, whereas Sb^V was retained in solution.

The preconcentration of Se from mineral water and beer samples has been achieved³⁸ using living bacteria Lactobacillus plantarum followed by continuous powder introduction into MIP-OES. The use of bacteria immobilised on silica permits on-column preconcentration of Se with minimum sample pretreatment by pH adjustment. A LOD of 52 ng g⁻¹ corresponded to 0.06 ng mL⁻¹ in the sample solution, implying a preconcentration factor of 1000. Dai et al.³⁹ developed a new type of cross-linked chitosan with diethylene triamine (DCCTS). The results indicated that the DCCTS could concentrate and separate Se^IV at pH 3.6; the maximum adsorption efficiency was 94%, and the maximum adsorption capacity was 42.7 mg g⁻¹.

Wu et al.⁴⁰ preconcentrated and speciated two analytes, inorganic As and Sb in waters by SPE coupled with HG-AFS. The speciation scheme involved the on-line formation and retention of the ammonium pyrrolidine dithiocarbamate complexes of As^III and Sb^III on a single-walled carbon nanotube packed micro-column. The total As and total Sb were determined by the same protocol after As^V and Sb^V were reduced by thiourea, with As^V and Sb^V concentrations obtained by difference. Anthemidis et al.⁴¹ formed an on-line chelate complex of Pb and Cd with ammonium diethyldithiophosphate (DDTP), and retained these complexes on the surface of reversed-phase poly(divinylbenzene-N-vinylpyrrolidone) co-polymeric beads, prior to elution with methanol and FAAS determination. The proposed method was well suited to detect the target elements at concentration levels below the maximum allowed concentrations endorsed by the European Framework Directive (2008/105/EC) in inland and coastal waters. Kalyan et al.⁴² preconcentrated Hg^II and U^VI ions using an itaconic acid functionalised adsorptive membrane. The micro-gel was formed by in situ photo-polymerisation of itaconic acid (ICA) along with co-monomer acrylamide and crosslinker pentaerythritoltetraacrylate (PETA) in pores of the host membrane. Afkhami et al.⁴³ developed a new SP for Pb^II and Cr^III preconcentration by immobilising 2,4-dinitrophenylhydrazine on nano-alumina coated with sodium dodecyl sulfate. The ions were eluted from the sorbent with a mixture of nitric acid and methanol. The FAAS detection limit for Pb^II and Cr^III was 0.43 and 0.55 μg L⁻¹, respectively, and the maximum preconcentration factor was 267.

Magnetic solid phase microextraction on a microchip combined with ETV-ICP-MS has been used for the determination of Cd, Hg, and Pb in cells.⁴⁴ Gamma-mercaptopropyltrimethoxysilane modified silica-coated magnetic nanoparticles were synthesised and used for the extraction of Cd, Pb, and Hg. Under an external magnetic field, these magnetic nanoparticles self-assembled in microchannels to form a solid phase packed column which enabled enrichment of Cd, Hg, and Pb by a factor of >40, and resulted in improved limits of detection for Cd (0.72 ng L⁻¹), Hg (0.86 ng L⁻¹), and Pb (1.12 ng L⁻¹) using ETV-ICP-MS. Similarly, Zhang et al.⁴⁵ immobilised iminodiacetic acid on mesoporous Fe₃O₄·SiO₂ microspheres and utilised the material for the fast and selective magnetic solid phase extraction of Cd, Mn, and Pb in environmental and biological samples. The microspheres were separated from the aqueous sample solution by applying an external magnetic field. Mashhadizadeh and Karami⁴⁶ preconcentrated Ag, Cd, Cu, and Zn using magnetic nanoparticles coated by 3-(trimethoxysilyl)-1-propantiol and modified with 2-amino-5-mercapto-1,3,4-thiadiazole. Under the optimal conditions, high concentration factors (194, 190, 170, and 182) were achieved for Ag, Cd, Cu, and Zn.

A poly(N-isopropylacrylamide) gel as a novel polymer coating for fibre-in-tube capillary microextraction has been used⁴⁷ for trace determination of Co, Ni, and Cd followed by on-line ICP-MS detection. The PNIPA coated fibre-in-tube capillary can be used for more than 150 times without decreasing the extraction efficiency. The method was used for the determination of Co, Ni, and Cd in human urine specimens. Huang and Hu⁴⁸ synthesised a titania hollow fibre membrane via a template method, coupled with a sol–gel process, for the online ICP-MS determination of 4 elements, Cd, Co, V, and Ni in human serum samples. Zhao et al.⁴⁹ adsorbed the rare earths, La, Ce, Nd, and Y on di(2-ethylhexyl) phosphoric acid resin in the presence of complexing agent, EDTA, with a FI-based means of sample introduction. MIP-AES detection limits were 1.09 μg L⁻¹ (La), 3.31 μg L⁻¹ (Ce), 2.05 μg L⁻¹ (Nd), and 1.25 μg L⁻¹ (Y). Li et al.⁵⁰ preconcentrated 6 noble metals (Ru, Rh, Pd, Pt, Ir and Au) using FI-online displacement SPE via magnetic immobilisation of mercapto-functionalised magnetite microspheres onto the inner walls of a knotted reactor. The sample throughput of the developed method for ICP-MS was about 20 samples h⁻¹. The recovery of all 6 metals was maintained at 90% after 140 successive cycles.

Baytak et al.⁵¹ preconcentrated 6 elements, Cr, Cu, Fe, Mn, Ni, and Zn from water samples on a minicolumn of yeast (Yamadazyma spartinae) immobilised on TiO₂ nanoparticles. The column, containing 100 mg sorbent, offered the capacity to preconcentrate up to 500 mL of sample solution to achieve an enrichment factor of 250 using 2 mL of 5% v/v HNO₃ as eluent.

1.1.1.2 Liquid–liquid extraction. Three papers reported the use of ultrasound-assisted liquid–liquid microextraction. In their work, Wang et al.⁵² used ultrasound-assisted dispersive LL microextraction (UA-DLLME) combined with LC-MS for the determination of organoarsenic compounds in edible oil. The compounds studied were DMA, MMA, and 3-nitro-4-hydroxyphenyl arsenic acid (Roxarsone). The optimum conditions were found to be 4 min of ultrasonic extraction using 1.25 mL of mixed solvent with 50 μL of buffer solution. In another study⁵³ ETAAS was used for detection of As following ultrasound-assisted ionic LLE. As^III was extracted as an APDC complex from 10 mL sample into a 50 μL volume of ionic liquid, 1-hexyl-3-methylimidazolium hexafluorophosphate, with the aid of sonication in an ultrasonic bath. The detection limit was 0.01 μg L⁻¹ and the enhancement factor was 208. Molaakbari et al.⁵⁴ used UA-DLLME to preconcentrate Rh prior to measurement by FAAS. The Rh was complexed with 2-(5-bromo-2-pyridylazo)-5-diethylamino phenol as chelating agent, and an ultrasonic bath containing the ionic liquid, 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, at room temperature, was used to extract the analyte.

Two papers reporting the speciation of Hg by dispersive liquid–liquid microextraction combined with HPLC-ICP-MS have been published by the same group. MeHg and Hg^II were speciated from water samples⁵⁵ by complexing them with sodium diethyldithiocarbamate, and then extracting into carbon tetrachloride using DLLME. Under the optimized conditions, enrichment factors of 138 and 350 for MeHg and Hg^II were obtained from 5.00 mL sample. The detection limits of the analytes (as Hg) were 0.0076 ng mL⁻¹ for MeHg and 0.0014 ng mL⁻¹ for Hg^II. In the second paper, the authors speciated Hg compounds from liquid cosmetic samples.⁵⁶ Hg^II, MeHg, and EtHg were complexed with APDC then the complexes were extracted into 1-hexyl-3-methylimidazolium hexafluorophosphate using DLLME. Enrichment factors of 760, 115 and 235 were obtained for Hg^II, MeHg, and EtHg, respectively, for a 5.0 mL sample size. Liquid-phase microextraction has been proposed⁵⁷ as an attractive tool in combination with XRF spectrometry for multielement analysis (Fe, Co, Zn, Ga, Se, and Pb). Since the X-ray beam can be focused on a small spot size and simultaneously DLLME produces a very small drop ranging from 2 to 30 μL, the combination of these two techniques is a very promising tool for multielement trace and ultratrace analyses. For this work, the authors used DLLME with APDC as a chelating agent.

Zeeb and Sadeghi⁵⁸ preconcentrated Zn^IIvia ionic-liquid cold-induced aggregation DLLME followed by FAAS. The ionic-liquid 1-hexyl-3-methylimidazolium hexafluorophosphate is dispersed into a heated sample solution containing sodium hexafluorophosphate as a common ion source. The solution is then placed in an ice bath upon which a cloudy solution forms due to the decrease in solubility of the ionic liquid. Zn is complexed with 8-HQ and extracted and this enriched phase is dissolved in a diluting agent and determined using FAAS, with LOD of 0.18 μg L⁻¹ for Zn. Zhang and Fan⁵⁹ separated and preconcentrated Sb^III and total Sb by DLLME, prior to determination by ETAAS with Rh as a permanent modifier. At pH 2, Sb^III was complexed with N-benzoyl-N-phenylhydroxylamine and extracted into fine droplets. The LOD for Sb^III was 0.03 μg L⁻¹. Serafimovska et al.⁶⁰ described a modified highly effective liquid phase semi-microextraction (LSME) procedure for preconcentration of inorganic Sb in waters by ETAAS. Sb^III species were selectively extracted as dithiocarbamate complexes from 100 mL sample into 250 μL xylene at pH 5–8. Total Sb was determined using the same extraction system over a sample acidity range 0–1.2, without the need for pre-reduction of Sb^V to Sb^III.

Durukan et al.⁶¹ described their solidified organic drop microextraction and FI-FAAS method for the determination of Cu in water samples. The complexing agent was 3-amino-7-dimethylamino-2-methylphenazine. Similarly this technique has been used⁶² to preconcentrate Cu in cereal samples and for the preconcentration of Se^IV prior to determination by HG-AFS.⁶³

1.1.1.3 Cloud-point extraction. A simple and rapid preconcentration technique using cloud point extraction (CPE) has been described⁶⁴ for the determination of As^V and total inorganic As (As^V and As^III) in water samples, with FAAS determination. As^V formed an ion-pairing complex with Pyronine B in the presence of cetyl pyridinium chloride (CPC) at pH 8.0 which was extracted into the non-ionic surfactant Triton X-114, this surfactant-rich phase was then separated and diluted with 1.0 M HNO₃ in methanol. A preconcentration factor of 120 was obtainable. Baig et al.⁶⁵ evaluated the accumulation of As in maize (Zea mays L.) grain shoots and roots from growing media by CPE. Ammonium pyrrolidine dithiocarbamate and Triton X-114 were used to perform the CPE and preconcentration of As. The use of Triton X-114-based CPE has been reported⁶⁶ as an efficient separation approach for the speciation analysis of silver nanoparticles (AgNPs) and Ag^I in antibacterial products and environmental waters. Real sample analysis indicated that even though the manufacturers claimed nanosilver products, AgNPs were detected in only three of six antibacterial products tested. Sun and Wu⁶⁷ preconcentrated ultra-trace Bi in human serum by CPE prior to ICP-OES determination. The method was based on the complexing Bi^III with 8-HQ with Triton X-114 as the non-ionic surfactant. An enrichment factor of 81 was obtained for the preconcentration of Bi^III from a 25 mL specimen size. The LOD for Bi was 0.12 μg L⁻¹. Yuan et al.⁶⁸ preconcentrated Hg^II by dithizone, followed by micelle-mediated extraction of the complex using Triton X-114. Foaming, which is always observed when Hg⁰ vapour is generated in the presence of a surfactant, was strongly reduced by using SnCl₂ as the reducing agent and an in-house designed gas-liquid separator. Finally, a mixed-micelle CPE procedure has been developed⁶⁹ for the separation and preconcentration of Pt from geological matrices. Pt was determined by ICP-MS. The method was based on the electrostatic interaction of the chloroanionic Pt with positive charge quaternary ammonium head groups of mixed-micelles. These mixed-micelles, formed by micelle–micelle interaction of Aliquat-336-Triton X-114, provide sufficient electrostatic attraction to extract the hydrophilic platinum into the small mixed-micelle rich phase.

1.1.2 Nebulisation. The novel application of the combination of an inline electrothermal vaporisation (ETV) and nebulisation source for ICP-MS has been described.⁷⁰ The nebuliser was utilised to sustain wet plasma conditions during ETV introduction, 200 mL min⁻¹ gas flow was passed through the nebuliser and merged with the ETV transport line at the torch. Using the inline-ETV source, improved detection limits were obtained for elements typically suppressed by polyatomic interferences using a nebuliser.

A high performance concentric nebuliser (HPCN) operating at sample uptakes rates of <10 μL min⁻¹, was described by Inagaki et al.⁷¹ The HPCN had a triple tube concentric structure, and consisted of a fused silica capillary mounted in the centre of the nebuliser body. When the capillary position was recessed by 20–50 μm with respect to the nebuliser nozzle tip, a liquid jet was produced inside the nozzle by a flow focussing effect. The sensitivity in ICP-MS with the HPCN was over two-fold higher that those with commercially available microflow nebulisers. Matrix effects in ICP-OES have been compensated for using three novel multiple sample introduction systems based on Flow Burring® technology.⁷² When the multi-nebulisation systems were employed, the main difference, compared to routine nebulisation, was the significant reduction in ‘resource consumption’ (i.e., samples, reagents, and time).

Paredes et al.⁷³ studied the characteristics of aerosol generation for 26 concentric nebulisers; 17 of the A-type, 6 of the C-type, and 3 of the K-type. For a given nebuliser design, the gas exit cross-sectional area was shown to critically influence the aerosol characteristics. The aerosol generation mechanism was explored and it was concluded that, for A-type nebulisers the nebulisation is more efficient than C and K-type.

A demountable capillary microflow nebuliser (d-CMN) was described by Cheng et al.⁷⁴ It consisted of a nebuliser body, a fused silica capillary with a tapered tip, and a PTFE adapter. The demountable construction of the d-CMN permitted that a blocked or broken solution capillary could be easily replaced. The low self-aspiration rate (4.8 μL min⁻¹), and the analytical characteristics comparable to commercial microflow nebulisers made the d-CMN a good choice for coupling capillary electrophoresis and microbore HPLC to ICP-MS.

An air-segmented flow-injection method has been developed⁷⁵ developed based on the use of a high temperature single pass spray chamber and the injection of a sample plug into an air carrier stream, to mitigate non-spectral interferences caused by organic solvents and petroleum products, and to reduce solvent loading in ICP-OES. The evaluated sample introduction systems were a 12 cm³ inner volume single pass spray chamber (also called a torch integrated sample introduction system) with and without heating, and a 40 cm³ cyclonic spray chamber as reference. For a 5 μL sample plug and a temperature of 200 °C, evaporation of the sample was complete before it entered the plasma, thereby reducing interferences.

The analytical potential of a coupled continuous-microflow ultrasonic nebuliser triple-mode micro-capillary system has been investigated⁷⁶for the determination of iodine in biological samples by direct iodine vapour generation-ICP-OES. The reaction products were obtained by mixing the sample and reaction products at the quartz oscillator, converting liquids into aerosol at the entrance to the spray chamber. The accuracy of the method was verified using NIST 1549 and 1566b SRM's, and a simple external calibration.

1.2 Chemical vapour generation

In their IUPAC Technical Report “Mechanisms of chemical generation of volatile hydrides for trace element determination”, D'Ulivo et al.⁷⁷ focussed on the rationalisation and clarification of fundamental aspects related to chemical hydride generation (CHG): (i) mechanism of hydrolysis of borane complexes; (ii) mechanism of hydrogen transfer from the borane complex to the analytical substrate; (iii) mechanisms through which the different chemical reaction conditions control the CHG process; and (iv) mechanism of action of chemical additives and foreign species. This report provides the tools to explain the reactivity of a CHG system and clarifies several controversial aspects and eliminates erroneous concepts in CHG.

Recent achievements in CHG hyphenated with ICP-OES and MIP-OES have been reviewed,⁷⁸ including new universal sample introduction interfaces operating as both conventional nebulisers and as gas-liquid phase separators; vapour generation from transition and noble metals; HG from slurry samples; and hyphenation with chromatographic techniques. Another review⁷⁹ highlighted the lower limits of detection achievable in HPLC speciation analysis when the technique is coupled with a CVG system. The development of a HPLC-CVG techniques for ultratrace-elemental speciation in a variety of matrices, and the application of these techniques was discussed. The use of AAS, OES, AFS, and MS techniques as detectors following chemical vapour generation in slurry samples has been evaluated⁸⁰ and practical application of the slurry chemical vapour generation technique to biological, environmental and food samples was discussed.

1.2.1 Chemical vapour generation-fundamental studies. Lopez-Molinero et al.⁸¹ demonstrated that the volatilisation of Sb^III with bromide could be stimulated by the presence of iodide. The mixture of halides showed a significant effect that enhanced the chemical vaporisation. The kinetic mechanism was characterised and adjusted to a first order kinetic under the experimental conditions. It could be demonstrated that the vaporisation took place in two different periods.

A new design of CVG integrated atom trap atomiser has been proposed⁸² for FAAS. The overall efficiency of the CVG process was found to be ∼50%, allowing for a 200-fold sensitivity enhancement for Ni when a 2 min in situ pre-concentration time was employed.

An automatic mesofluidic CV-AFS method incorporating a lab-on-a-valve (LOV) manifold for Hg determination has been developed.⁸³ The LOV integrated a readily exchangeable commercially available sorptive microcartridge, a microscale reaction chamber for in-line vapour generation, and a membrane-less gas–liquid separator. Hg^II was adsorbed onto the surfaces of reversed-phase co-polymeric Oasis™ HLB beads, containing a balanced ratio of hydrophilic and lipophilic monomers, namely poly-divinylbenzene-co-N-vinylpyrrolidone. The applicability and reliability of the automated miniaturised method were ascertained through the analysis of spiked environmental waters.

The suitability of different borane reductants and acids has been investigated⁸⁴ for the generation of vapour from AsII, BiIII, and SbIII, prior to ICP-OES detection. Effects of the HCl, CH₃COOH, and C₆H₈O₇ acids as well as the NaBH₄. H₃B–NH(CH₃)₂, and H₃B–NH₂C(CH₃)₃ reductants on efficiency of the As, Sb, and Bi HG were investigated and discussed. Similarly, Duan and Wang⁸⁵ investigated the use of sodium tetradecahydroundecaborate (NaB₁₁H₁₄) as an alternate reductant to NaBH₄ for Au, Pt, Pd, Co, Zn, and Mn chemical vapour generation.

1.2.2 Vapour generation of individual elements.
1.2.2.1 Arsenic. Automated methodologies for measuring As in environmental have been reviewed.⁸⁶ with the focus on methods exploiting multicommutation flow techniques coupled to HG-AFS. Taurkova et al.⁸⁷ reported loss of trimethylarsine and dimethylarsine upon passage through a Nafion membrane dryer, depending on dryer dimensions, while arsine and methylarsine did not exhibit significant losses. A dryer based on sodium hydroxide pellets was proposed as a ‘safe ‘alternative for drying all arsines. A sensitive method has been developed⁸⁸ for inorganic arsenic speciation in dietary supplements using slurry-sampling HG-AAS.

Currier et al.⁸⁹ demonstrated that HG-cryotrapping-AAS could be used to quantify both methylarsonate (MAs^III) and dimethylarsinite (DMAs^III) in livers of mice exposed to inorganic As. No sample extraction was required, thus limiting MAs^III or DMAs^III oxidation prior to analysis. The limits of detection were below 6 ng g⁻¹As of tissue, making this method suitable for studies examining low exposures to inorganic As. Roman et al.⁹⁰ also studied As concentrations in tissue specimens. Specifically, they studied the As behaviour in cardiovascular tissues from an As exposed coronary heart disease patient group from Antofagasta, Chile, against a small unexposed As coronary heart patient group. Measurement for total As was made by HG-AAS, and speciated As by HPLC-ICP-MS. The As concentrations in auricle, and mammary artery tissues were significantly different between both groups of patients. Also, it was demonstrated that auricle is an ‘As^III target tissue’.

Several papers were published in which arsenic was determined with one or more vapour-forming elements. For example, HG-ICP-OES was employed⁹¹ for the determination of As and Sb in fly ash samples. The determination of Sb was significantly interfered by HF in the digestion step, but the interference could be eliminated by the addition of boric acid. An As recovery of 96% for NIST SRM 1633b was obtained following a two-step ultrasound-assisted digestion. Likewise As and Sb have been determined⁹² simultaneously by HG-AFS with a dielectric barrier discharge as the hydride atomiser. The low-temperature and atmospheric-pressure micro-plasma was generated in a quartz cylindrical configuration device, which was constructed of an axial internal electrode and an outer electrode surrounding the outside of the tube. The method was validated against quartz sandstone (GBW07106) and argillaceous limestone (GBW07108) reference materials. The basic principles and application of HG multi-channel-AFS for soil analysis have been described.⁹³ As, Bi, Te, and Se were determined simultaneously; method detection limits in the soil matrix were 0.19 μg g⁻¹, 0.10 μg g⁻¹, 0.11 μg g⁻¹, and 0.08 μg g⁻¹ for As, Bi, Te, and Se, respectively. Zhang et al.⁹⁴ determined the same four elements in tea leaves by HG-AFS. The operating parameters of an in-house made HG-AFS were optimised, along with the hydride forming chemistry.

1.2.2.2 Gold. Arslan et al.⁹⁵ studied the generation of an ‘analytically useful’ volatile form of Au. The FI-based generation was performed in a dedicated generator consisting of a special mixing apparatus and gas-liquid separator design in the presence of surfactants (Triton X-100, and Antifoam B) and diethyldithiocarbamate. A gold radioactive indicator of high specific activity, together with AAS measurements, was used to quantitatively track the transfer of analyte to the atomiser, indicating a vapour generation efficiency of 11.9 ± 0.1%.^198,199 Transmission electron microscopy measurements proved the presence of Au nanoparticles of diameter of approximately 10 nm and smaller, which were transported from the generator by the flow of carrier Ar.
1.2.2.3 Cadmium. Cd vapour generation, based on HCl, cobalt chloride, thiourea and KBH₄, has been used to determine Cd in biological samples.⁹⁶
1.2.2.4 Cobalt. UV photochemical vapour generation (photo-CVG) was adapted for the determination of ultratrace Co by AFS.⁹⁷ Co volatile species can be generated when the buffer system of formic and formate containing Co^II is exposed to UV radiation. Factors affecting the efficiency of photo-CVG were investigated in detail; type and concentration of low molecular weight organic acid, buffer system, UV irradiation time, temperature, carrier and hydrogen gas flows. With 4% v/v HCOOH and 0.4 M HCOONa buffer solution, 150 s irradiation time, and 15 W low-pressure Hg lamp, a generation efficiency of 23–25% was achieved.
1.2.2.5 Iron. Welna et al.⁹⁸ generated volatile species from Fe^II and Fe^III (along with Cr^III, CrO₄²⁻, Mn^II and MnO₄⁻) by reaction with NaBH₄ in HCl, CH₃COOH and citric acids with ICP-OES used for detection. A detection limit of 25 ng mL⁻¹ for Fe^II was obtained.
1.2.2.6 Germanium. ICP-OES has been combined with chloride generation for the determination of inorganic Ge in health foods.⁹⁹ 12 M HCl was used as the carrier solution, and 6 M HCl in the sample solutions to generate volatile GeCl₄.
1.2.2.7 Mercury. A dielectric barrier discharge (DBD) has been used¹⁰⁰ to introduce Hg solution, with or without formic acid, into a low temperature argon plasma. Mercury vapour generated in the DBD was separated from the liquid and swept into an ICP-OES for determination. The vapour generation of Hg could be significantly enhanced with the addition of formic acid, or reduced in the presence of chloride ions or oxidising substances of high concentration.

Several authors reported the use of a photochemical reactor for the generation of Hg vapour. Gao et al.¹⁰¹ used diethyldithiocarbamate (DDTC), not only as a chelating reagent to form a hydrophobic compound for Hg preconcentration, but also as a reductant for in situ photochemical vapour generation and desorption of Hg from a coiled UV reactor. By controlling the desorption and photochemical vapour generation conditions, the proposed method can be applied for rapid speciation of Hg^II and MeHg⁺. Angeli et al.¹⁰² developed a photochemical method for the online oxidation of p-hydroxymercurybenzonate (PHMB), an organic Hg specie widely used for mercaptan and thiolic compound labelling. An integrated online UV/microwave chemical reactor led to the quantitative conversion of PHMB and thiol-PHMB complexes to Hg^II, with a yield between 91% and 98%. Gil et al.¹⁰³ noticed a remarkable improvement in sensitivity when stopped-flow was applied to their PVG system, suggesting that kinetics play an important role in the photo-induced reduction of Hg^II to Hg⁰. In the presence of acetic acid, a LOD of 0.3 μg L⁻¹ could be obtained, which represented a 6-fold improvement with respect to that obtained without stopped-flow.

A novel solution cathode glow discharge (SCGD) induced vapour generation system for the HPLC-SCGD-AFS determination of Hg^II, MeHg⁺ and EtHg⁺ has been developed.¹⁰⁴ The decomposition of organic species and the reduction of Hg^II could be completed in one step with the proposed system. The system was simple in operation, and eliminated the need for auxiliary redox reagents.

Electrolytic cold vapour generation (ECVG) with AFS detection has been reported¹⁰⁵ for the determination of trace concentrations of Hg in waters. Several buffer solutions were used to improve the detection limit by a factor of 1–2 more than in a conventional ECVG system. A phosphate buffer solution increased the signal intensity of Hg vapour from electrolytic generation on the Pt cathode, and reduced the impact of cathode erosion on the stability of signal intensity. Under the optimised conditions, the detection limit for Hg in waters was found to be 0.27 ng L⁻¹.

1.2.2.8 Lead. A quartz tube (QT) atomisation cell in conjunction with high-resolution continuum source (HR-CS) HG-AAS has been evaluated for the determination of Pb.¹⁰⁶ A full two-level factorial design was performed to characterise the effects of NaBH₄, potassium hexacyanoferrate^III and nitric acid reagents. The optimal experimental conditions were determined using a Box Behnken design. The limit of detection for Pb in drinking water was 0.13 μg L⁻¹. Accuracy was confirmed by NIST 1643d, Trace Elements in Natural Water, and the method was applied to the determination of Pb in Sao Paulo River samples. Saenz et al.¹⁰⁷ employed an electrochemical Pb volatile species generation system, and FAAS for the determination of Pb in urine samples. The in-house reaction cell consisted of a reaction compartment housing a glassy carbon cathode and platinum wire anode. The limit of detection was 11 μg L⁻¹. A method for the simultaneous determination of trace Pb, Sn, and Cd in biological samples by CVG coupled with non-dispersive AFS has been described.⁹⁶ The LOD for Pb was 0.071 ng mL⁻¹ and the method was validated against Chinese certified biological reference materials.
1.2.2.9 Antimony. Serafimovska et al.¹⁰⁸ studied the influence of organic additives on stibine generation; EDTA, carboxylic acids, amino- and hydroxocarboxylic acids, monosaccharide and humic acids. EDTA, tartaric, citric and malonic acids, fructose and N,N-bis(2-hydroxyethyl)-glycine were the most appropriate reaction media for the selective determination of Sb^III. The pre-reduction of Sb^V to Sb^III in the presence of organic ligands was found to be quantitative with L-cysteine (1% m/v) as reductant. An analytical application to the determination of Sb^III and Sb^V in waters, tea infusions, EDTA soil/sediment extracts using continuous flow HG-AAS was demonstrated. The speciation of Sb in soils is strongly hampered because the most efficient extractant reported in the literature (oxalic acid) strongly inhibits the generation of stibine by Sb^V. The use of a post-column on-line reduction system with L-cysteine reagent, to reduce the detection limit of Sb^V has been proposed,¹⁰⁹ with LOD for Sb^V and Sb^III in oxalic acid (0.25 M) improved from 0.3 and 0.1 μg L⁻¹ to 0.07 and 0.07 μg L⁻¹, respectively. Zih-Perenyi et al.¹¹⁰ employed a selective extraction technique to fractionate traffic-related Sb compounds with ETAAS detection. Most traffic-related Sb air pollutants are derived from brake dust, which contains Sb₂S₃, which is used as a friction material, and its high-temperature oxidation products Sb₂O₃ or Sb₂O₄. Citric acid solution proved to leach the whole Sb₂O₃ content while extracting less than 10% Sb₂S₃ and no Sb₂O₄ at all. Sb₂O₃ and Sb₂S₃ traces were soluble in 6 M HCl. In related studies Sb, as a traffic-related element, was determined in size-fractionated road dust samples collected in Buenos Aires using ICP-MS and FI-HG-AAS;¹¹¹ a procedure has been proposed for the speciation of inorganic Sb in airborne particulate matter using slurry sampling and HG-AAS with quartz tube atomisation,¹¹² with LOQ of 0.3 and 0.2 μg L⁻¹ for total Sb and Sb^III respectively.

Reyes et al.¹¹³ described a non-chromatographic speciation method for the determination of Sb^III, Se^V (and Te^IV and Te^VI) in cereal samples. Ultrasound assisted extraction was employed, along with HG-AFS determination. H₂SO₄ was found to be the most suitable extraction media (>90% for the total content of Sb) achieved without species interconversion.

1.2.2.10 Selenium. Li et al.¹¹⁴ coated nanometer TiO₂ particles onto the inner wall of a T-shaped quartz tube atomiser for the determination of Se (and Hg) by HG-AAS. The performance of the quartz tube improved the linear calibration range (10.0–70 ng mL⁻¹ to 5.0–100.0 ng mL⁻¹ Se) and the interference from the coexistence of As could be eliminated.

Electrochemical HG has been employed¹¹⁵ to determine Se (and As) in drinking waters with AAS detection. Three types of electrolytic cells were constructed and optimised. Two cells (thin-layer and tubular) were finally chosen for their low inner volume and high hydride generation efficiency (ca. 90%). HG headspace SPME with ICP-OES for the determination of Se (and As) has been investigated,¹¹⁶ wherein hydrides were generated from the headspace of the sample solution in a 10 mL septum-sealed vial and collected onto a polydimethylsiloxane/Carboxen SPME fibre, then the fibre was transferred into a thermal desorption unit and Se was directly introduced to either an argon ICP or helium MIP.

Sun and Feng¹¹⁷ speciated organic and inorganic Se in Se-enriched eggs by HG-AFS by precipitating albumen with trichloroacetic acid to effect separation, with LOD of 0.07 μg L⁻¹.

1.2.2.11 Tin. An electrochemical hydride generation system, with a polyaniline-modified lead cathode, has been developed for the determination of Sn by AFS.¹¹⁸ The Sn fluorescence signal intensity was improved evidently as the polyaniline membrane could facilitate the transformation process from Sn⁰ to SnH₄ and prevent the aggregation of Sn atoms on the Pb electrode surface.

1.3 Solids

Methods of solids analysis are either direct, where vaporisation, atomisation and excitation of the sample occurs together, such as arc/spark ablation, glow discharge analysis and secondary ion mass spectrometry; or indirect, where vaporisation is separate to atomisation and/or excitation, such as laser ablation or thermal vaporisation coupled with a separate atomisation/excitation source.

1.3.1 Direct methods. Shao¹¹⁹ revisited the idea of direct introduction of powdered samples into an ICP. Internal standards were added to powder samples in solution form and the dried powder samples dispersed using a high speed Ar stream in a custom-made powder dispersion device (PD). The construction and analytical performance of the PD are described. Calibration curves were obtained that were linear around 3 orders of magnitude for a set of synthetic geological standards. Curves of intensity ratio versus concentration for a further set of CRMs of geological sediments with different matrices among the standards were found to be scattered and possible errors in the method PD-ICP analysis were discussed. An additional sample preparation step involving wet-grinding to overcome such errors was proposed.

For a number of years, high-resolution continuum source AAS has been presented as a potential revolution in the field, in particular for direct analysis of complex samples. Resano and Garcia-Ruiz have attempted to evaluate the advantages of the approach, particularly for direct solid sampling.¹²⁰ Three solid sampling applications are discussed, each one highlighting one of the main advantages of this technique. The review also attempts to clarify some misconceptions on the true potential of the instrumentation that is currently commercially available, such as its performance for multielemental analysis.

1.3.1.1 Arc and spark. No publications of note have been published in this well established area of direct solids analysis during the time period covered by this review.
1.3.1.2 Glow discharge. Advances in instrumentation and the parallel development of robust analytical methodologies have fuelled a significant growth of analytical applications with GD techniques. In fact GDs with detection by OES and MS have become fast, comparatively simple and reliable tools for materials characterization at the nanoscale. A critical description of latest advances and presently available GD-OES and GD-MS instrumentation (commercial prototype and laboratory equipment) has been prepared.¹²¹ Analytical strategies developed for the analysis at the surface and for concentration depth profile analysis of thin and ultrathin layers with GDs are also discussed. Selected representative applications and trends of GD-OES and GD-MS techniques for nanometer range analysis (e.g., nanolayers of two-dimension nanostructured materials and molecular depth profiling of polymer-based coatings) are briefly described confirming the increasing analytical value of GD-OES and GD-MS techniques in the nanotechnology field. Although GDMS is best known for its utility in trace element analysis, it can also be used to provide chemical speciation information. GDMS has been applied¹²² to the direct speciation of three iron oxide species in solid state samples. Parameters such as sampling distance, temporal regime, discharge gas pressure, pulse frequency, and duty cycle must be optimised carefully to enable the speciation. For the iron oxides, species specific variations in ratios between Fe⁺ and FeOH⁺ signal intensities provide the ability to discriminate between FeO, Fe₂O₃ and Fe₃O₄.

For several decades and for many applications, rf-GDOES has proved to be a powerful tool for the direct solid bulk materials and in-depth profile analysis. Rf-GD is generally operated in continuous mode, but the interest in pulsed rf-GD) is steadily increasing. Alberts et al.¹²³ have attempted to assess the analytical performance of pulsed rf-GD-OES. The technique was carefully evaluated for the analysis of three conducting materials with different matrices (copper, steel and aluminium) and for bulk homogeneous non-coated glasses of varying thicknesses (1, 1.8 and 2.8 mm). The comparison was performed in terms of crater shapes, sputtering rates and emission yields, demonstrating that increased emission yields could be obtained in pulsed mode working with small pulse widths (i.e. at high pulse frequency and relatively short duty cycle). In addition, pulsed rf-GDs was applied to depth profiling analysis of a thin gold layer (approximately 30 nm) deposited on conductive and insulating substrates and an improvement in depth resolution, compared with continuous rf-GDs was observed. Successful analysis of samples including commercial tinplates and multilayered glass was also performed. Pulsed GD with TOFMS detection has also been used¹²⁴ for the characterisation of nanostructured materials. Two types of nanostructured materials were successfully studied proving that pulsed rf-PGD-TOFMS allows for fast and reliable depth profile analysis as well as for the detection of contaminants introduced during the synthesis process.

The coupling of GDs as ion sources for TOFMS chemical analysis has been extensively investigated during the last two decades. In particular, the last five years have seen a number of important instrumental advances in GD-TOFMS as well as a significant increase in the description of unique analytical applications. Pereiro et al.¹²⁵ provide a useful review covering instrumental developments and recent applications of GD-TOFMS, both for elemental and molecular direct solid analysis and for analytes in gaseous phase. The potential of GD-TOFMS for use in analytical research is also critically discussed.

1.3.1.3 Secondary ion mass spectrometry. SIMS remains one of the most powerful techniques utilized in analytical geochemistry. Wiedenbeck¹²⁶ described the key strength of SIMS as its capacity to provide trace element and isotope data at sampling sizes which are not approached by other methods. As compared to LA-ICP-MS, SIMS commonly provides a total sampling mass some 10 to 500 times smaller; this feature can be the deciding factor as to whether an analytical objective is technically achievable. Additional strengths of SIMS lie in the areas of depth profiling and trace element imaging. Though perhaps not as commonly used in the geosciences, these two operational modes represent unique capabilities of SIMS.

1.3.2 Indirect methods.
1.3.2.1 Laser ablation. The basic principles and recent developments of LA-ICP-MS as a method for the element- and isotope-selective trace analysis of solid materials has been reviewed.¹²⁷ The paper covers aerosol formation/transportation process, quantification issues, and technical aspects concerning the system configuration and ICP operating conditions. The performance of femtosecond (fs) LA-based analyses as one of the most important advancements made over the past few years is discussed. The benefits offered by fs-LA in comparison to LA using nanosecond (ns) laser sources are demonstrated on the basis of oxide layer and silicate glass analyses with different applied calibration strategies. A further review of the use of LA for elemental analysis over the past decade¹²⁸ details the three types of technique that are widely used. In particular, the application of high irradiance laser ionization MS for the rapid determination of elements in solids is discussed.

LA in liquid (LAL) has been evaluated¹²⁹ as a sampling technique for solids. LAL is widely used at present for the micro fabrication or production of nano-particles in various materials. In this study, a solid sample was placed into deionized water, and LA was carried out within the water using a Ti:S femtosecond laser operating at fundamental wavelength (780 nm). The laser-induced sample particles were trapped and collected in the ambient water. Elemental and isotopic fractionation during the LAL sampling was investigated. Five isotopic ratios were measured on the resulting sample particles obtained with various ablation conditions (fluence and repetition rate) using ICP-MS. The data obtained demonstrates the potential of the LAL technique for elemental and isotopic analyses with the careful optimization of laser ablation conditions.

A procedure enabling multi-point calibration of LA-ICP-MS for the analysis of powdered solid samples has been developed.¹³⁰ CRMs available in the form of powders, e.g., soils, sediments or ashes, were used for this purpose. Stable, homogeneous and mechanically resistant targets with immobilized powder were prepared by mixing powdered solids with zinc oxide, which were then solidified by 2-methoxy-4-(2-propenyl)phenol via the formation of a zinc complex. Prepared targets were subjected to LA for multi-element ICP MS measurements. For a number of elements: Al, Ba, Co, Cr, Fe, Mg, Mn, Pb, Sb, U and V, the linear correlation coefficients between the mass of the CRM and the intensity of the signal were above 0.99. The accuracy of the proposed calibration procedure, with the use of matrix matched calibration standards, was demonstrated by successful analysis of the CRMs.

2. Instrumentation and fundamentals

2.1 Instrumentation

This section covers the development of new instrumentation, such as the development of completely new techniques, novel developments in atom cells, excitation sources, monochromators, mass spectrometers and detectors. Also included in this section is a discussion of new research into the attenuation of interferences by instrumental methods.

A new tungsten coil atomiser for AAS and AES has been evaluated¹³¹ which provides a more isothermal environment around the atomizer, more concentrated atomic cloud and a smaller background signal, resulting in LODs of e.g. 0.6 ppm for Cu (324.7 nm) by AAS and 4.5 ppm for Cr (425.4 nm) by AES. A portable liquid electrode plasma (LEP) atomic emission spectrometer has been described.^132,133 The instrument requires no plasma gas and has been used for in situ analysis of preconcentrated extracts such as 1 M HCl soil leachate, with RSD of 20%. Plasma excited AAS has been developed for on-line analysis of industrial waters,¹³⁴ in which samples are injected into, and atomised in, a nitrogen plasma jet. It is not clear from the abstract how this facilitates AA measurements with LOD of 0.25 ppm for Cu. A wavelength calibration method for flat-field grating spectrometers has been developed which provides information about the alignment of optical elements as well as higher calibration accuracy.¹³⁵

Developments in detectors for AES and MS continue, with a report¹³⁶ on the evaluation of a fourth-generation focal plane detector, containing 1696 Faraday-strip detectors fitted to a Mattauch-Herzog MS, detailing LODs in the single to tens of ng L⁻¹ range for most elements, LDR of 10⁹ and IR precision of better than 0.02%. In a related paper,¹³⁷ a similar detector has been evaluated for use with SIMS and parallel detection of ²⁹Si and ²⁸SiH or ¹⁸O and ¹⁶OD at a sensitivity level of 230 counts s⁻¹. A so-called ion CCD detector has been developed¹³⁸ by replacing the semiconductor part of the CCD pixel with a conductor, and used in conjunction with Mattauch-Herzog MS for the detection of electrons and charged biomolecular ions. A high resolution Rutherford Backscattering method has been developed¹³⁹ for the determination of trace elements in thin films of blood dried on a carbon substrate with lod of 50 ppm for Fe. A solar-blind UV detector, based on a Zr_xTi_1−xO₂ thin film prepared by a sol–gel method, has been developed¹⁴⁰ with dark current of less than 7 nA at 5 V bias and responsivity rate of 470 A W⁻¹ at 270 nm.

Miniaturisation is a theme which has been prominent in recent years.¹⁴¹ A low power CCP micro-torch (30 W, 13.56 MHz, 0.5 L min⁻¹ Ar) has been coupled with a commercially available microspectrometer for multi-element analysis by AES, with LODs in the range 0.003 and 1.5 μg mL⁻¹ for Li and Mn respectively.¹⁴² Staying with the mini theme, a novel detector for GC, based on plasma excitation and AES, has been developed¹⁴³ which comprises a microhollow cathode surrounded by an array of elliptically shaped collectors milled out of aluminium which couple light, via fibre-optics, into a spectrometer.

The attenuation of interferences is a topic which continues to exercise researchers. A collision reaction interface (CRI) has been utilized¹⁴⁴ for the reduction of spectral interferences in ICP-MS, by introduction of hydrogen through the sampler or skimmer cones for correcting polyatomic interferences on ⁷⁵As⁺ and Se isotopes in matrixes containing up to 2% v/v chloride. A different approach¹⁴⁵ to correction of isobaric interferences in ICP-MS has been taken whereby the ArX⁺ signals are used to correct for variations in the signal of the interfering rather than the analyte ion; the authors claiming comparable results to those obtained with a collision–reaction interface.

Multivariate optimisation has been used to minimize interferences in Ar–N₂ mixed gas ICP-MS,¹⁴⁶ with 0.13% v/v N₂ in the plasma gas and 0.11% in the central channel as a sheath gas around the nebulizer gas flow found to be optimal for interference-free determinations in the presence of 0.1 M Na.

Online electrothermal heating in the temperature range between 900 and 1200 °C has been used¹⁴⁷ for attenuating isobaric interferences in LA-ICP-MS by up to 10³ for volatile interfering elements such as Ag and Cd.

A standard addition and indicative dilution method has been developed which consists of addition of a standard solution to a sample and then successive dilution of this until the signal is equal to that produced by the undiluted sample.¹⁴⁸ The authors claim that this approach works better than the normal standard additions approach providing the interferences are not multiplicative.

2.2 Fundamentals

Coverage is confined to a consideration of fundamental studies related to instrumental methods used primarily for analytical atomic spectrometry, and excludes studies related to, e.g., astronomy and sputtering sources except where particularly relevant.

The father of GFAAS, Boris L'Vov, has written a history of the formation and development of the theory of solid-state decomposition reactions.¹⁴⁹ The essay is based on a discussion of the mechanism of congruent dissociative vaporization of a solid with simultaneous condensation of the supersaturated vapour of the low-volatility product. The review covers a 30-year period (1981–2010), beginning with basic experimental studies in the decomposition process by ETAAS and QMS and ending with measurements of the decomposition kinetics by thermogravimetric analysis. The conclusion contains remarks and recommendations based on the L'Vov's long term participation in both the development of ETAAS and in the theory of solid-state reactions.

A variety of analytical techniques were used to characterise the condensed phase beryllium species occurring on graphite and tungsten platforms in the presence of aluminium and silicon matrices.¹⁵⁰ The solid residue was viewed by SEM, while the chemical composition was probed by ED-XRF, FTIR spectrometry and Raman microanalysis. Beryllium oxide phases were found to be the predominant species over a wide temperature range, persisting up to about 1800 °C on the graphite platform. Beryllium metal species were also identified at high temperatures (1500 °C), but the transformation of beryllium oxide to beryllium was influenced by the amount and localised behaviour of concomitant species on the platform surface. For graphite platform atomisation, aluminium and silicon concomitants are present as metal oxides. Other silicon species, such as silicon carbide, were found mainly at temperatures above 900 °C. Little or no beryllium oxide was found on tungsten platforms up to 1800 °C, although there was evidence of some beryllium alloyed to tungsten. Tungsten from the platform supports some hydration forming different tungsten oxidation states (W⁶⁺, W⁵⁺, W⁴⁺). Also, at 900 °C, silicon was present as an oxide, but also as elemental silicon, silicon carbide, and silicon alloyed to tungsten forming tungsten disilicide at the surface interface. When tungsten platform atomisation was used for samples containing silicon, evidence of degradation of the graphite tube through formation of carbon clusters and nanostructures was more easily noticeable and evaluated by Raman spectrometry.

Ilyashenko et al.¹⁵¹ describe a model used for the verification of measurements made in ETAAS. The basis of the approach involves comparing the atomisation parameters obtained from samples and from calibration standards. The closer the atomisation behaviour in sample and standard, the more reliable the results are perceived to be. CRMs were analysed for heavy metal content to illustrate the feasibility of the method.

Effective simultaneous multi-element ETAAS demands rigid requirements of the design and operation of the atomiser. The system must provide a high degree of atomisation for the selected analytes, independent of the vaporisation kinetics and heating ramp residence time of atoms in the absorption volume and absence of memory effects from major sample components. Use of a low resolution spectrometer with a continuum radiation source (for simultaneous multi-element ETAAS) offers reduced sensitivity compared with traditional ETAAS. However, this reduction can be compensated, at least partially, by creating high density of atomic vapour in the absorption pulse. This has been achieved¹⁵² by using a longitudinally heated graphite atomiser, 18 mm in length and 2.5 mm in internal diameter fitted with a 2–4.5 mg ring shaped carbon fiber yarn collector. The collector is located next to the sampling port and provides a large substrate area that helps to keep the sample and its residue in the central part of the tube after drying. The collector also provides a “platform” effect that delays the vaporization and stipulates vapour release into an absorption volume that has reached a stabilised gas temperature. This furnace system allows direct simultaneous determination of a group of elements within 3–4 order concentration range. Limits of detection are close to those for sequential single element determination in FAAS.

Few fundamental studies of plasmas have been published during the review period, presumably reflecting the maturity of these atom cells. An LTE modelling approach has been used¹⁵³ to determine T_ion and n_e in an argon ICP from MS measurements of ion ratios and their dependence on ionisation potentials, with results of 7000–8000 K and 10¹⁵ cm⁻³ respectively. Similarly, refinements in the method for the determination of T_gas using polyatomic ions in ICP-MS have been published,¹⁵⁴ with several main conclusions. First, excited electronic states should be included for ArO⁺, neutral NO, and O₂; second, a 10% error in the solvent load, sample gas flow rate, vibrational constant, rotational constant or measured ion ratio produces only a 1 to 3% error in T_gas; third, a 10% error in dissociation energy results in a 10% error in T_gas; and high temperature corrections to the partition functions can generally be neglected. The temperature of an argon-hydrogen MIP operated at 915 MHz was determined to be ca. 7000 K,¹⁵⁵ though the intended use was for deposition of silicon coatings. PCA has been used to study intensity enhancements of 34 atomic spectral lines belonging to 33 elements and 35 ionic spectral lines belonging to 23 elements in an oscillating DCP.¹⁵⁶ Ionisation energies, oxide bond strengths, energies of vaporisation, atomisation, and excitation were also considered in the analysis, resulting in negative correlation between enhancements and first ionisation energies for atomic lines; and positive correlation with the sum of the first ionisation energies and oxide bond energies for ionic lines, with four distinctive groups of elements.

A novel method for the determination of pressure in rock vapour plumes has been proposed¹⁵⁷ which makes use of line broadening of Fe I at 381.58 nm and Ca I at 646.26 nm, with Stark broadening predominant for >1% ionization whereas resonance and van der Waals broadening predominates for <1% ionization.

3. Chemometrics

For the purposes of this review, chemometrics relates to a range of mathematical techniques and algorithms used for the interpretation of analytical data, correction of interferences, optimization and instrument calibration.

A number of papers describing the application of chemometrics for comparisons of datasets are normally published during the review period, and this year is no exception. There is generally very little novelty in this. However, the interested reader is directed to one review of the application of ICP-MS in plant science, which includes the use of chemometrics on multi-element datasets.¹⁵⁸

Some new chemometric algorithms have been used in spectroscopy, such as the Tikhonov regularization which is a multivariate calibration method with numerous variants such as ridge regression, which has been used for near-infrared, ultraviolet visible, and synthetic spectral data sets.¹⁵⁹ A new method called sequential PCA has been used to identify the atmospheric sources of trace elements (determined by ICP-MS) in rainwater.¹⁶⁰ The method of angle constrained alternating least squares has been used¹⁶¹ to enhance contrast in either the spectral or concentration domain by obtaining unique solutions for variables in mixture datasets such as m/z values in mass spectrometry. A neural network has been used in conjunction with AES analysis of emission from a plasma processing chamber to detect leaks, presumably because the spectral emission characteristics change.¹⁶²

A generalized calibration method for analytical chemistry has been proposed¹⁶³ which combines sample dilution and standard additions such that, at each stage of sample dilution, the result is estimated from six values which respond differently to errors caused by interference and non-linearity. A related paper describes its use for the determination of Cr.¹⁶⁴

4. Isotope analysis

4.1 Reviews

The Commission on Isotopic Abundances and Atomic Weights (CIAAW) of the IUPAC published its last update of the isotopic compositions of the elements as determined by isotope-ratio mass spectrometry in 2009,¹⁶⁵ which included a critical evaluation of the literature and a table of the isotope abundances for a particular sample and representative isotope abundances and uncertainties that cover variations in normal terrestrial materials.

Several reviews of a general nature have been published relating to ICP-MS¹⁶⁶ and both ICP-MS and GD-MS.¹⁶⁷ The development of magnetic resonance mass spectrometry (MRMS) and radio frequency mass spectrometry (RFMS) has been reviewed.¹⁶⁸ These instruments have been around since the ‘50s (and are distinct from FT-ICR-MS instruments) and have been used to determine many fundamental physical constants such as the magnetic moment of the proton in nuclear magnetons and tritium half-life.

Many reviews focus on specific fields of research, namely: isotope ratio mass spectrometry (IRMS) in the earth and environmental sciences;¹⁶⁹ the application of LA-ICP-MS in general¹⁷⁰ and for use in geological research;¹⁷¹ the use of ICP-MS for provenancing purposes;¹⁷² the study of fractionation effects for non-traditional or heavy isotopes;^173,174reference materials for geological research;^175,176 and the application of SIMS in geological research;¹²⁶

4.2 Isotope dilution analysis (IDA)

Isotope dilution analysis is a method used for quantification whereby an isotopically enriched spike of the analyte is added to the sample, and the modified isotopic ratios can be measured and used to determine the concentration of the analyte. When the detector is a mass spectrometer (as is often the case) the technique is referred to as isotope dilution mass spectrometry (IDMS). This review will cover advances in IDA for determination of total concentrations of elements using atomic emission, absorption, fluorescence and mass spectrometry, but not including radiochemical techniques.

The use of IDMS for the certification of reference materials has been reviewed,¹⁷⁷ the relative merits of ICP-MS and TIMS compared, and sources of errors, bias and IR correction models discussed.

The accuracy and precision achievable using IDMS are exemplified by its use in an international project to determine Avogadro's constant using MC-ICP-MS,¹⁷⁸ wherein the atoms in two 1 kg crystal spheres of isotopically enriched silicon were ‘counted’. Several papers have been published which report the steps in this process.^179–181 The molar mass of Si in an artificial silicon crystal material highly enriched in ²⁸Si was determined to be 27.97697027(23) g mol⁻¹ with u_rel = 8.2 × 10⁻⁹ (k = 1), resulting in a value of 6.02214082(18) × 10²³ mol⁻¹ for Avogadro's constant.

A novel method for the determination of Ni in natural waters has been published¹⁸² which involved the extraction of Ni into MIBK as a Ni-diethyldithiocarbamate complex and direct analysis using nitrogen MIP-MS which had been doped with isotopically enriched oxygen, presumably to act as the isotopic spike for the O-containing Ni complex.

Accelerator mass spectrometry (AMS) for IDA continues to grow in popularity now that more compact systems are available. Methods have been developed for: the determination of tritium,¹⁸³ an isotope which occurs at extremely low concentrations as a by-product of nuclear reactors and in landfill leachate; and for the measurement of ¹²⁶Sn, via the formation of SnF₂ as the target and selection of SnF₃⁻ molecular ions, yielding a sensitivity of 1.92 (±1.13) × 10⁻¹⁰ for the ¹²⁶Sn/Sn ratio.

Several methods have been developed for absolute protein quantification by ID-ICP-MS using isotopically labelled tags. For example, Eu labelled metal coded affinity tags (MeCATs) have been used to label peptides¹⁸⁴ with subsequent IDA for quantification. A separate group has also used ¹⁵³Eu labelling by conjugating proteins with an element tag (1,4,7,10-tetraazacyclododecane-1,4,7-trisacetic acid-10-maleimidoethylacetamide loaded with europium).¹⁸⁵ Contrastingly, LA-ICP-MS has been used for species-specific IDA of transferrin in human serum using an isotopically enriched ⁵⁷Fe-transferrin complex after separation by gel electrophoresis. The authors reported ‘good agreement’ with a serum certified reference material (ERM-DA470k/IFCC) with RSD of between 0.9 and 2.7%.

Online spiking for IDA is often used for these type of analyses where a chromatographic separation is performed prior to detection using ICP-MS.³ A method has been proposed¹⁸⁶ whereby a post-column, online double spike is introduced and the mass flow of sample and spike solution is measured with a balance, thereby eliminating errors due to changes in viscosity.

Developments in geological research include a method for the simultaneous ID-ICP-MS determination of Ca, Mg and Sr using a mixed spike of ²⁵Mg, ⁴³Ca and ⁸⁷Sr,¹⁸⁷ with the long-term precision of 0.4% (2s) RSD achieved for the calculation of Mg/Ca and Sr/Ca ratios in calcium carbonate.

IDMS is widely used in the nuclear industry, for which reliable isotopically enriched standards are required. The IRMM has undertaken an intercalibration exercise¹⁸⁸ using various Pu spike reference materials for isotopes ²³⁹Pu, ²⁴⁰Pu, ²⁴²Pu and ²⁴⁴Pu and present results with uncertainties calculated using the GUM protocol.

4.3 Isotope ratio analysis (IRA)

Isotope ratio analysis is a method used to obtain precise measurements of isotopic abundance, used primarily for geochronology, isotope tracer studies and isotopic fingerprinting. This often involves novel developments in sample preparation and introduction methods.

4.3.1 New developments. There has been some activity in the development of new instrumental methods for IRA. A prototype instrument based on off-axis integrated cavity output laser spectroscopy has been used to measure the carbon and oxygen isotope composition of CO₂ gas evolved from the acidification of carbonate minerals,¹⁸⁹ by utilizing the absorption spectra of ¹²C¹⁶O¹⁶O, ¹³C¹⁶O¹⁶O, and ¹²C¹⁶O¹⁸O in the near-infrared wavelength region, and yielding precision (1s) of better than 0.15 ppth and 0.6 ppth for δ¹³C and δ¹⁸O respectively. Electrodeless discharge lamps (EDLs) have been prepared¹⁹⁰ to study the emission spectra of ²³²U using μg amounts of solid uranium oxides and aqueous solutions of uranium nitrate to form uranium tetraiodide by a high temperature iodination reaction in the presence of Ne. A new method called laser ablation molecular isotopic spectrometry (LAMIS) has been developed¹⁹¹ to perform optical isotopic analysis in ambient air, from a distance and without sample preparation. The technique, which is a modification of LIBS, uses radiative transitions from molecular species of H, B, C and O, vaporized from the sample, which have larger isotopic shifts relative to atomic spectra.

Research into methods for improving the accuracy and precision of IR measurements continue. The correction of instrumental mass fractionation effects is a perennial problem in IRMS. One study¹⁹² investigated calculation methods for direct internal correction by TIMS and MC-ICP-MS for elements having at least two internal reference isotopic ratios. In the former case, methods based on both power and exponential laws were derived, whereas in the latter only an exponential law was considered due to the larger mass fractionation effects. A model and equations have been developed to determine the maximum level of contamination that can be tolerated for IRMS using SF-ICP-MS.¹⁹³ The authors concluded that the often used 1000 : 1 sample-to-contaminant concentration limit can sometimes underestimate the error compared to their model calculations. The effect of sulfate on δ³⁰Si and δ²⁹Si measurements in natural waters by MC-ICP-MS has been studied,¹⁹⁴ with the recommendation that samples are matrix matched by the addition of an excess of sulfate to prevent mass bias. Odd-even isotope, mass independent W isotopes has been observed in MC-ICP-MS,¹⁹⁵ possibly related to the high first ionization potential of W, implying that other elements with I.P. > = 8 eV may also exhibit mass independent instrumental bias. Fractionation during IRA using LA-MC-ICP-MS of ¹⁴⁷Sm and ¹⁴⁴Nd has been corrected¹⁹⁶ using an independently defined isochron for an Archean monazite standard, for 16–37 μm single spots, with comparable precision and accuracy to TIMS.

Compensation for detector dead-time is another important factor which has been found to be voltage dependent for an electron multiplier,¹⁹⁷ with only a 2 ns disparity between actual and used dead-time giving rise to significant non-linearity. The method of averaging a number of ratios collected over the course of a measurement in SIMS has been shown to cause a positive bias,¹⁹⁸ and the authors propose the use of Beale's estimator when the bias from the summed counts is unacceptably large.

A metrological triangle has been investigated¹⁹⁹ for the measurement of isotope amount ratios of Ag, In and Sb using MC-ICP-MS, wherein Sb and In are calibrated against Ag by construction of three pairs of interelement isotope amount ratios, the so-called ‘Harvard Method’.

Broadening the bandwidth of the first laser in three-colour, three-photon ionisation RIMS, from a bandwidth of 1.8 GHz to about 10 GHz, has been found to decrease laser-induced isotopic fractionation during ²³⁵U : ²³⁸U IR measurement from 10% to 0.5%.²⁰⁰

Somewhat worryingly, the first international inter-laboratory comparison of AMS for measurement of ³⁶Cl : Cl ratios in AgCl found that ³⁶Cl : Cl data from two individual AMS laboratories can differ by up to 17%!²⁰¹

High spatial resolution analysis is an area which attracts much attention. Laser assisted atom probe microscopy has been used to achieve depth and lateral resolution of 5–15 nm thick ²⁸Si and ³⁰Si enriched layers of single crystalline silicon,²⁰² thereby enabling the construction of a three-dimensional image. IRA of sub-micrometer Pu particles of NBL SRM-947 has been performed²⁰³ for nuclear forensics using ICP-MS after chemical separation of Pu and Am. Similarly, IRA of sub-micron uranium oxide particles using TIMS,^204,205 and the development of software that can aid in the initial location and IRA of sub-micron U particles using SIMS²⁰⁶ have been reported.

One of the methods used most widely to achieve high spatial resolution is laser ablation (LA). One novel LA-IRMS method²⁰⁷ for the measurement of δ¹³C, with 50 μm resolution along a piece of hair, involved quantitative combustion of the ablated particles to CO₂, with subsequent cryo-focusing, before detection using IRMS. Often, transient signals result from this mode of sample introduction so particular protocols are required to achieve accurate and precise IR measurements. One such²⁰⁸ utilised individual synthetic fluid inclusions of known Pb and Tl isotopic compositions, prepared from dissolved NIST SRM 981 with or without SRM 997, enclosed in quartz by a hydrothermal crack annealing technique, and analysed using 193 nm LA-MC-ICP-MS to yield external precision of ±0.011% (2s) for ²⁰⁸Pb : ²⁰⁶Pb.

Ion microprobe MS is another technique used to achieve high spatial resolution. Extremely small samples present particular challenges to the analyst; this has been addressed²⁰⁹ by the development of a sample holder disk with multiple holes, in which epoxy disks containing a single unknown sample and a standard grain can be cast and polished. Chondrule particles were located within the 500 μm and 1 mm radius of the centres of the respective sized holes for δ¹⁷O measurement.

The development of new IRA methods for particular elements is an area of considerable research interest. In particular, IRA of ¹¹B : ¹⁰B isotope amount ratios has been reported for TIMS,²¹⁰ using the Cs₂BO₂⁺ ion for measurement and the addition of mannitol to suppress B volatilisation during pretreatment. Similarly, SF-ICP-MS has also been used for IR measurement of δ¹¹B.²¹¹ A demountable DIHEN nebuliser has been used with MC-ICP-MS for IR measurement of B isotopes,²¹² the obvious advantage being that the solution does not come into contact with glassware so washout time was reduced ten-fold, and sensitivity improved by a factor of 2–5 compared with the traditional Scott/cyclonic spray chamber and nebuliser arrangement. Precision of 0.25 ppt (2s) was achieved for measurement of ¹¹B : ¹⁰B isotope amount ratios.

The use of SIMS for IR of O, S and Fe isotopes has been reported,²¹³ with routine precision of 0.3 ppt for measurement of ¹⁸O : ¹⁶O isotope amount ratios using MC Faraday cup detectors and a 10 μm Cs⁺ beam.

AMS has been used in the search for long-lived isotopes of the superheavy element roentgenium (Rg, Z = 111) in gold.²¹⁴ The authors observed two and nine events for ²⁹¹Rg and ²⁹⁴Rg respectively resulting in an abundance in the 10⁻¹⁵ range, however, high background signal hampered positive identification as Rg isotopes.

High resolution ICP-AES has been used for IRA of U isotopes,²¹⁵ by analysis of ²³⁵U at 424.412 nm and ²³⁸U at 424.437 nm in depleted and enriched samples. The authors cite the lack of requirements for matrix separation and mass bias correction as advantages compared to more sophisticated MS methods, and propose that the method can be used for rapid, fit for purpose IR measurement with reasonable accuracy (<1.5%) and precision (1%). Extractive electrospray ionisation (EESI) MS has been used²¹⁶ for rapid IR measurement of ²³⁵U : ²³⁸U in uranyl nitrate samples prepared from natural waters, ores and soils by nitric acid extraction, with 0.21–0.25% accuracy and 1.54–1.81% RSD.

The addition of 250 ppm Ge and 500 ppm Re with colloidal silica was used to enhance the ion beam intensity for IRA of Pb using TIMS,²¹⁷ while methods have been developed for IRA of K (TIMS)²¹⁸ and Mg.²¹⁹

There have been some new developments in sample introduction methods. A novel sample cone, the jet cone, has been tested with MC-ICP-MS for IRA of Hf and Pb at ‘sub-ng’ levels, with RSD for Pb isotope amount ratios ranging between 0.08% and 0.36% depending on isotopic abundance and concentration. A torch integrated sample introduction system has been used²²⁰ with liquid flow rates as low as 10–15 μL min⁻¹ for IR-MC-ICP-MS. The authors found that flow rate had a large effect on mass discrimination and recommend the use of a syringe pump to avoid errors arising from free aspiration, which were typically 0.05 ppth for 1–2% fluctuations at 10 μL min⁻¹. The sample introduction system for IRA of Os has been modified²²¹ by using a small glass connector to transfer OsO₄ vapour directly after dissolution and in situ distillation from a Carius tube into a nebuliser without any prior separation procedure.

Several radiochemical methods have been adapted for MS. One such is the determination of ²²⁸Ra : ²²⁶Ra isotope amount ratios in seawater samples by MC-ICP-MS after purification by ion exchange.²²² Better precision and low LOD was reported compared to the radiochemical method.

4.3.2 Geological studies. Developments in this field fall into two broad categories, namely isotopic fractionation and geochronology.

Fractionation of isotopes during geological processes can occur for heavy elements so there is considerable research on the development of new IR methods for a wide range of isotope systems. The use of MC-ICP-MS for fractionation studies of heavy elements has been overviewed,²²³ with particular emphasis on Hg. The development of methods for the precise IR measurement of Mg isotopes has attracted much recent attention and has been reviewed.²²⁴ The use of the double-spike technique has been used in conjunction with MC-ICP-MS to investigate the fractionation of Zn isotopes,²²⁵ with ±0.05 ppth (2s) precision for δ⁶⁶Zn (JMC Lyon Zn), in soil samples to determine plant availability; and δ⁵³Cr (NBS 979),²²⁶ in carbonates in continental crust and the terrestrial mantle with ±0.031 ppth precision (2s).

There have been a number of publications on the use of LA-ICP-MS for IRA of B isotopes in situ,^227,228 rather than the traditional method of solution analysis by TIMS which requires lengthy sample preparation. Numerous publication on the use of B isotope ratios as a pH proxy have appeared, but few detail advances in instrumental or methodological developments. One paper of note²²⁹ utilizes ¹¹B MAS NMR and EELS to determine the speciation of trigonal B(OH)₃ and tetrahedral B(OH)₄⁻. Ion microprobe SIMS is capable of extremely high spatial resolution, and has been used to study fractionation of Fe isotopes,²³⁰ with 0.2–0.3 ppth (2s) precision compared with 0.1 ppth for MC-ICP-MS for δ⁵⁶Fe measurements in magnetite. High-resolution multiple sulfur isotope analysis in sandstone has also been performed using nano-SIMS to determine δ³³S (VCDT) values between −1.65 and +1.43 ppth and δ³⁴S between −12 and +6 ppth in pyrite grains over only 5–10 μm.

Isotope ratio analysis is widely applied for geochronology. There have been numerous papers on the application of LA-MC-ICP-MS,²³¹ SIMS and SHRIMP²³² to the U–Pb dating of zircons because of the extremely high in situ spatial resolution afforded by these techniques, however, these are primarily applications, and few new instrumental developments are reported. One exception is the use of SIMS for U–Th dating of fine-grained zircons at <5 μm scale²³³ by using Gaussian mode primary O₂⁻ and O⁻ probes of 5.2 μm and 4.5 μm i.d. respectively, with beam intensities of ca. 100 pA, yielding precision of 1–2%. An algorithm for U–Pb IR uncertainty propagation has been developed²³⁴ which, the authors claim, incorporates all input uncertainties and correlations without limiting or simplifying covariance terms to propagate them though intermediate calculations. This should allow a more transparent method of uncertainty propagation without making assumptions about the various contributions.

A new method has been developed for IRA of Ar, the gas being extracted from the sample by successively heating the samples with a continuous infrared laser, pre-concentrated into a capillary, separated by GC, then analysed by IR-MS at the 10⁻¹² g level.

4.3.3 Stable isotope tracer studies. Stable isotopes are often used as tracers during in vivo experiments to determine uptake or metabolic pathways, or to study the source and fate of environmental pollutants. There have been very few new developments in this field, although, the use of stable isotope tracers to study the course and fate of Hg in the environment has been reviewed (110 references).²³⁵ One novel development is the use of ¹⁸O labelling and SIMS for the ‘direct observation’ of oxygen/oxide ion motion at the cathode/interlayer/electrolyte interfaces in an oxide fuel cell.²³⁶ A similar approach has been used to study the electrochemically active region of oxygen incorporation into yttria stabilised zirconia by means of ¹⁸O tracer incorporation on dense Pt thin film microelectrodes and analysis by TOF-SIMS.²³⁷

4.3.4 Isotopic fingerprinting. IRA is often used to elucidate the origin of archaeological artefacts, extra-terrestrial and biological specimens, for food authentication, or in nuclear forensics.

The use of ICP-MS and IRA for provenancing studies has been reviewed.²³⁸ MC-ICP-MS has been used for high precision IRA of Mg to determine the absolute composition of the Earth's mantle, and hence that of bulk silicate earth,²³⁹ yielding values of ²⁵Mg : ²⁴Mg = 0.126896 ± 0.000025 and ²⁶Mg : ²⁴Mg = 0.139652 ± 0.000033. The authors propose that this provides a basis for comparison with suspected extra-terrestrial specimens. A combination of MC-ICP-MS and IRMS has been used for IRA of Sr and O respectively²⁴⁰ to determine the origin of green coffees after PCA, with good discrimination between South American and islands point of origin. This statistical approach is also evident in a study in which Q-ICP-MS was used²⁴¹ to determine ¹¹B : ¹⁰B and ⁸⁷Sr : ⁸⁶Sr isotope amount ratios to distinguish between South African wines. The use of B isotope ratios together with elemental concentrations and LDA was found to be the most successful approach.

In the nuclear forensics field, isobaric interference of ²³⁸U on ²³⁸Pu has been reduced by using 0.4 to 1.5 mL min⁻¹ CO₂ as a reactive gas in a collision-reaction cell coupled with MC-ICP-MS,²⁴² thereby facilitating IRA of U and Pu isotopes for nuclear waste management, with only 0.38% and 0.11% disparity from a chemical separation method respectively. Staying with the nuclear theme, a method has been developed²⁰⁴ using SEM to identify uranium oxide particles <1 μm in diameter and then TIMS to determine the U and Pu isotope amount ratios at the fg level.

5. Glossary of abbreviations

Whenever suitable, elements may be referred to by their chemical symbols and compounds by their formulae. The following abbreviations may be used without definition. Abbreviations also cover the plural form.

8-HQ	8-Hydroxy quinoline
AA	Atomic absorption
AAS	Atomic absorption spectrometry
AAS	Atomic absorption spectrometry
AFS	Atomic fluorescence spectrometry
AMS	Accelerator mass spectrometry
APDC	Ammonium pyrrolidine dithiocarbamate
CCD	Charge coupled detector
CCP	Capacitively coupled plasma
CHG	Chemical hydride generation
CNT	Carbon nano-tube
CPE	Cloud point extraction
CRI	Collision reaction interface
CRM	Certified reference materials
CS	Continuum source
CV	Cold vapour
CV-AAS	Cold vapour atomic absorption spectrometry
CVG	Cold vapour generation
DCP	Direct current plasma
DDTC	Diethyldithiocarbamate
DIHEN	Direct injection high efficiency nebulizer
DLLME	Dispersive liquid–liquid microextraction
DMA	Dimethylarsinic acid
DPC	Diphenylthiocarbazone
ECVG	Electrolytic cold vapour generation
EDTA	Ethylenediaminetetraacetic acid
EDXRF	Energy dispersive X ray fluorescence
EELS	Energy electron loss spectroscopy
ETAAS	Electrothermal atomic absorption spectrometry
ETV	Electrothermal vaporization
FAAS	Flame atomic absorption spectrometry
FI	Flow injection
FIA	Flow injection analysis
FTIR	Fourier transform infrared spectroscopy
GC	Gas chromatography
GD	Glow discharge
GD-MS	Glow discharge mass spectrometry
GD-OES	Glow discharge optical emission spectroscopy
GF	Graphite furnace
GFAAS	Graphite furnace atomic absorption spectrometry
GUM	Guide to the expression of uncertainty in measurement
HG	Hydride generation
HG-AAS	Hydride generation atomic absorption spectrometry
HPCN	High performance concentric nebuliser
HPLC	High-performance liquid chromatography
ICP	Inductively coupled plasma
ICP-AES	Inductively coupled plasma atomic emission spectrometry
ICP-MS	Inductively coupled plasma mass spectrometry
ICP-OES	Inductively coupled plasma optical emission spectrometry
IDA	Isotope dilution analysis
IDMS	Isotope dilution mass spectrometry
IR	Isotope ratio
IRA	Isotope ratio analysis
IRMM	Institute for Reference Materials and Measurements
IRMS	Isotope ratio mass spectrometry
IUPAC	International Union of Pure and Applied Chemistry
LA	Laser ablation
LA-ICP-MS	Laser ablation inductively coupled plasma mass spectrometry
LEP	Liquid electrode plasma
LIBS	Laser induced breakdown spectroscopy
LIPS	Laser induced plasma spectroscopy
LLME	Liquid–liquid microextraction
LOD	Limit of detection
LOQ	Limit of quantification
LTE	Local thermal equilibrium
MA	Methylarsonic acid
MC-ICP-MS	Multicollector inductively coupled plasma mass spectrometry
MIBK	Methyl isobutylketone
MIP	Microwave induced plasma
MIP-AES	Microwave induced plasma atomic emission spectrometry
MIP-MS	Microwave induced plasma mass spectrometry
MIP-OES	Microwave induced plasma optical emission spectroscopy
MMA	Monomethyl arsenic
MRMS	Magnetic resonance mass spectrometry
MS	Mass spectrometry
MWCNT	Multi-wall carbon nanotubes
n e	Electron number density
NBL	New Brunswick Laboratory
NIST	National Institute of Standards and Technology
NMR	Nuclear magnetic resonance
OES	Optical emission spectroscopy
PCA	Principal component analysis
ppb	Parts per billion
ppm	Parts per million
PTFE	Poly(tetrafluoroethylene)
QMS	Quadrupole mass spectrometry
QT	Quartz tube
rf	Radio frequency
RFMS	Radio frequency mass spectrometry
RIMS	Resonance ionization mass spectrometry
RSD	Relative standard deviation
SEM	Scanning electron microscopy
SF-ICP-MS	Sector field inductively coupled plasma mass spectrometry
SHRIMP	Sensitive high mass resolution ion microprobe
SIMS	Secondary ion mass spectrometry
SPE	Solid phase extraction
SRM	Standard reference material
T e	Electron temperature
T gas	Gas temperature
TIMS	Thermal ionization mass spectrometry
T ion	Ionization temperature
TMAH	Tetramethylammonium hydroxide
TOFMS	Time of flight mass spectrometry
USN	Ultrasonic nebuliser
v/v	Volume per volume
VCDT	Vienna Canyon Diablo Troilite
w/v	Weight per volume
XRF	X-ray fluorescence.

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