On the determination of platinum group elements in

Analytica Chimica Acta 489 (2003) 245–251

On the determination of platinum group elements in environmental materials by inductively coupled plasma mass spectrometry and microwave digestion Rumyana Djingova a,∗ , Heike Heidenreich b , Petya Kovacheva a , Bernd Markert b a

Faculty of Chemistry, University of Sofia, 1 J. Bouchier Blvd., 1164 Sofia, Bulgaria b IHI-Zittau, Markt 23, D-02763 Zittau, Germany

Received 25 February 2003; received in revised form 26 May 2003; accepted 3 June 2003

Abstract The interferences from Cd, Cu, Hf, Pb, Sr, Zn, Zr and Y on the inductively coupled plasma quadrupole mass spectrometry (ICP-MS) determination of Pt, Pd, Rh, Ru and Ir in geological (Pt-ore SARM-7, abundance range for platinum metals 0.07–3.74 ␮g/g) and environmental samples (sediment JSd-2 abundance range for Pt and Pd 0.0167–0.021 ␮g/g; road dust and plant sample) are evaluated using model solutions, real samples and comparison to inductively coupled plasma atomic emission spectrometry (ICP-AES) results. Pt, Rh, Ru and Ir can be determined usually after introduction of corrections for the interference in all investigated materials though in sediments the direct determination of Pt might be a problem depending on the actual Hf concentrations. The direct determination of Pd (after microwave-assisted acid digestion) is possible in ores using all investigated isotopes (105 Pd, 106 Pd, 108 Pd), in plants using 108 Pd and correction for the interferences of Zr, Mo and Cd, and not possible in sediments and road dust. Therefore, we developed a procedure for isolation of Pd using its diethyl-dithio-carabamate (DDTC) complex. The detection limits for Pt, Pd and Ir are 0.015 ng/g, and for Ru and Rh 0.03 ng/g. © 2003 Elsevier B.V. All rights reserved. Keywords: ICP-MS; Platinum group elements; Sample preparation; Environmental samples

1. Introduction In the last decade the determination of platinum group elements (PGE), mainly platinum, but also rhodium and palladium, has received increasing attention due to their release into the environment as a result of intensive use of catalytic converters in cars, e.g. [1–13]. Elevated Ir abundances have already been established [14,15]. Ir is most likely deposited ∗ Corresponding author. E-mail address: [email protected] (R. Djingova).

along with Pt, Pd and Rh as impurity in car catalysts and also is being used in some new diesel car catalysts [14,16]. Ru is mostly used in caustic soda and chlorine production. The general problems in the determination of PGE, irrespective of the analytical method used, are the low concentrations (especially background ones) and the lack of certified reference materials (CRM) for quality control [3,5,17]. The majority of available reference materials are different types of minerals, ores, etc. (such as SARM-7), and recycled monolith autocatalyst (NIST-SRM-2557) or car exhaust catalyst material (IMEP 11), where the

0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0003-2670(03)00716-5

246

R. Djingova et al. / Analytica Chimica Acta 489 (2003) 245–251

concentrations of PGE are considerably higher (in the higher mg/kg range) than in environmental materials (where normally ␮g/kg and lower concentrations are detected), and the matrix composition and forms of PGE are very different [17]. Thus, the methods validated against these CRMs may not necessarily provide reliable data in the case of road dust, soils, sediments and plants. The sample preparation for PGE analysis varies in dependence on the method used, as well as on the material. Classical silicate analysis with HF and HClO4 , and alkali fusion are widely used approaches together with the specific fire assay technique and Te precipitation [3,18–21] for geological and pedological samples. However, besides demanding larger samples, all of these techniques are very time consuming and are prone to either loss or contamination of samples, and are very dependant on the analytical skill of [22–24], and, therefore, are not very appropriate for environmental analysis. Microwave digestion with different acid mixtures however was reported to lead to incomplete dissolution of PGE in the case of some geological materials and demands further alkali fusion of the residue [23], whereas the results obtained for car exhaust fumes are satisfactory [25]. For voltammetric determinations mainly high-pressure ashing (HPA) with nitric and hydrochloric acid are preferred to destroy any organic material in the samples, e.g. [2,6,9]. Inductively coupled plasma quadrupole mass spectrometry (ICP-QMS) is one of the most promising techniques for PGE determination at trace level having generally very low detection limits [2,3,5]. A very serious problem in this case however is the spectral overlap of HfO+ ions with all Pt isotopes and of ZrO+ , YO+ , SrO+ , MoO+ and Cd isotopes with Pd [2,3,5,17,24–26] and of Pb, ArCu+ , RbO+ and SrO+ with 103 Rh [27]. These interferences of course may also be totally different depending on the matrix, the equipment and the measurement conditions and again validation of a method against one type of material does not automatically mean that the procedure is correct. The aim of the present paper is to investigate the influence and significance of the probable interferents in the determination of Pt, Pd, Rh, Ru, and Ir in environmental materials and to propose an ICP-MS method following microwave oven assisted digestion.

2. Experimental 2.1. Samples For interference determinations single element standard solutions (Merk, CertiPure) with concentrations 1 g/l were diluted with 3% HNO3 to obtain the following concentrations: Sr (100 mg/l); Zn, Cu, Y, Zr, Hf, Mo, Cd, Pb, Rb (10 mg/l each). Mixed solutions of all these elements in the above concentrations; and mixed solutions with addition of Pt-metals in concentrations 10–30 ␮g/l to the interfering elements were investigated. The following CRMs were analyzed: SARM-7 (Platinum ore, SA Bureau of Standards, Pretoria, SA) [28] and JSd-2 (Stream Sediment, Geological Survey of Japan, Geochemical References Samples, Tsukuba, Ibaraki, Japan) [29] together with road dust collected during 1999 along highway A-1 in Germany. Moss, collected also in 1999 in the city of Saarbrücken, Germany, was subjected to analysis. The sampling and sampling preparation procedures of the road dust and plants are described in details in [15]. Plant samples (leaves) were cut at 1 cm above ground and mechanically cleaned from dust and soil particles by blowing with air stream. After air drying in a clean room the samples were oven dried for 4 h at 85 ◦ C. The dust samples were sieved through 2 mm PTFE sieve, homogenized in a PTFE ball mill and oven dried for 4 h at 85 ◦ C. All samples were collected after 14 days dry period. (Water content has been determined for all samples.) 2.2. Digestion of samples Sample digestion of CRMs and road dust was done according the following digestion procedure: 0.1–0.2 g of the sample are mixed with 3 ml 37% HCl and 0.5 ml 65% HNO3 in PTFE vessels and the mixture is left for 15 min. Afterwards 1 ml 48% HF are added and microwave digestion (Microwave MLS 1200 mega, MLS GmbH, Leutkirch, Germany) is performed. The conditions are presented in Table 1. After that the solution is evaporated on a sand bath (temperature not exceeding 95 ◦ C) in the same vessels and the residue is dissolved in 1 ml mixture of 2.5% HCl and 1.3% HNO3 . The solution is diluted to 50 ml with bidistilled water in nalgene flasks and is measured immediately.


247

Table 1 Programs for microwave digestion of the samples (MLS 1200 mega, MLS GmbH, Leutkirch, Germany)

Table 2 Measurement conditions for ICP-MS (Perkin-Elmer Sciex ELAN 6000)

Steps

Time (min)

Power (W)

1 2 3 4 5 Vent

5 5 5 10 5 5

250 400 500 650 300

Argon plasma gas flow Auxiliary gas flow Nebulizer gas flow Lens voltage ICP RF power Pulse stage voltage Integration time Dwell time Acquisition mode Peak pattern

Microwave digestion: max. t = 220 ◦ C; max. pressure = 35–40 bar.

Plants were digested according the following procedure: 0.2 g of the sample is mixed with 4 ml 65% HNO3 and 1.5 ml 37% HCl. The mixture stays for 10 min, and afterwards microwave-assisted digestion is performed (Microwave MLS 1200 mega, MLS GmbH, Leutkirch, Germany). In the investigated samples digestion was complete but in case of incomplete digestion 0.5 ml 48% HF should be added. All reagents: HCl, HNO3 , HF, KJ, chloroform were of grade suprapure, Merck (Darmstadt, Germany) and diethyl–dithiocarbamate was p.a., product of Sigma, Sigma–Aldrich, Dorset UK. 2.3. Separation of Pd For the separation of Pd from the samples the following procedure was developed: After digestion of the sample 1 ml 37% HCl and 0.5 ml 1% KJ are added and the sample is shaken for 5 min. Afterwards the pH is adjusted to 4 with an acetate buffer solution and 1 ml 1% diethyldithiocarbamate is added and again the solution is shaken for 5 min. Extraction with 1 ml chloroform is performed for 2 min and the solution is transferred to a beaker. The chloroform is evaporated on a sand bath, 1 ml mixture of 37% HCl and 0.3 ml 65% HNO3 are added and evaporation to incipient dryness is done. The residue is dissolved in 1 ml mixture of 2.5% HCl and 1.3% HNO3 . The solution is diluted to 50 ml with bidistilled water in nalgene flasks and measurements are done immediately at 1:2 dilution. 2.4. Instrumentation Measurements are performed using a quadrupole ICP-MS spectrometer Perkin-Elmer Sciex ELAN

Measured mass numbers Sample uptake rate Number of runs Rinse time Rinse solution

15 l/min 1.2 l/min 0.93 l/min 9.5 V 1100 W 1500 V 2000 ms 50 ms Peak hop One point per mass at maximum peak 99, 101, 102, 103, 105, 106, 108, 191, 193, 194, 195 2 ml/min 6 180 s 3% HNO3

6000 with cross flow nebulizer. The spectrometer is optimized to provide maximal intensity for Rh+ , and minimal values for CeO+ /Ce+ (below 0.03%) and Ba2+ /Ba+ (below 0.03%). External calibration is performed using mono and multielement standard solutions of PGE. The optimum measurement conditions are presented in Table 2. Additional validation of the results (besides analysis of CRM) was done by ICP-AES after anion-exchange separation. The procedure is described in details in [30]. 2.5. Correction for interference The following equation was used for introducing the correction for interferences: Scorr = Smeas − (Sinter A) where Scorr is the corrected signal of the analyte, Smeas the measured signal, Sinter the signal of the interfering element and A is the % of formation of the respective interfering species (e.g. oxide). For calculations the values of A are determined in model solutions with appropriate concentration of the interfering element and measured at the working conditions in Table 2. To determine Sinter the interfering elements are measured in each sample and the isotope abundances are accounted for.

248


3. Results and discussion 3.1. On the digestion procedure In respect to procedures reported in the literature, e.g. for microwave digestion of geological materials prior to ICP-MS [23], or for HPA digestion of environmental materials prior to voltammetric analysis [9] the one proposed in the present work has considerably higher ratio HCl:HNO3 = 6:1 ([23]—3:1, [9]—1:8), which has been done keeping in mind the recommendation of Beamish [31] for the determination of PGE in geological materials. 3.2. Interference investigation using model solutions Table 3 presents the well-known potential interferences in the determination of Pt metals. To investigate the individual importance of each interferent a lot of single element solutions of the interfering elements and mixed solutions of the interfering elements with and without Pt-metals, and in ratios (interfering elements to platinum metals) near the ones in plants, sediments and geological materials have been analyzed. Thus the percentage of formation of all interfering MO+ , MOH and M2+ species has been experimentally determined under the present working conditions and the results are presented in Table 3, column 4 as relative % from the concentration of

the interfering element. The error in the determination of the respective Pt-metal is given in column 5, Table 3. In this case as concentrations for the calculation of the error values from Markert [32], and own investigations [15] are considered. The data in Table 3 indicate that the determination of Ru (by 101 Ru and 102 Ru) is interfered by Sr (1–20% error) whereas Pb, Rb and Zn do not influence significantly the analysis. The only isotope of rhodium 103 Rh is also interfered by SrO+ both in plants (20% expected error) and road dust (ca. 10% error) and the introduction of correction of this interference is necessary. No significant interference form Cu, Rb and Pb has been established under the present working conditions, while in [27] serious interferences from Cu and Pb have been reported in airborne samples up to 75%. On the basis of the data presented in Table 3 the conclusion that the direct determination of Pd (after digestion) in dust is not possible is confirmed, whereas in plants 108 Pd (and not the most popular 105 Pd) might be used after introduction of correction for interferences from Zr, Mo and Cd. (108 Pd has been also used in [33] but only correction for Cd has been considered.) The determination of Pt after correction for HfO+ is possible using both 195 Pt and 194 Pt, but 195 Pt should be preferred. The analysis for Ir should be done using 191 Ir the error introduced from HfO+ on 193 Ir is too high.

Table 3 Isotopes used for the analysis and interferences in the ICP-MS determination of platinum metals in some environmental samples Element

Mass

Ru Ru Ru Rh Pd Pd Pd Ir Ir Pt Pt

99 101 102 103 105 106 108 191 193 194 195

Expected interferences

64 Zn35 Cl+ , 59 Co40 Ar + 85 RbO+ , 84 SrOH

102 Pd, 86 SrO+ , 204 Pb2+

206 Pb2+ , 40 Ar 63 Cu+ , 87 Sr 16 O+ , 87 Rb16 O+ 40 Ar 65 Cu+ , 35 Cl + , 89 Y16 O+ , 88 Sr 16 O1 H 3 106 Cd, 40 Ar 66 Zn+ , 90 Zr 16 O+ , 89 Y16 O1 H 108 Cd, 40 Ar 68 Zn+ , 92 Zr 16 O+ , 92 Mo16 O+ 175 Lu16 O+

177 Hf 16 O+ 178 Hf 16 O+ 179 Hf 16 O+

Established interferences as relative % from the concentration of the interfering element

Error (relative %) in the determination of a platinum element at “normal” concentration of the interfering element

– 0.0004% Sr 0.005% Sr 0.003% Sr 4.6% Y, 0.063% Sr, 0.004% Cu 2.4% Zr, 0.7% Y, 0.36% Cd 0.83% Zr, 0.08% Mo – 0.4% Hf 1.2% Hf 0.5% Hf

– Dust—1, plants—2 Dust—15, plants—20 Dust—10, plants—20 Dust, plants > 100 Dust > 100, plants > 100 Dust > 100, plants—13–15 – Dust > 100, plants > 70 Dust—15–20, Plants—10–15 Dust—8–12, plants—2–10

R. Djingova et al. / Analytica Chimica Acta 489 (2003) 245–251 Table 4 Concentration of platinum elements and certified values for platinum and interfering elements in SARM-7 and JSd-2 [28,29] (ng/g) Element

SARM-7 (certified)

SARM-7 (experiment)

Pt Pd Rh Ru Ir Cd Cu ␮g/g Hf ␮g/g Pb ␮g/g Sr ␮g/g Zn ␮g/g Zr ␮g/g Y ␮g/g

3740 1530 240 430 74 130 1000 2.0 24 50 23 10 5

4000 1520 223 400 70

JSd-2 (certified)

JSd-2 (experiment)

16.7 21.1 – – – 3060 1117 2.7 146 202 2056 111 17.4

15b (2) 700c 10.5d (1.5) 2.4 (0.4) 0.95 (0.15)

249

Table 5 Experimental results for the concentrations of platinum elements, interfering elements in road dust and plant sample and results from ICP-AES analysis after chemical separation [30] (ng/g) Element

Road dust ICP-MS

ICP-AES

Moss ICP-MS

ICP-AES

a

Pt Pd Rh Ru Ir Cd Cu ␮g/g Hf ␮g/g Pb ␮g/g Sr ␮g/g Zn ␮g/g Zr ␮g/g Y ␮g/g

280a (29)b 652c 25d (2) 1.8 (0.3) 1.1 (0.2) 2400 290 4.65 122 109 533 153 10.5

307 (60) 107 (7) – – – 2600 305 4.2 120 115 520 – 12

30 (2) 2.4 (0.3) 5.4 (0.5) 0.9 (0.1) 0.10 (0.03) 2000 10 0.10 21.1 45 5 4.46 0.20

34 (8) >5 – – – 2200 11.2 0.115 23 44.2 5.3 – 0.23

b

a

(100)a (25) (13) (15) (5)

In brackets: standard deviation from six replicate analysis. After correction for HfO+ . c No correction. d After correction for SrO+ .

3.3. Analysis of geological materials, sediments, road dust and plant samples Table 4 presents the results for the Pt-metals and interfering elements in SARM-7 [28] and JSd-2 [29] together with the certified or information values. In the case of SARM-7 no corrections have been necessary. The concentrations of Pt-metals are very high whereas the concentrations of interfering elements are the lowest from all investigated materials. The results from the analysis of JSd-2 in Table 4 once again confirm the conclusion that Pd cannot be determined directly after digestion in sediments even after corrections since the values may exceed more than 100 times the information value depending on the Pd isotope used for analysis. The correction for the interference of HfO+ resulted in about 40% decrease in Pt result, which is a strong indication that in sediments (and soils) where the actual values for Pt are too low (below 50 ng/g), chemical isolation of Pt from the matrix might be necessary depending on the actual Hf concentration. Table 5 gives the results from the analysis of a road dust sample (collected in Germany in 1999) and a plant sample (moss sample from Saarbrücken, Germany, 1999) and for comparison the results for Pt and Pd from ICP-AES analysis [15,30]. In road dust the

After correction for HfO+ . In brackets: standard deviation from six replicate analysis. c No correction. d After correction for SrO+ . b

determination of Pt was possible after correction for HfO+ introducing about 10% error (the same value is reported in [27]). The direct determination of Pd after digestion is also not possible and the results cannot even be used for information (as suggested in [27]) because due to the interferences of SrO+ , SrOH, YO+ , ZrO+ , ArCu+ and Cd2+ on the investigated Pd isotopes the concentrations exceed more than six times the ones we obtained by ICP-AES after chemical separation and preconcentration [30]. In the case of Rh correction for Sr interference has been necessary while the interference from Pb, Cu, and Rb has been negligible. The smallest interferences have been established in the analysis of plant material although the values for the Pt-metals are the lowest but the content of the interfering elements are also the lowest. Determination of Pd is possible by using 108 Pd and after correction for Zr and Cd, and the interference of HfO+ on 195 Pt amounts to about 2–3%. The correction of Sr on Rh is also negligible. These conclusions have been confirmed after analysis of additional road dust and plant samples [15]. The detection limits (3σ criterion) of the method are 0.015 ng/g for Pt (195 Pt) and Ir (191 Ir), and 0.03 ng/g for Ru (99 Ru and 101 Ru) and Rh (103 Rh). The precision

250


is below 5% at 0.1–4 ␮g/g concentration range and 10–15% at 0.1–10 ng/g range. The accuracy (relative error %) amounts to 7% for Pt and Rh, and 5% for Ru (see Table 4). 3.4. Determination of Pd As demonstrated in Tables 3–5 and acknowledged by many authors (e.g. [2,14,27]) the error in the ICP-MS determination of Pd demands other approaches outside the purely instrumental one in road dust and sediments mostly due to the very high Zr, Y and Cd concentrations. One should keep in mind that the interference from Zr and Cd is very strong in road (and tunnel) dust and soils near by the highways due the fact that Zr concentrations there are also high because of traffic [34]. Though 105 Pd is not interfered by ZrO+ (the results using 105 Pd are two times lower than using the other two isotopes) the interference from YO+ is significant Y concentration being two to three orders higher than Pd in the road dust. To solve this problem we have proposed a simple method for isolation of Pd in environmental samples and its consequent measurement by ICP-MS as DDTC complex using the information in [35] for synthesis of pure Pt and Pd–DDTC complexes for HPLC determinations. The present procedure is described in the experimental part above. Though DDTC complexes are very typical for many elements under the conditions described no complex formation of the major interfering elements has been established and the results obtained for Pd in road dust and JSd-2 are presented in Table 6. Although there was no problem in the determination of Pd in SARM-7 it is included in Table 6 to demonstrate the applicability of the procedure to geological materials. Both results for SARM-7 [28] and JSd-2 [29] agree well with the certified and information values, and the result for road dust is between 2 Table 6 Results from the analysis of Pd after chemical separation (ng/g) Element Pd Certified value [28,29] or ICP-AES value [30] a

SARM-7 (21)a

1444 1530 (32)

JSd-2

Road dust

21 (2) 21.2b

92 (3) 107 (7)c

In brackets: standard deviation from three replicate analysis. Non certified value. c ICP-AES determination after ion exchange procedure [30]. b

and 14 times lower than the direct determination after digestion (depending on the isotope). The comparison of the present result for road dust with the data, obtained by ICP-AES after anion-exchange [30] shows a good agreement. The detection limit (105,106,108 Pd) amounts to 0.015 ng/g. The precision of the method varies between 1.5% (at ␮g/g concentration level) and 10% (at ng/g concentration level). The accuracy of the determination is better than 5% for all investigated materials.

4. Conclusions A microwave digestion procedure is developed for ICP-MS determination of PGE in sediments and road dust using HCl:HNO3 :HF mixture = 6:1:1. The investigation on the interferences in the ICP-MS determination indicates the possibility for direct determination of Ru, using 101 Ru and 99 Ru, and Rh after correction for SrO+ in environmental materials. For Ir determination the use of 191 Ir may be recommended. The necessity to introduce corrections for the HfO+ interference on 194 Pt and 195 Pt is demonstrated for sediment and dust samples, but the direct determination is possible in geological and plant samples. The direct determination of Pd is possible in geological materials using all isotopes, in plants using 108 Pd and not possible in sediments and road dust. Therefore a simple procedure for chemical separation of Pd using its DDTC complex is developed. In all cases SARM-7 demonstrated different behavior than the investigated environmental materials. The present method permits reliable determination of PGE in many environmental matrices using the most distributed version of ICP-MS the quadrupole. Although in principle isotope dilution ICP-MS, high resolution ICP-MS and sector field ICP-MS are expected to have advantages in the determination of PGE, they suffer from the same interferences, discussed in the present paper besides being much more expensive To overcome the interferences usually offor on-line matrix separation is done with ID-ICP-MS [36,37]. In the case of HR-ICP-MS sometimes resolutions over 10 000 are necessary to eliminate interferences [36]. ICP-SFMS with membrane dessolvatation system has been introduced [37,38] for analysis of


silica containing matrices whereas for Pt in biological materials very low detection limits have been obtained by this method [39]. Acknowledgements The authors are thankful to S. Korhammer (IHI-Zittau) for the fruitful discussions and suggestions. One of the authors (R. Djingova) is grateful to DAAD for a fellowship at IHI-Zittau. References [1] F. Alt, U. Jerono, J. Messerschmidt, G. Toelg, Microchim. Acta III (1988) 299. [2] S. Lustig, Platinum in the Environment Dissertation, Ludwig-Maximillians-Universitaet, Muenchen, 1997, p. 143. [3] M. Balcerzak, Analyst 22 (1997) 67R. [4] F. Zereini, B. Sketrupp, F. Alt, E. Helmers, H. Urban, Sci. Total Environ. 206 (1997) 137. [5] R. Barefoot, Environ. Sci. Technol. 31 (1997) 309. [6] F. Alt, H. Eschenauer, B. Mergel, J. Messeschmidt, G. Toelg, Fresenius J. Anal. Chem. 357 (1997) 1013. [7] E. Helmers, ESPR-Environ. Sci. Pollut. Res. 4 (1999) 100. [8] T. Hees, B. Wenklawiak, S. Lustig, P. Schramel, M. Schwarzer, M. Schuster, D. Verstraete, R. Dams, E. Helmers, ESPR-Environ. Sci. Pollut. Res. 5 (1998) 105. [9] E. Helmers, N. Mergel, Fresenius J. Anal. Chem. 362 (1998) 522. [10] J. Schaefer, J.-D. Eckhardt, Z.A. Berner, D. Stueben, Environ. Sci. Technol. 33 (1999) 3166. [11] E. Helmers, K. Kuemmerer, ESPR-Environ. Sci. Pollut. Res. 6 (1999) 29. [12] F. Zereini, F. Alt (Eds.), Anthropogenic Platinum-Group Element Emissions, Springer, Berlin, 1999, p. 310. [13] K. Pyrzynska, J. Environ. Monit. 2 (2000) 99N. [14] J. Ely, C. Neal, Ch. Kulpa, M. Schneegurt, J. Seidler, J. Jain, Environ. Sci. Technol. 35 (2001) 3816. [15] R. Djingova, P. Kovacheva, G. Wagner, B. Markert, Sci. Total Environ. 308 (1–3) (2003) 235. [16] M. Palacios, M. Moldovan, M. Gomez, in: F. Zereini, F. Alt (Eds.) Anthropogenic Platinum-Group Element Emissions, Springer, Berlin, 2000, pp. 3–14.

251

[17] P. Schramel, M. Zischka, H. Muntau, B. Stojanik, R. Dams, M. Gomez-Gomez, Ph. Quevauviller, J. Environ. Monit. 2 (2000) 443. [18] J.G. Sen Gupta, C.D. Gregoire, Geost. Newslett. 13 (1989) 197. [19] J.C. Van Loon, R.R. Barefoot, Determination of the Precious Metals—Selected Instrumental Methods, Wiley, Chichester, 1991. [20] M. Plummer, F. Beamish, Anal. Chem. 31 (1959) 1141. [21] H. Urban, F. Zereini, B. Sketrupp, M. Tarkian, Fresenius J. Anal. Chem. 352 (1995) 537. [22] I. Jarvis, M.M. Totland, K.E. Jarvis, Chem. Geol. 143 (1997) 27. [23] M. Totland, I. Jarvis, K. Jarvis, Chem. Geol. 124 (1995) 21. [24] M. Totland, I. Jarvis, K. Jarvis, Chem. Geol. 104 (1993) 175. [25] M. Moldovan, M.M. Milagros Gomez, M.A. Palacios, J. Anal. At. Spectrom. 14 (1999) 1163. [26] H. Munai, Y. Ambe, M. Morita, J. Anal. At. Spectrom. 5 (1990) 75. [27] B. Gomez, M.A. Palacios, M. Gomez, J.L. Sanchez, G. Morrison, S. Rauch, C.M. McLeod, S. Caroli, A. Alimonti, F. Petrucci, B. Bocca, P. Schramel, M. Zischka, C. Petterson, U. Wass, Sci. Total Environ. 299 (2002) 1. [28] R. Korotev, Geost. Newslett. 20 (1996) 217. [29] N. Imai, S. Terashima, S. Itoi, A. Ando, Geost. Newslett. 20 (1996) 165. [30] P. Kovacheva, R. Djingova, Anal. Chim. Acta 464 (2002) 7. [31] F.E. Beamish, The Analytical Chemistry of the Noble Metals, Pergamon Press, Oxford, 1966, p. 609. [32] B. Markert, Instrumental Element and Multi-Element Analysis of Plant Samples, Wiley, Chichester, 1996, p. 296. [33] J.S. Becker, D. Bellis, I. Staton, C.W. McLeod, J. Dombovari, J.S. Becker, Fresenius J. Anal. Chem. 368 (2000) 490. [34] E. Ebbinghaus, Dissertation, Universitaet Bremen, Bremen, 1994. [35] S.F. Wang, C.M. Wai, J. Chromatogr. Sci. 32 (1994) 506. [36] M. Müller, K.G. Heumann, Fresenius J. Anal. Chem. 368 (2000) 109. [37] G. Köllensperger, St. Hann, G. Stingeder, J. Anal. At. Spectrom. 15 (2000) 1553. [38] G. Köllensperger, St. Hann, G. Prinz, G. Stingeder, M. BujattiNarbeshuber, Fresenius J. Anal. Chem. 370 (2001) 559. [39] S. Zimmermann, Ch. Menzel, Z. Berner, J.-D. Eckhardt, D. Stüben, F. Alt, J. Messerschmidt, H. Taraschewski, B. Sures, Anal. Chim. Acta 439 (2001) 203.