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Sample Digestion Methods for the Determination of Traces of Precious Metals by Spectrometric Techniques

Sample Digestion Methods for the Determination of Traces of Precious Metals by Spectrometric Techniques

ANALYTICAL SCIENCES JULY 2002, VOL. 18 737 2002 © The Japan Society for Analytical Chemistry

Reviews Sample Digestion Methods for the Determination of Traces of Precious Metals by Spectrometric Techniques

Maria BALCERZAK

Department of Analytical Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland

Recent advances in digestion methods used in the analysis of precious metal samples by spectrometric techniques are reviewed. The applicability of a fire assay, a wet acid treatment, chlorination and alkaline oxidizing fusion to a quantitative recovery of metals from various materials is discussed. Data on the precious metal contents obtained by using particular digestion methods as well as UV-VIS spectrophotometry, atomic absorption spectrometry, atomic emission spectrometry and inductively coupled plasma mass spectrometry in the examination of various samples are tabulated.

(Received November 26, 2001; Accepted April 4, 2002)

1 Introduction 737 5 Oxidizing Fusion 746 2 Fire Assay 738 6 Conclusions 748 3 Wet Acid Treatment 739 7 References 748 4 Chlorination 741

sensitivity, selectivity and reliability have been carried out. 1 Introduction Spectrometric techniques, ultra-violet visible (UV-VIS) spectrophotometry, atomic absorption spectrometry (both flame The members of the group metals (PGM) (, (FAAS) and graphite furnace (GFAAS) techniques), inductively rhodium, , , and platinum) and gold coupled plasma combined with atomic emission spectrometry are called “precious” or “noble” metals. These names have (ICP-AES) or mass spectrometry (ICP-MS), are widely applied roots in the unique physical and chemical properties of the in the analysis of a variety of samples containing noble metals metals, owing to their low abundance and high economic value. over a large range of concentrations.3Ð14 Precious metals are colored and lustrous, exceptionally stable, UV-VIS spectrophotometry was historically the first hard, malleable, electrically resistant and inert to chemical instrumental technique used for the quantification of small attacks. amounts of noble metals in various materials. The technique Nobility and catalytic activity are unique properties of requires a quantitative conversion of the analytes into stable precious metals that result in their wide applications, e.g. as complexes that can act as the basis of spectrophotometric catalysts in various chemical processes, in electrical and measurements. The high chemical similarity of noble metals, electronic industries, and in jewellery.1 Growing demand for resulting in the formation of complexes of similar compositions rhodium, palladium and platinum has resulted from and properties, limits the direct application of UV-VIS autocatalysts production. Since 1978, platinum complexes, spectrophotometry in the analysis of multi-component samples. cisplatin and second-generation compounds, have been applied The developed methods are usually combined with separation in chemotherapy as effective anticancer drugs.2 The procedures.15 Recent generations of ultra violet-visible investigation of ruthenium complexes as an alternative to spectrophotometers that can operate in the derivative mode or platinum cancer inhibitors has recently become a subject of allow direct numerical processing of the absorption spectra of extensive studies. the examined mixtures have substantially extended the The large variety and complexity of examined materials, wide possibilities of spectrophotometric methods. Derivative concentration ranges to be determined (from ng gÐ1 and sub-ng spectrophotometry is a unique technique that allows the gÐ1 in geological, environmental and clinical samples to % elimination of the separation steps that are generally required levels in some industrial products), high inertness of noble prior to the detection of the analytes by classical metals towards many chemical reagents and high chemical spectrophotometric methods.16 Success in the application of similarities of numerous complexes formed make the choice of derivative spectrophotometry requires, however, the the analytical methodology for their determination in the sample simultaneous conversion of the metals into stable complexes. of interest a challenge. Extensive studies on the methods for the This can cause problems when examining multi-component determination of the metals in various matrices with satisfactory precious metal samples. The detection limits (DLs) offered by UV-VIS † E-mail: [email protected] spectrophotometry make the technique applicable to the 738 ANALYTICAL SCIENCES JULY 2002, VOL. 18

determination of relatively higher concentrations of the metals (SiO2), lead as a collector (PbO) and a reducing agent (µg gÐ1 levels) as compared with the other spectrometric (flour, starch).3,17Ð23 It results in the extraction of noble metals techniques, i.e. AAS, ICP-AES and ICP-MS. The latter into a metallic lead button produced by a reaction of PbO with a techniques can be applied to the determination of trace amounts reducing agent under fusion. The matrix elements react with (ng gÐ1 and sub-ng gÐ1) of the metals. They have been widely the flux component to form a slag that is subsequently used for the determination of precious metals in a large variety discarded. The lead button is placed on a cupel prepared from of samples, such as ores, rocks, industrial products, waste bone ash or magnetite. Upon heating at a temperature of about solutions and solids, as well as clinical, biological and 800 Ð 850ûC in an oxidizing atmosphere, the lead oxide and the environmental materials. ICP-MS is particularly suited to the non-noble metal are produced and absorbed into the determination of metals in various materials owing to the cupel where upon a bead of precious metals remains. The excellent detection limits (low pg levels), wide dynamic range, precious metal bead is treated with acids (HCl, HNO3) to extract possibility of accurate multi-element analysis and unique the analytes. capability of measuring isotopic ratios. The collection of precious metals from large samples (10 Ð 50 The direct application of spectrometric techniques to the g) of a complex matrix into a relatively small bead of simple detection of metals in complex matrices is limited due to metal alloys is the main advantage of the fire assay procedure. numerous interferences, both mutual and those from associated Success in the quantitative recovery of precious metals requires, base metals. Interference effects substantially arise in the however, an experienced and skilled assayer to optimize both analysis of non-homogeneous materials when the examination the flux composition and the fusion conditions. High amounts of large samples is required. Much consideration is to be taken of salts introduced to the sample are disadvantages of the fire for the elimination of interferences, particularly those from assay procedures. They may provide high procedural blanks common elements present in examined samples, when using the and difficulties in the direct analysis of the obtained solution by AAS detection technique. The interfering effects of matrix instrumental techniques. Another drawback of the classical fire elements may cause problems in the direct determination of assay using a lead collector is that it does not provide for an precious metals by the ICP-AES technique. Base elements, e.g. efficient recovery of all noble metals. Quantitative collection iron, copper, nickel, chromium, titanium, manganese and can be achieved for gold, silver, platinum and palladium. The vanadium, interfere with the ICP-AES signals of precious technique has found limited applications to the determination of metals.11 Numerous interferences limit the direct application of ruthenium, osmium, rhodium and iridium in complex matrices. ICP-MS to the quantification of metals in complex materials. The recovery of these metals is seriously affected by the flux Interferences from ions and polyatomic groups as well as from composition and experimental conditions. Serious losses of particular analytes can occur.4 ruthenium during the FA procedure were observed.24 Low concentrations of precious metals, particularly in Significant retention of the metal by the slag and the cupel geological and environmental samples, may require occurred. Losses of osmium due to retention by the slag or the preconcentration prior to detection. Volatilization, solvent volatilization of OsO4 formed during the cupellation were also 25 extraction, coprecipitation, sorption and chromatographic reported. Losses of ruthenium in the form of RuO4 during the methods are applied to separate precious metals from associated fusion and cuppelation were negligible. base metals, as well as particular analytes from their mixtures, Modifications to the lead fire assay procedure using silver or and to preconcentrate them to the levels detected by the used gold as collectors for the isolation of PGM from rocks, ores and instrumental technique. An effective combination of the minerals have been proposed.26Ð28 Combining the technique digestion procedure with the separation and detection steps with GFAAS allows the determination of gold, palladium, determines the reliability of the results. platinum and rhodium26 and of platinum, palladium, rhodium The problem of choosing the procedure for the digestion of and iridium27 down to ng gÐ1 levels. An alkaline cyanide the examined samples remains a fundamental one in spite of a solution has been used to dissolve noble metals from the silver huge amount of work done to develop accurate methods for the or gold beads.26 A successful application of spark-ablation ICP- determination of precious metals in various materials. The MS for the direct detection of platinum, palladium, rhodium and necessity of examining large samples, the inertness of noble iridium in the gold and silver beads was demonstrated.28 metals to chemical attack and the complexity of their properties The application of other iron-nickel-copper,29 copper,30,31 in solutions provide large difficulties in the quantitative tin,32,33 and NiS34 fire assay collectors for precious metals has conversion of the metals into soluble stable complexes. Fire been proposed. Nickel sulfide has been found to be the most assay (FA), a wet acid treatment, chlorination and oxidation effective collector for the quantitative recovery of all precious fusion are used for the decomposition of various noble metal metals from complex matrices and large samples.35Ð51 Recent samples and for the transformation of the metals into solutions. applications of NiS-FA in chemical speciation studies of PGM Recent applications of particular digestion methods in in geological samples can be mentioned.48,50 combination with UV-VIS spectrophotometry, AAS, AES and The NiS-FA procedure involves fusion of the sample with

ICP-MS for the examination of various materials are reviewed nickel, sulfur, Na2B4O7, Na2CO3 and SiO2 in a clay crucible at a in this paper. temperature of ca. 1000ûC. The NiS bead formed is separated from the slag, crushed and then dissolved in hydrochloric acid. The matrix elements are removed under an HCl treatment. The 2 Fire Assay insoluble precious metal sulfides are filtered and dissolved in HNO3 and HCl. A high procedural blank resulting from The FA digestion and preconcentration technique provides the chemicals required for the flux is the disadvantage of the NiS highest recovery of precious metals from a large number of fire assay method. complex matrices, such as ores, rocks, minerals, concentrates The recovery and losses of PGM (µg gÐ1 levels) from ore and soils. The classical lead assay involves fusion of the samples at different stages of the NiS-FA digestion procedure, examined sample at high temperatures (ca. 1100ûC) with a flux, including optimization of the button size and a dissolution 40 namely carbonate (Na2CO3), borax (Na2B4O7), silica technique, have been examined using ICP-MS detection. ANALYTICAL SCIENCES JULY 2002, VOL. 18 739

Among the three button sizes investigated, i.e., 2.5, 5 and 8 g, spectrophotometry have been presented.52Ð58 Mixtures of HCl, the 2.5 g button was found to be adequate for the full recovery HBr and Br2 were also used for the extraction of gold from of the metals (20 g samples). Losses of the metals, especially of geological matrices prior to AAS detection.59 platinum and palladium, during the NiS button dissolution step The high resistance of ruthenium, rhodium, osmium and were minimized by the application of coprecipitation with iridium to an attack by a mixture of acids, including aqua regia, tellurium. The effect of the collector mass on the recovery of limits the use of a direct wet digestion procedure to their PGM and gold in the analysis of ore (< µg gÐ1) and komatiite (< quantitative transformation into soluble species. The metals can 10 ng gÐ1) samples by NiS-FA combined with ICP-MS was also be transformed into solution by an aqua regia treatment when investigated.41 The recoveries of ruthenium, rhodium, accompanying (at low % levels) platinum and paladium60 or palladium and platinum from komatiite increased along with an common metals61 in alloys. Aqua regia is applied for the increase of the collector mass (0.5 Ð 5 g of Ni examined). They decomposition of various catalysts containing PGM.62Ð68 A were independent of the mass of the collector in the analysis of decrease in the amounts of nitric acid used is advantageous a PGM ore standard reference material (SARM-7). when determining ruthenium.69,70 It should be noted, however, The use of lithium tetraborate as a flux constituent in FA that catalysts usually contain small amounts of platinum metals digestion procedures has been examined.39 A lower, as dispersed on the surface of the support, such as alumina, silica, compared with sodium tetraborate, recovery rate (GFAAS silica-alumina or activated carbon. This significantly facilitates detection) from geological standard reference materials, SARM- the dissolution of the metals under a wet acid treatment. A non- 7, CHR-Pt+ and CHR-Bkg, except for chromitites with PGM quantitative recovery of the metals can occur in the analysis of abundances in the µg gÐ1 range, has been achieved. Lithium complex materials that may contain them occluded within the tetraborate was chosen earlier as a preferred reagent for grains. There is evidence of a 20 Ð 40% recovery of platinum, chromium-rich materials, since it improved the dissolution of rhodium and osmium, and 1 Ð 10% of iridium from ores71 and the chromite grains.35 55 Ð 87% of platinum from soils72 digested by an acid treatment,

The applicability of the FA technique to the examination of aqua regia or HF/HClO4 and HNO3/HCl, respectively, followed precious metal samples, the fluxing agents, equipment, by ICP-MS detection. Average recoveries of 46 Ð 55% for materials and detection techniques used have been described in platinum, 61 Ð 78% for gold, 61 Ð 88% for palladium, 73 Ð 94% detail.3 Recent FA procedures used in combination with the for rhodium, 30% for ruthenium and 44% for iridium by ICP- spectrometric detection techniques in the analysis of various MS have been reported for ore samples (10 g) digested with materials are presented in Table 1. aqua regia under heating in a microwave (MW) oven.73 There are opinions that in the analysis of geological samples a direct aqua regia treatment can be used only for a rapid preliminary 3 Wet Acid Treatment evaluation of the precious metals content.10 Combined digestion procedures, a wet acid treatment followed by alkaline oxidizing The digestion of various precious metal samples under a wet fusion of an undissolved residue, are often applied for acid treatment has been widely examined. The method is improving the recovery of the metals.74Ð76 simple, fast and inexpensive. Mixtures of HCl, HNO3, HClO4, The use of hydrofluoric acid in the digestion procedures is 74,76Ð85 HF and H2O2 are commonly applied. The ratio of the sample preferable, particularly when examining geological and weight to the volume of acids and an appropriate sample mesh environmental materials.72,86Ð94 High-pressure systems and size are important factors, particularly in the analysis of microwave heating are widely applied for improving the complex matrices. Incomplete wetting of the samples, or digestion of such samples. A pressure (PTFE bomb) occlusion of the analytes within the grains, can result in a non- decomposition method using an HNO3 and HF mixture has been quantitative recovery of the metals. The application of HF is successfully applied in the analysis of manganese crust and generally required for the complete digestion of geological and geological samples for the content of gold, platinum, palladium 77 more complex environmental materials, e.g. dusts, soils and and rhodium (ng and pg levels) by GFAAS. HF and HNO3 sediments. Hydrofluoric acid attacks the silicate phases and dissolution followed by an aqua regia treatment in a high- facilitates the liberation of the analytes. The purity of the acids pressure digestion bomb (Paar Instrument Co.) (24 h at 150ûC) used should be controlled so as to avoid high blank values. has been applied to the determination of ruthenium, rhodium, Sub-boiling distillation of the commercially available acids can palladium, iridium and platinum (fg and pg levels) in geological result in the reduction of blanks. samples (ICP-MS detection).84 The concentrations of PGM and An open-vessel wet acid treatment, high-pressure gold (0.01 Ð 11.3 µg gÐ1) in copper-nickel ores, corresponding decomposition systems, microwave heating and a Carius tube well with the certified values, were determined by GFAAS technique are incorporated in the digestion procedures. The use using an autoclave decomposition technique along with HCl, 82 of high-pressure systems and microwave heating significantly HF and HNO3. Hydrofluoric acid and aqua regia have also accelerates the decomposition of the samples and leaching been used for the digestion of soil matrices when determining 94 analytes. palladium at ng levels by GFAAS. Mixtures of HNO3, HF and The acid extraction effectiveness strongly depends on the H3BO3 and a pressure ashing device have recently been applied chemical of individual metals, their concentration and to digest road dust (a candidate reference material) prior to the the kind of matrix. Noble metals exhibit a high resistance to detection of platinum, palladium and rhodium (ng gÐ1 levels) by single mineral acids. Palladium and rhodium are the only various techniques.91 Samples of 50 Ð 70 mg were subjected to metals attacked by hot HNO3 and boiling H2SO4, respectively. the digestion procedure. Boric acid was used to solubilize any Aqua regia is used for the dissolution of palladium, platinum insoluble fluorides. The application of pressure ashing and and gold. various acids (HNO3, HClO4, HF and aqua regia) for the Recent successful applications of an aqua regia treatment for decomposition of dust samples, prior to the determination of the determination of gold and palladium in various (geological, platinum by ICP-MS, was examined earlier.89 The procedure anode slime, catalyst and airborne particulate matter) samples with HNO3, HClO4 and HF was recommended for the analysis by AAS (both flame and graphite furnace) or UV-VIS of samples containing platinum bound into the silica matrix. 740 ANALYTICAL SCIENCES JULY 2002, VOL. 18

Table 1 Fire assay digestion procedures combined with spectrometric techniques in the analysis of precious metal samples

Sample Element Collector/ Separation Detection Concentration DL Reference (weight) determined flux technique technique

Concentrate PGM, Au Pb Pb distillation, AES 0.059(Os) Ð 28(Pd) ng gÐ1 18 (1 Ð 50 g) sorption g tÐ1 (Polyorgs IV) Products of PGM (except Pb extraction AAS 0.5 ng gÐ1 Ð 0.1 19 complex Os), Au µg gÐ1 (Pt, Pd, composition Rh, Au) (5 Ð 10 g) 5 ng gÐ1 Ð 0.1 µg gÐ1 (Ir, Ru)

Ð1 Ores Pt, Pd, Rh, Au Pb/Na2CO3, AES 0.43(Pt) Ð 30.0(Pd) 0.1 Ð 1 µg g 20 Ð1 (2.5 g) Na2B4O7, SiO2 g t

Geological Au, Pd, Pt Pb ICP-MS 2(Au), 0.1(Pt), 21 reference 0.5(Pd) materials ng gÐ1

CuS Ag, Au PbO/Na2CO3, FAAS 0.5 Ð 300(Au), 22 concentrates KNO3, SiO2 25 Ð 1500(Ag) g tÐ1 Urban road Pt Pb ICP-MS 0.35 Ð 32.7 23 dust, soils ng gÐ1 <0.30 Ð 7.99 ng gÐ1

Rocks Pd, Pt, Rh NiS/Na2CO3, GFAAS 0.25(Rh) Ð 5.7(Pt) 36 Ð1 (SARM-7) Na2B4O7, SiO2 mg l (5 Ð 10 g)

Geological PGM, Au NiS/Na2CO3, Te coprecipit. ICP-MS 0.5 Ð 1.26 37 Ð1 (15 g) Na2B4O7, SiO2 µg l

Rocks PGM, Au NiS/Na2CO3, Te coprecipit. ICP-MS 55(Os) Ð 3846(Pt); 0.09 Ð 2.1 38 Ð1 (SARM-7), Na2B4O7, SiO2 1.8(Os) Ð 82.2(Pd) ng g chromitite ng gÐ1 (5 Ð 15 g)

Geological PGM NiS/Na2CO3, Te coprecipit. ICP-MS 0.07(Ir) Ð 3.29(Pt) 0.5 40 Ð1 Ð1 (SARM-7) (except Os) Na2B4O7, SiO2 µg g ng g (20 g)

Silicate rocks Ru, Pd, Ir, Pt NiS/Na2CO3, anion-exchange isotope dilution 31.3(Ir) Ð 370(Pt) 42 Ð1 (5 Ð 10 g) Na2B4O7 (ID) ICP-MS ng g

Geological Ru, Rh, Pd, Ir, NiS/Na2CO3, ICP-MS 0.01 Ð 1.57 43 Ð1 reference Pt, Au Na2B4O7, SiO2 ng g materials (1 g)

Geological PGM NiS/Na2CO3, Te coprecipit., ICP-MS 0.01 Ð 0.39 44 Ð1 reference Na2B4O7, SiO2 OsO4 ng g materials distillation (20 g)

Geological PGM, Au NiS/Na2CO3, laser ablation 6.9(Os) Ð 249(Pd) 0.2(Os, Ir) Ð 7(Pt) 45 Ð1 Ð1 reference Na2B4O7 (LA) ng g ng g materials high resolution (15 g) (HR) ICP-MS

Geological PGM, Au NiS/Na2CO3, LA ICP-MS 1.7(Au), 3.3(Pd), 46 reference Li2B4O7 8.3(Pt), materials 1.3(Os), 1(Rh), (10 Ð 15 g) 5(Ru), 0.7(Ir) ng gÐ1 Rock reference PGM, Au NiS ICP-MS 1(Rh) Ð 23(Au) 47 materials pg gÐ1 (50 g) Continued ANALYTICAL SCIENCES JULY 2002, VOL. 18 741

Table 1 continued

Sample Element Collector/ Separation Detection Concentration DL Reference (weight) determined flux technique technique

Geological Ru, Rh, Pd, Ir, NiS Te coprecipit. ICP-MS 21(Ru), 3(Rh), 49 Pt, Au 9(Pd), 2(Ir), 13(Pt),53(Au) pg gÐ1

Geological PGM, Au NiS/Na2CO3, LA ICP-MS 0.2(Os, Ir) Ð 7(Pt) 51 Ð1 reference Na2B4O7 ng g materials (10 Ð 15 g)

Recent examples of the successful application of aqua regia been the subject of most studies.106Ð116 The direct digestion in alone for the digestion of some environmental materials, such as HNO3 (borosilicate tubes, ca. 100ûC) had been used prior to the road dust, soil and urban river sediments (0.20 Ð 0.25 g samples, detection of platinum in blood, urine and tissue speciments by high pressure asher (HPA) or microwave heating) for the ICP-MS (DL of 0.1 µg lÐ1) and GFAAS (DL of 10 µg lÐ1).106 determination of PGM (ng gÐ1) by ICP-MS can, however, be Samples of tissue107 and cell cultures exposed to cisplatin109 mentioned.95,96 Aqua regia and HF were used to digest airborne were prepared for analysis by HPLC ICP-AES or ICP-MS by particulate matter prior to the detection of platinum, palladium dissolving in HNO3, followed dry ashing at 450ûC, or direct Ð3 97 and rhodium (pg m ) by ICP-MS. A mixture of H2SO4 + digestion in HNO3 (2 h heating at 70ûC), respectively. Mixtures CrO3 has been proposed for the decomposition of carbonaceous of HNO3 and HCl have been used for the digestion of tobacco rocks prior to the determination of PGM by GFAAS.98 and bean samples (high-pressure PTFE bombs),108 and tree bark 115 Highly efficient acid (HNO3, HCl) digestion procedures for (MW heating) prior to the detection of platinum by GFAAS various types of rocks including silicates and sulfides (0.1 Ð 5 g (DL of 0.3 ng Pt) and ICP-MS (double focusing, DL of 0.03 ng samples) have been developed using a Carius tube design.99Ð102 gÐ1; quadrupole, DL of 0.2 ng gÐ1), respectively. Digestion in

In the Carius design, an acid treatment is accomplished in a HClO4 and HNO3 preceded the determination of platinum in sealed thick-walled Pyrex tube at high temperature (240ûC) and cisplatin and carboplatin by HPLC,110 in plant tissue111 and in elevated pressure for ∼12 h.99 High efficiency in leaching mouse liver and corn leaves112 by the ICP-MS technique. analytes and low procedural blanks have been achieved. A Microwave digestion procedures using HNO3 and H2O2 have modified Carius design incorporating a liner of high-purity been developed for the determination of platinum in wine quartz glass inside the outer borosilicate shell allowed a (GFAAS)114 as well as fish liver and mussel soft tissue (ICP- reduction of the blank values by a factor of 10 Ð 100, as MS).116 The results for the platinum content in wine were in compared with the standard NiS-FA sample-preparation agreement with those obtained using dry mineralization. technique.101 A preliminary desilification procedure with HF is Rhodium in biological materials (bovine liver, non-fat milk usually added to the sample-preparation step for examining powder and oyster tissue) has been determined by GFAAS (DL Ð1 materials with silicate matrices. of 16.5 ng ml ) after digestion with HNO3 and H2O2 (120ûC, 3 The Carius tube method has been found to be particularly h).117 Ultra-violet photolysis has been proposed for the suitable for the complete recovery of osmium which is easily determination of physiological levels of palladium, platinum, 99,100 118 oxidized to volatile OsO4. A comparison of the iridium and gold in human blood and palladium and platinum 119 conventional Teflon vessel and the Carius tube digestions of in urine to minimize the amounts of the reagents (HNO3, various types of rocks, including silicates, sulfides and metals H2O2) used in the digestion procedures. The application of showed that the latter technique liberates more osmium from microwave and ultrasonic HNO3, H2O2 digestion procedures for most matrices and is more robust for measuring the isotopic the transformation of palladium and platinum from airborne composition of the samples. Recently, >80% recovery of dust collected on glass fiber filters120,121 and platinum from osmium (µg gÐ1 levels, ICP-MS detection) from iron meteorites, tunnel dust122 into solutions has been reported. using the atmospheric pressure apparatus and a longer by a Only dilution with acids was satisfactory prior to the detection factor 10 decomposition time, has been achieved.103 The Carius of platinum and ruthenium in biological fluids by ETAAS.123Ð125 tube dissolution provided better results for platinum, palladium Samples of human plasma, ultrafiltrate plasma, saliva and urine and rhodium by 0.16%, 0.43% and 1.00%, respectively in the have been diluted with 0.2 M HCl + 0.15 M NaCl123 or 0.2% 124 used autocatalysts, as compared with hot-plate dissolution HNO3 +0.1% Triton X-100. Dilution with water has been 104 (HNO3, HClO4 and HF). A comparison of a high-pressure used in the determination of platinum in urine and plasma by asher and Carius tube technique for the digestion of chromitites the ICP-AES technique.107 and other geological materials prior to the detection of The examples of wet acid digestion procedures recently ruthenium, palladium, rhenium, osmium, iridium and platinum developed for the analysis of various precious metal samples (ng gÐ1 levels) by ICP-MS has recently been presented.105 A using spectrometric detection techniques are presented in Table higher temperature (320ûC) than that which can be reached in 2. HPA has been considered to be the advantage of the technique.

The wet acid treatment (HNO3, HNO3 and HCl, HNO3 and HClO4, HNO3 and H2O2) has been successfully applied to the 4 Chlorination total digestion of clinical as well as some biological materials. Until now, the determination of ultra-traces of platinum has Platinum group metals and gold undergo a chlorine attack, 742 ANALYTICAL SCIENCES JULY 2002, VOL. 18

Table 2 Wet acid digestion procedures for precious metal samples examined by spectrometric techniques

Sample Element Separation Detection Dissolution Concentration DL Reference (weight) determined technique technique

Ore Au aqua regia extraction FAAS 0.45 Ð 1.07 52 (1 Ð 10 g) g tÐ1 Anode slime Au, Pd aqua regia anion-exchange UV-VIS 996(Au) Ð 3.7(Pt) 53 (50 Ð 100 mg) spectrophot. µg gÐ1

Ore Au aqua regia chelation GFAAS 0.65 µg lÐ1 54 (2 g) chromatogr.

Geological Au, Pd aqua regia chelation GFAAS 20(Au) Ð 0.35(Pd) 55 (0.5 g) chromatogr. ng mlÐ1

Geological Au aqua regia anion-exchangeFAAS 7.23 µg gÐ1 46 µg lÐ1 56 (1 g) 0.071% anode slime (0.1 g) Airborne Pd aqua regia chelation GFAAS 0.2 Ð 14.6 57 particulate chromatogr. pg mÐ3 matter (filter) Ore, catalyst Pd aqua regiaUV-VIS 193 Ð 503 0.07 58 (2 Ð 5 g) spectrophot. µg gÐ1 µg mlÐ1

Cu alloys PGM aqua regia GFAAS 1.5 × 10Ð4 Ð 61 (0.2 Ð 15 mg) 0.03% Catalysts Pt, Ir aqua regia UV-VIS 0.3(Pt) Ð 62 (3 g) derivative 0.2(Ir)% spectrophot.

Co3O4 Pt, Rh aqua regia Pt extractionFAAS (2.07 Ð 2.54) × 63 Ð2 catalysts (followed H2 10 (Rh), (2 g) reduction) (1.10 Ð 1.63) × 10Ð2(Pt)%

Catalyst Pt aqua regia UV-VIS 35.20 Ð 35.50 64 (0.1 g) spectrophot. µg Pt/C catalyst Pt, Pd, Rh aqua regia/ FAAS 3(Pd) Ð 40(Rh) 66 (50 mg) NaOH fusion µg lÐ1 Autocatalyst Pd, Pt aqua regia UV-VIS 0.0102(Pd) 67 (0.5 Ð 1 g) derivative 0.207(Pt)% spectrophot. Pt-Sn/MgO Pt HCl, aqua regia ETAAS 18.5 Ð 19.8 68 catalyst (MW) mg gÐ1 (20 Ð 30 mg) Pt/C, Pt-Ru/C Pt, aqua regia, UV-VIS 16.7(Pt); 69 catalysts Pt, Ru HCl + HNO3 derivative 22.2(Pt), (2 Ð 20 mg) (6 + 1) spectrophot. 3.4(Ru)%

Pt-Ru-Ge Ru HCl + HNO3 UV-VIS 0.49% 70 catalyst (6 + 1) spectrophot. (5 Ð 17 mg) Geological PGM, Au aqua regia ICP-MS 6010(Pd) Ð 2(Rh) 71 (chromitite, µg kgÐ1 silicates) (10 g) Continued ANALYTICAL SCIENCES JULY 2002, VOL. 18 743

Table 2 continued

Sample Element Separation Detection Dissolution Concentration DL Reference (weight) determined technique technique

Ð1 Soil Pt HF, HClO4 Te coprecipt.ID ICP-MS 3.77 Ð 4.05 0.08 ng g 72 Ð1 (2 g) HNO3, HCl ng g Ore PGM, Au aqua regia ICP-MS 73 (10 g) (MW) Ores, PGM (except HF, aqua cation- GFAAS µg gÐ1, 74 Ð1 concentrates, Os) regia/Na2O2 exchange or ng g mattes, silicate fusion Te coprecipit. and iron- formation rocks (2 Ð 5 g) Geological Ru, Rh, Ir, Pt aqua regia cation- ICP-MS 1.3(Rh) Ð 11(Pd) 75 Ð1 (1 g) (MW)/Na2O2 exchange ng g fusion Ores, rocks Au, Pt, Pd, Ir, aqua regia, ion-exchangeICP-MS 0.5(Rh) Ð 4.0(Au) 76 Ð1 (SARM-7) Rh HF/Na2O2 ng g (0.25 g) fusion

Geological Au, Pt, Pd, Rh HNO3, HF Se coprecipit.GFAAS ng Ð pg levels 77 (0.5 Ð 1.5 g) (PTFE bomb) Geological Au, Pd, Pt, Rh aqua regia, Hg coprecipit.GFAAS 227(Rh) Ð 1580 0.3(Au) Ð 0.5(Pd) 78 (SARM-7) HF (Pt) ng kgÐ1 (0.5, 5 g) ng gÐ1

Ð1 Copper Au HCl, H2O2, reductive FAAS, 2 Ð 20 0.1 µg g 79 Ð1 concentrate HNO3, HF coprecipit. ICP-AES µg g (5 g) Silicate rocks, PGM, Au HF, aqua regia cation- GFAAS µg gÐ1, 3.7 pg(Au) Ð 80 ores, exchange ng gÐ1 80 pg(Pt) metallurgical samples (5 g)

Ore Au HNO3, HClO4, sorptionFAAS 81 (0.5 Ð 1 g) HF, aqua regia

Cu-Ni ores PGM, Au HCl, HF, HNO3 extractionGFAAS 0.01(Ru) Ð 11.3(Pd) 0.001(Au) Ð 82 (1 Ð 5 g) µg gÐ1 0.06(Os) µg gÐ1

Rocks, ores Au, Pt, Pd aqua regia, Br2, ICP-MS 0.01 Ð 0.06 83 (2 g) HF, Rh ng mlÐ1

GeologicalRu, Rh, Pd, Ir, HF, HNO3, aqua cation- ICP-MS fg, pg levels 84 Pt regia exchange (Paar bomb)

Ð1 Sulfide ore Rh, Pd, Pt HNO3, HCl, solid phase FAAS 2.0 Ð 5.1(Rh), 3 Ð 8 ng g 85 (5 Ð 10 g) HF/Na2O2 extraction 5.8 Ð 6.3(Pd), fusion 1.1 Ð 2.5(Pt) µg gÐ1 Cu alloys aqua regia 70 Ð 240(Rh), (2 Ð 5 g) 1000 Ð 2800(Pd), 310 Ð 900(Pt) µg gÐ1

Sediments Pt, Ir HNO3, HClO4, anion-exchangeGFAAS 0.01 Ð 1(Pt) 86 (0.5 Ð 1 g), HNO3, 0.0009 Ð 7.4(Ir) Ð1 nodules, HF/Na2O2 ng g organisms fusion (3 Ð 5 g)

Airborne Pt HF, HClO4, cation- ICP-MS 0.014 Ð 0.184 0.005 87 particulate aqua regia exchange µg gÐ1 µg gÐ1 matter (1.5 g) Continued 744 ANALYTICAL SCIENCES JULY 2002, VOL. 18

Table 2 continued

Sample Element Separation Detection Dissolution Concentration DL Reference (weight) determined technique technique

Plants Au aqua regia, HFICP-MS 0.04 88 (0.25 g) ng mlÐ1 Dust Pt aqua regia, ICP-MS 67.9 Ð 68.2 0.5 Ð 10 89 Ð1 Ð1 (0.1 Ð 1 g) HNO3, HClO4, ng g ng l HF Airborne Pd, Pt, Rh aqua regia, HF HR ICP-MS 21.2 Ð 85.7(Pd), 0.2(Rh) Ð 1.0(Pd) 92 particulate (MW) 7.8 Ð 38.8(Pt), pg mÐ3 matter 2.2 Ð 5.8(Rh) (filter) pg mÐ3

Road dust 102 Ð 504(Pd), 0.5(Rh) Ð 2.5(Pd) (100 mg) 14.4 Ð 62.2(Pt), ng gÐ1 1.9 Ð 11.1(Rh) ng gÐ1

Road dust Rh, Pd, Pt HNO3, HCl, HF HR ICP-MS 9(Rh), 17(Pd), 0.1(Rh), 93 (100 mg) (MW) 63(Pt) 0.5(Pd), 0.4(Pt) ng gÐ1 ng gÐ1 Soil Pd aqua regia, HF solvent GFAAS 7.2 Ð 58.6 94 (5 g) extraction ng gÐ1 Road dust, soil Pt, Pd, Ru, Ir aqua regia anion-exchange ID ICP-MS 0.16(Ir) Ð 47(Pt) 0.15(Pt), 0.075(Pd), 95 (0.2 g) (HPA) 0.1(Ir) Ð 87(Pt) 0.015(Ru, Ir) ng gÐ1 ng gÐ1

Sediments Rh, Pd, Pt aqua regia HR ICP-MS 0.67(Rh) Ð 0.6(Rh) Ð 7.8(Pd) 96 (0.25 g) (MW) 472(Pd) ng lÐ1 ng gÐ1 Airborne Pt, Pd, Rh aqua regia, HF ICP-MS n.d. Ð 9.3(Rh) 0.6(Pt), 97 particulate (MW) 3.0 Ð 15.5(Pt) 3.3(Pd), matter pg mÐ3 0.9(Rh) (filter) ng lÐ1

Carbonaceous Au, Pt, Pd, Rh, H2SO4, CrO3 GFAAS 98 rocks Ir, Ru ICP-AES (1 g) Rocks Os aqua regia distillation MS ng gÐ1 99 (silicates, (Carius tube) sulfides), metals (0.1 Ð 5 g)

Geological Ru, Pd, Ir, Pt HCl, HNO3 ion-exchange ID ICP-MS 1 Ð 15 101 (5 g) (Carius tube) pg gÐ1 Komatiite Pd, Ir, Ru, Re, acids solvent ICP-MS 3(Os, Ir) Ð 15(Pt) 102 reference Os, Pt, (Carius tube) extraction and pg gÐ1 material ion-exchange Iron meteorites Os HCl ID ICP-MS 0.02 Ð 50 103 (0.1 Ð 2 g) (atmospheric µg gÐ1 pressure)

Autocatalysts Pt, Pd, Rh, Pb HCl, HNO3, ICP-MS 697.4, 1131(Pt), 104 (0.1 g) (Carius tube) 326, 233.2(Pd), (HF) 51.2, 135.1(Rh) µg gÐ1

Geological Ru, Pd, Re, Os, HCl, HNO3 anion-exchange ID ICP-MS 0.15(Ir) Ð 70(Pd) 0.012(Re, Os) Ð 105 (2 g) Ir, Pt (HPA) ng gÐ1 0.77(Pt) ng

Ð1 Blood, plasma Pt HNO3 ICP-MS 0.1 µg g 106 (0.5 Ð 2 ml), GFAAS 0.2 Ð 10 µg gÐ1 tissue (1 g) Continued ANALYTICAL SCIENCES JULY 2002, VOL. 18 745

Table 2 continued

Sample Element Separation Detection Dissolution Concentration DL Reference (weight) determined technique technique

Tobacco, Pt HNO3, HCl electro- GFAAS 2.6 0.3 ng 108 beans, slag, (PTFE bomb) deposition 0.12 Ð 0.55 dust 0.050 (0.1 Ð 2 g) 0.012 µg gÐ1

Cisplatin, Pt HClO4, HNO3 HPLC 0.02 Ð 2.5 0.11 ng 110 carboplatin µg mlÐ1

Urine, plants, Pt HNO3, HClO4 adsorption ICP-MS 1 pg 111 soil, dust HNO3, HClO4, chromatogr. (0.1 g) HF

Ð1 Ð1 Mouse liver, Pt HNO3, HClO4, ICP-MS 180 ng g Ð 2.8 8 Ð 25 ng l 112 (100 mg) aqua regia µg gÐ1 corn leaves (50 mg)

Ð1 Mice tumor Pt HNO3 GFAAS 30 Ð 1000 3 µg l 113 tissue µg lÐ1 (0.1 g)

Wine Pt HNO3, H2O2 GFAAS 100 pg Pt 114 (MW)

Tree bark Pt HNO3, HCl ICP-MS 0.07 Ð 5.4 0.03 Ð 0.2 115 (0.5 g) (MW) ng gÐ1 ng gÐ1

Ð1 Fish liver, Pt HNO3, H2O2 ICP-MS 0.1 Ð 2.3 0.2 ng g 116 mussel tissue (MW) ng gÐ1 (300 mg)

Ð1 Bovine liver, Rh HNO3, H2O2 GFAAS 16.5 ng ml 117 non-fat milk, oyster tissue (1 g)

Airborne dust Pd, Pt HNO3, H2O2 electro-deposition GFAAS 0.023 Ð 0.08(Pd) 120 (filter) (MW) 0.27(Pt) µg lÐ1

Ð1 Airborne Pd HNO3, H2O2 extraction UV-VIS 0.077 Ð 19.6 0.007 mg l 121 porticulate (ultrasonic spectrophot. µg gÐ1 matter bath) (filter), automobile catalyst (0.5 g)

Ð1 Ð1 Tunnel dust Pt HNO3, H2O2 anion-exchange ICP-MS 13.1 ng g 0.17 ng g 122 (0.3 Ð 0.4 g) (MW)

Autocatalysts Pt, Pd, Rh HNO3, HF, ICP-MS 2250(Pt) Ð 7.4(Pd) 0.0015(Rh) 126 (0.05 Ð 0.25 g) HCl, or µg gÐ1 0.012(Pt) H2SO4, H3PO4, 0.26(Pd) aqua regia µg lÐ1

Ð1 Car exhaust Pt, Rh, Pd HNO3, HF, HCl ICP-MS ng km 127 fumes (MW) (filter) Automobile Pd HF, HCl UV-VIS 0.96 Ð 1.96 128 catalyst spectrophot. mg gÐ1 (0.1 g)

yielding binary chlorides and salts, both dissolved in weak selective spectrometric techniques can be accomplished directly hydrochloric acid.4,10,129,130 The chlorination products can be in the obtained solutions without any further chemical separated from resistant and water-insoluble components of the treatment. examined samples by filtration. The detection of the metals by Three types of chlorination procedure are used: “direct” 746 ANALYTICAL SCIENCES JULY 2002, VOL. 18

Table 3 Chlorination procedures in combination with spectrometric detection in the analysis of precious metal samples

Sample Element Chlorination Temp. Detection Dissolution Concentration Reference (weight) determined agent (time) technique

Rocks PGM, Au NaCl, Cl2 580ûC 10% HCl ICP-MS 0.7 Ð 37 129 (25 g) (3.5 h) ng gÐ1

Ð1 Rock pulp PGM, Au NaCl, Cl2 580ûC 10% HCl ICP-MSng g 130 (250 g) (3.5 h)

Ð1 Sulfide PGM, Au NaCl, Cl2 500 Ð 600ûC AES, 95 ng g Ð 131 concentrates, (1.5 h) AAS 2.78% metal coated (Pt, Pd, Rh) glass wool (1 g)

Ð5 Metallurgical HCl + KMnO4 160ûC AAS, 5 × 10 (Rh) Ð 132 samples (2 Ð 3 h) ICP-AES 30.09(Pd)% (0.1 Ð 2 g) (50 mg) CCl4, CuCl2 800ûC (2 h)

Ore PGM HCl, Cl2 250ûC 2 M HCl FAAS0.072(Ru) Ð 133 (0.1 Ð 0.3 g) (10 h) 85.8(Pt)%

Catalyst Ru Cl2 750ûC HCl UV-VIS 0.350 Ð 0.367% 134 (2 g) (6 h) spectrophot.

chlorination in the presence of large amounts of alkali chloride, the presence of an oxidizing agent, such as “wet” chlorination at elevated temperatures and pressures in a or nitrate. mixed with sodium peroxide, or sealed tube containing HCl and an oxidizing agent, and “dry” sodium peroxide alone, are most frequently used as a flux. chlorination by hot chlorine passing over a sample usually Fusion operation is usually performed at 450 Ð 600ûC for 15 Ð 60 mixed with a small amount of NaCl in an open tube at 500 Ð min. The melt is dissolved in water and acidified with 600ûC. Dry chlorination has such advantages as large samples hydrochloric acid for converting the analytes into chloro- (up to 25 Ð 250 g) that can be submitted for analysis, low complexes, which serve as the basis of most separation and concentration of salts and low blank levels (less than 0.1 ng gÐ1 determination methods. Large amounts of salts and for most of the elements). contaminations introduced to the sample, including those from Recoveries >90% of nanogram amounts of PGM and gold the wall of the crucible attacked by the flux, are the occurring in the form of native metals, natural alloys and sulfide disadvantages of fusion procedures. The application of the minerals in rock pulps were achieved by dry chlorination method is restricted to small sample weights (0.5 Ð 2 g). combined with ICP-MS.129,130 The examination of reference Silver crucibles are often applied in fusion procedures. Silver rock pulps showed that the results obtained by the dry that partially passes into the melt during the fusion operation chlorination method are comparable to, or even better than, can be quantitatively removed in the form of AgCl after those from the fire assay technique. acidifying the melt with HCl. A nickel crucible can also be Direct chlorination was earlier considered to be a promising used. The application of glassy carbon crucibles in the method for the analysis of a variety of complex commercial geological sample fusion procedures has been reported.135 In samples, including automotive catalysts, using AES and AAS.131 order to protect the crucible from an attack by the sodium Chlorination in two closed system: (in autoclaves with a peroxide flux, a layer of sodium carbonate was applied to line mixture of HCl and KMnO4 (0.1 Ð 2 g samples) or in sealed the bottom and walls of the crucible before fusion. silica ampoules with carbon tetrachloride (or CuCl2) (50 mg Alkaline oxidizing fusion is an effective way for the samples)) has been applied for the decomposition of dissolution of ruthenium, osmium, rhodium and iridium, which metallurgical samples (copper-nickel-sulfide ores, slages and are metals having a high resistance to acid attack. The method platinoid concentrates) and a subsequent determination of PGM is widely used to prepare ruthenium standard solutions. (µg gÐ1 Ð % levels) by AAS or ICP-AES.132 No losses of Metallic powder ruthenium is completely oxidized to soluble ruthenium and osmium from ore samples submitted to sodium ruthenate, Na2RuO4, under a treatment with a mixture of 136 2Ð chlorination in sealed ampoules (250ûC, 10 h) have been NaOH + Na2O2 at 450 Ð 600ûC. Stable RuOHCl5 and 133 4Ð observed. A combination of chlorination (in Cl2 stream) with Ru2OCl10 complexes are formed after acidifying the obtained UV-VIS spectrophotometry has been used for the determination ruthenate solution with hydrochloric acid.137,138 of ruthenium at % levels in a spent catalyst.134 Fusion with sodium peroxide was successfully used to The chlorination procedures used for the determination of decompose various geological samples containing ruthenium, precious metals in various samples are summarized in Table 3. rhodium, palladium, iridium, platinum and gold.135,139Ð142 The detection of the metals by GFAAS or ICP-MS techniques has been preceded by separation with tellurium coprecipitation135,141 5 Oxidizing Fusion or by anion-exchange chromatography.139,140,142 The latter method requires a quantitative transformation of compounds Precious metals are readily attacked by alkaline hydroxides in occurring in alkaline media into defined, stable anionic chloro- ANALYTICAL SCIENCES JULY 2002, VOL. 18 747

Table 4 Oxidizing fusion of precious metal samples prior to spectrometric detection

Sample Element Temp. Separation Detection Flux ConcentrationDL Reference (weight) determined (time) technique technique

Geological Pt, Pd, Ru, Ir Na2O2 200ûC Te coprecipit. ICP-MS 0.15(Ir) Ð 0.3 Ð 2 135 (0.5 g) (15 min) 3792(Pt) ng gÐ1 490ûC (1 h) ng gÐ1 Ruthenium Ru NaOH dull-red heat UV-VIS 75.95% 136 oxide (RuO2), (15 min) differential (RuO2) ruthenites spectrophot. 27.1 Ð 28.3% (30 Ð 60 mg) (ruthenites)

Ð1 Silicate rocks Pt, Pd, Au Na2O2 550 Ð 600ûC anion- GFAAS µg g 139 (SARM-7) (1 h) exchange (0.5 g)

Rock Ru, Pd, Ir, Pt Na2O2, NaOH 600ûC anion- ICP-MS 6 Ð 10 0.2 Ð 0.5 140 (0.6 Ð 1.5 g) (30 min) exchange ng ng gÐ1 meteorite (60 mg)

Ð1 Geological Ru, Rh, Pd, Na2O2 700ûC Te coprecipit. ICP-MS ng g 1 Ð 9 141 (1 Ð 20 g) Ir, Pt, Au (10 min) pg gÐ1

Ð1 Impact PGM Na2O2 650ûC anion- ICP-MS ng g 142 breccias (30 min) exchange (1 g)

Cu-Ni Os Na2O2, NaOH extraction GFAAS 0.07 Ð 0.5 143 concentrates gtÐ1 (0.5 Ð 2 g)

Secondary Au Na2O2 800ûC ICP-AES 58.80% 145 raw materials (20 min) (Au-Ag-Cu) (0.1 Ð 0.5 g)

Metallurgical Pt Na2CO3, 700ûC sorption FAAS 1.21 Ð 303 146 Ð1 samples Na2O2 (10 min) µg g (0.5 g)

complexes that can be used in separation procedures. The techniques in the examination of various precious metal samples formation of hydroxochloro-complexes on acidification with are presented in Table 4. HCl provided low and variable results for the content of The application of fluorooxidants, such as liquid bromine platinum and gold, of 46% and 76%, respectively, in the fluoride (BrF3) and molten potassium tetrafluorobromate 139 analysis of silicate rocks. The percolation of chlorine gas (KBrF4), for the dissolution of precious metals has recently been through the sample solution was applied to improve the extensively examined by Mit’kin and co-workers.147Ð152 The conversion of PGM into chloro-complexes of high affinity to method is based on the treatment of samples (e.g. ores, anionic resin.142 concentrates, process materials) having an appropriate volume

Alkaline (Na2O2, NaOH) fusion has been used to digest with liquid BF3 with a subsequent transformation of fluoro- copper-nickel concentrates prior to the determination of complexes formed into chlorides under HCl action. Quartz or osmium.143 The extraction separation of osmium prior to the glassy carbon crucibles are used in the decomposition detection by GFAAS has been applied. Earlier experiments procedures. The insoluble residue is decomposed by a showed that metallic osmium could be quantitatively treatment with excess KBrF4 at 350 Ð 400ûC for 2 Ð 4 h with transformed into a soluble osmate by fusion with NaOH.136 The subsequent processing using 2 Ð 4 M HCl. An oxygen stream at low stability of osmate solutions limits the application of the 650 Ð 700ûC can also be used for the oxidation of an insoluble technique in the analysis of real samples. residue after a BrF3 treatment of the samples. The possibility to Alkaline oxidizing fusion is often combined with a wet acid minimize the losses of PGM organometallic compounds due to treatment for the decomposition of acid resistant components of volatilization by the use of fluorooxidants in the decomposition the examined materials.74Ð76 Any residue not dissolved in acids step has been demonstrated.148 The applicability of oxidizing is submitted to Na2O2 fusion and, after dissolution, is combined fluorination with BrF3 and alkaline fusion with NaOH, Na2O2 with the main sample. The application of the sodium peroxide and Na2CO3 to the determination of ultra-low levels of gold and fusion procedure for the preparation of an automobile catalyst PGM (AAS detection) in resistant geochemical materials, for the determination of platinum, palladium and rhodium by including chromites, molybdenites and ultrabasic ores, has ICP-MS has recently been examined.144 The method provided recently been considered.153 100% recoveries of all metals. It was recommended as an alternative to the fire assay sample digestion method. Oxidizing fusion procedures combined with spectrometric 748 ANALYTICAL SCIENCES JULY 2002, VOL. 18

6 Conclusions 7 References

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