Analytica Chimica Acta 436 (2001) 79 85

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1 Analytica Chimica Acta 436 (2001) Determination of platinum group elements in impact breccias using neutron activation analysis and ultrasonic nebulization inductively coupled plasma mass spectrometry after anion exchange preconcentration Xiongxin Dai a, Christian Koeberl a,, Heinz Fröschl b a Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria b Arsenal Research, Chemical Analyses, Faradaygasse 3, A-1030 Vienna, Austria Received 11 October 2000; received in revised form 17 January 2001; accepted 19 February 2001 Abstract An anion exchange procedure has been optimized for the analysis of platinum group elements (PGEs) in geological samples at low abundances by adding two stages: chlorination and replicate sorption onto a small anion exchange column. Combined with neutron activation analysis (NAA) and ultrasonic nebulization inductively coupled plasma mass spectrometry (USN-ICP-MS) determination, this procedure is evaluated by replicate analyses of PGEs (Ru, Pd, Os, Ir, Pt), Au, Ag and Re in two reference materials, WMG-1 and WITS-1. The suitability of the method for actual geological samples was demonstrated by analyzing target rocks and breccias from the Bosumtwi impact crater, Ghana Elsevier Science B.V. All rights reserved. Keywords: Platinum group elements; Anion exchange; NAA; ICP-MS; Impact breccias 1. Introduction The detection of an extraterrestrial component in impact-derived rocks (e.g. in impact breccias and melt rocks) by geochemical analysis, especially the measurement of elevated concentrations and interelement ratios of the platinum group elements (PGEs), is a challenging analytical problem of great importance, because the presence of such a component is of diagnostic value [1 5]. However, as it is difficult to determine abundances of the PGEs at ppb or sub-ppb levels in geological samples, concentration proce- Corresponding author. Tel.: ; fax: address: christian.koeberl@univie.ac.at (C. Koeberl). dures are necessary for the chemical separation of the PGEs from the matrix. Variable major and trace element composition of actual rocks, reagent blank, and sample heterogeneity in terms of PGE distribution limit the accuracy and sensitivity of analytical data. Anion exchange preconcentration has been widely applied to separate the PGEs from various geological samples [6 8]. In our previous studies, an -amino pyridine resin preconcentration procedure for iridium determination was presented, and the adsorption behavior of iridium on the resin was studied [9]. In the present study, based on [9], an optimized anion exchange preconcentration procedure was developed and checked on two international geological standards. Its validity for actual rock samples was tested by determining the contents of the PGEs in impact /01/$ see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S (01)

2 80 X. Dai et al. / Analytica Chimica Acta 436 (2001) breccias and target rocks from the Bosumtwi crater (Ghana) by neutron activation analysis (NAA) and ultrasonic nebulization inductively coupled plasma mass spectrometry (USN-ICP-MS). 2. Experimental 2.1. Samples and materials Samples and international reference materials The present method was tested by analyzing a suite of actual impact breccias and target rocks from the Bosumtwi impact crater (Ghana, West Africa). For details on the crater, location, and samples types, see [10]. The rocks, including eight target rocks, three locally melted graywackes and five impact glass-bearing breccias (suevites), were powdered in a low-contamination automatic agate mortar for the PGEs analyses. Two geological reference materials, WMG-1 (a mineralized gabbro PGE standard reference material) [11] and WITS-1 (a silicified komatiite and ultramafic rock from the Barberton area, South Africa) [12], were analyzed to obtain data on precision and accuracy of the method Reagents Dowex 1X8 anion resin (Cl -form; mesh; Fluka Chemicals, Switzerland) was used for the separation of the PGEs and Au from the matrix elements. Resin cleaning was performed by soaking three times with 6 M HCl solution, followed by washing with deionized water until neutrality of the eluate was reached. The resin was stored in 0.4 M HCl solution before use. Other reagents used were sodium peroxide (granular, high purity grade, MERCK, Germany) and concentrated hydrochloric acid (high purity grade, MERCK, Germany) Chemical standards Multi-elements standards of the PGEs Au, Ag, and Re were prepared from high-purity chemical reagents (NH 4 ) 2 RuCl 6, (NH 4 ) 2 RhCl 6, (NH 4 ) 2 PdCl 6, (NH 4 ) 2 OsCl 6, (NH 4 ) 2 IrCl 6 and (NH 4 ) 2 PtCl 6 (all from Fluka Chemicals, Switzerland), and high purity gold and silver metal wires, and rhenium metal powder (ÖGUSSA, Austria). A reasonable amount of the above-mentioned reagents was dissolved in 2 ml of concentrated hydrochloric acid (except Au in aqua regia, and Ag and Re in concentrated nitric acid), and evaporated to near dryness. Then, concentrated HCl or HNO 3 acid was added. The acid solution was transferred into a volumetric flask, and diluted with deionized water to the acidity of 0.01 M HCl or HNO 3 as a stock solution. For NAA standards, 25 l of the fresh stock solutions were sequentially pipetted on six square filter papers (0.8cm 0.8 cm). Between each pipetting step the previously applied solutions were dried with an IR lamp under clean conditions at about 70 C. Finally, the filter papers were sealed in small high-purity quartz vials. As the double neutron capture reaction of 197 Au interferes with the determination of Pt via 199 Au, the Au standard was prepared separately. For ICP-MS standards, the stock solutions were diluted with deionized water. Finally, four multi-elements standard solutions with the concentration levels of the PGEs, Au, Ag, and Re at about 0.01, 0.1, 1.0 and 10 ppb, respectively, were prepared, and the acidity of the solutions was about 3% HNO Anion exchange preconcentration experiments All samples were treated by an anion exchange preconcentration procedure as detailed below. The contents of the PGEs, Au, Ag, and Re in two reference materials, WMG-1 and WITS-1, were measured to in duplicate to examine the accuracy and precision of the procedure. The blank of the preconcentration procedure was also examined, assuming a sample weight of 1.0 g Dissolution of the sample by alkali fusion Sample dissolution was performed using the sodium peroxide fusion technique. Powder splits of 1.0 g of the samples were weighed and mixed with five times the amount of sodium peroxide in a 50 ml high-purity pyrolytic graphite crucible (PGC, provided by the Institute of Chemical Engineering and Metallurgy, China), covered with a graphite lid, and fused at 650 C for 30 min in a muffle furnace. We did not use a zirconium crucible, because it would induce a significant increase in the blank levels of the PGEs [13]. After fusion, concentrated HCl and deionized water were used to dissolve the fused cake, and the solu-

3 X. Dai et al. / Analytica Chimica Acta 436 (2001) tion was transferred into a 50 ml PTFE beaker. This step requires careful attention to avoid sputtering of the sample out of the crucible. Any small amount of melt, which might have splashed onto the lid during the fusion, was also washed into the beaker with concentrated HCl and deionized water. Afterwards, the total solution was evaporated to dryness under an infrared lamp. The residue was then dissolved in 25 ml of 0.4 M HCl solution. A small amount of a colloidal suspension, most likely aluminosilicic acid, still remained at this stage. In order to remove the suspended remnant, the solution was centrifuged at 3500 rpm for 10 min. Then, the residue was discarded after being washed and centrifuged for three times with 8 ml of 0.4 M HCl. The removal of the residue does not lead to any losses of the PGEs, because the PGEs remain in the acidic solution. Finally, the supernatant with an acidity of 0.4 M HCl was transferred into a PTFE beaker Chlorination It is well known that the sorption ability of anion exchange resin is quite different for various species of the PGEs [6,14]. The higher oxidation states of the PGEs are more strongly sorbed by anion resins, whereas lower oxidation states are more weakly bound. Thus, in order to obtain the best retention efficiency, it is necessary to convert the PGEs to the higher oxidation states before the addition of the samples to the anion exchange column. For this purpose, chlorine gas is an optimal oxidizing agent with very low blank levels. In this step, chlorine gas (directly prepared by the reaction of concentrated HCl and MnO 2 powder, which is easier to handle than chlorine gas tanks) was vigorously percolated through the sample solution for 3 min. The solution was then sealed and occasionally agitated for more than 12 h, to ensure the best oxidation efficiency Addition of sample onto column After the chlorination step, the sample was passed through the anion exchange column at a flow rate of 2mlmin 1. About 1 g of anion resin was filled into a quartz glass column (7 mm 50 mm) to a height of 20 mm, and the solution flow rate was controlled using a peristaltic pump. Then, the solution was introduced into the column again for a second sorption cycle, in order to avoid possible loss due to the leakage of the PGEs on a relative small column with coarse-grained resin. Afterwards, the column was rinsed with 10 ml of 1 M HCl and washed with a large quantity of deionized water to neutrality of the eluate Sample preparation for NAA determination All the resin was removed with deionized water from the column and transferred onto a filter paper. Then, the filter paper was packed into a 5mm 50 mm-sized quartz vial. To avoid the possible irradiation decomposition of the resin and filter paper, the sample was heated at 200 C for 6 h. Finally, the vial was sealed for long-term neutron irradiation Resin-ashing and dissolution for USN-ICP-MS determination For the USN-ICP-MS measurement, a separate batch of the samples was prepared by the same preconcentration method. After the PGEs were adsorbed onto the anion exchange column, the resin was rinsed out of the column with little deionized water and transferred into a 10 ml quartz crucible, dried, and ashed at 550 C for 4 h. Afterward, the ash was dissolved with 0.5 ml of concentrated HCl at boiling temperature. Then the HCl solution was evaporated to near dryness. After cooling, 0.5 ml of concentrated nitric acid was added. With the complete dissolution (at this time, only very little ash residue is not dissolved), the solution was filtered and the residue was carefully washed with 2% of HNO 3 solution. The solution was then transferred into a 25 ml volumetric flask. Finally, this solution, with an acidity of about 3% HNO 3, was used for the USN-ICP-MS analyses Neutron activation analysis (NAA) All samples, reference materials, procedural blank, and chemical standards were irradiated for 10 h at a thermal neutron flux of ncm 2 s 1 at the Tank WWR reactor of the Atomic Energy Research Institute (Budapest, Hungary). Three chemical standards were used to examine the reproducibility of the measurements, as well as to determine the neutron flux gradient in the vertical direction, which was found to be 15% over the length of the irradiation container. After a cooling period of 3 days, the samples were transported to the activation analysis laboratory of the

4 82 X. Dai et al. / Analytica Chimica Acta 436 (2001) Table 1 Nuclear data for the PGEs, Au, Ag and Re used for NAA determination in this study (from [16]) Element Isotope Half-life -ray energy (kev) Interfering nuclide and reaction Ru 103 Ru 39.3 day U fission Pd 109 Pd 13.7 h 88.0 Ag 110m Ag day 657.8, Re 186 Re 3.78 day Os 191 Os 15.4 day Ir 192 Ir 74.3 day 316.5; Cr (320 kev) Pt 199 Au 3.14 d Au(n, ) 198 Au(n, ) 199 Au; Ca 47 Sc interference (160 kev) Au 198 Au 2.70 day Institute of Geochemistry, University of Vienna. After decontamination, the samples were counted repeatedly using a high pure germanium (HPGe) detector (EG&G, ORTEC), with 48% relative efficiency and energy resolution of 1.82 kev at the 1332 kev peak of 60 Co. The first measurements (4 days after irradiation, counting time about 1000 s), led to the determination of Pd, Re, Au, Pt, and Ir. The second counting cycle was done for 3000 s for each sample, after 10 days of cooling, for the determination of Re, Au, Pt, Os, Ir, and Ru. The third counting cycle was done for 5000 s for each sample, after 20 days of cooling, for the determination of Os, Ir, and Ru. For data acquisition and reduction, we used Nuclear Data VAX/VMS spectroscopy application and nuclide identification programs (see [15] for more details). The nuclides and -rays used in this study are given in Table 1. The final results were obtained not only by averaging data from several counts, but also from different -lines in each count USN-ICP-MS determination We used a Perkin-Elmer/SCIEX Elan 5000A with a CETAC U-5000AT Ultrasonic Nebulizer sample introduction system for the ICP-MS analyses. The average operating parameters of the ICP-MS for the duration of this study are given in Table 2. Before sample analysis, PGE, Au, Ag, and Re standard solutions in the ppb range were measured to establish external calibration curves. The following isotopes, chosen for the absence of isobaric interferences, were used in these measurements: 101 Ru, 103 Rh, 105 Pd, 107 Ag, 185 Re, 189 Os, 193 Ir, 195 Pt, and 197 Au. Table 2 ICP-MS operating parameters Forward power (W) 1200 Reflected power (W) <5 Plasma gas flow-rate (l min 1 ) 15 Auxiliary gas flow-rate (l min 1 ) 1.0 Nebulizer gas flow-rate (l min 1 ) 1.0 Sample uptake flow-rate (ml min 1 ) 1.4 Number of replicates 7 The detection limits of the PGE measurements by the present USN-ICP-MS setup were found to be in the low (pg g 1 ) range and no obvious interfering species were observed, in agreement with results given in [17]. 3. Results and discussion 3.1. Reference materials results The optimized anion exchange preconcentration procedure was evaluated by analyzing two geological reference materials, WMG-1 and WITS-1. The analytical results of the PGEs, Au, Ag, and Re in the reference materials by NAA and USN-ICP-MS are summarized, and compared with certified reference values and other literature data, in Table 3. No Rh value is reported, because, on the one hand, the half-life of the useable radionuclide is too short for NAA determination, and on the other hand, for the USN-ICP-MS measurement, we found that Rh was obviously lost during the sample pretreatment; a similar significant loss of Rh (up to 40%) was also observed by Jarvis et al. [8]. This loss might be caused by changes in the speciation of Rh in the HCl solution, and some

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6 84 X. Dai et al. / Analytica Chimica Acta 436 (2001) of these species only weakly adsorbed by the anion exchange resin. For most measured elements, our analytical results agree well with the certified values. However, the following observations were made: 1. For the NAA determinations, the determination of Pt (via the kev 199 Au line) suffered from a strong interference from the low-energy tail of the 47 Ca 47 Sc peak at 160 kev. This interference amounts to 90% of the 199 Au counts at the kev -line in the standard reference materials and most natural samples that contain a few weight percent of Ca, and makes the proper Pt analysis by NAA impossible in our experiments. Therefore, no Pt data by NAA are reported in this study. 2. For USN-ICP-MS analysis, we found that Os and Rh were more or less completely lost during the pretreatment of the sample. This loss might be caused by the volatility of osmium as OsO 4 at a high temperature during the resin-ashing stage. 3. For both of the reference materials, one of the duplicate Os values (obtained by NAA) is lower than the reference values. It is possible that in these cases a small fraction of the Os was lost during the pretreatment stage of the samples. 4. The Au data by NAA do not agree well with the certified values and are quite variable, possibly because Au tends to be heterogeneously distributed in some geological samples. In our study, a relatively small sample mass (1.0 g) was used. Thus, the influence of the nugget effect could be important. Alternatively, for WITS-1, the high procedural blank might have led to the increase in the Au values. For ICP-MS analysis, no Au value can be reported due to bad linear regression correlation of the external calibration curve of the gold standards in the ppb range in our measurements. In addition, the procedural blanks were also examined and are listed in Table 3. The procedural blanks for this method are dominated by impurities from the alkali fusion reagents, i.e. Na 2 O 2 and concentrated HCl [13]. Relatively high blanks of Pt and Au, 0.5 and 0.6 ppb, respectively, were found and might derive from the concentrated HCl that was not distilled for the further purification before using to dissolve the fused cake of sample during alkali fusion. However, blank corrections for Au and Pt are not straightforward, because the volumes of concentrated HCl used for the dissolution of the samples were not completely identical. In particular, much more concentrated HCl was added for some sample types (e.g. granite), because the fused cakes were difficult to dissolve. It is possible that for this reason our Pt values for WITS-1 are somewhat higher than the reference value (Table 3). For the other elements, the blanks are low, and no obvious interferences can be found in the present samples Analyses of impact breccias and target rocks from Bosumtwi crater, Ghana To test the method on some real geological samples, a suite of Bosumtwi crater samples, including target rocks, impact breccias, and impact glasses (see [10] for details), were analyzed, and the results are also reported in Table 3. A geochemical interpretation of these data is beyond the scope of the present paper and will be given elsewhere. The reason for discussing the data here is to (a) evaluate the procedure on some of the samples it was developed for, and (b) to compare the results from different analytical methods. Often in the literature on meteoritic components in impactites, different authors come to different conclusions (see [5] for a review), but no discussion is usually made on the effects of the different analytical methods. For our experimental conditions, we observe that the determination of the contents of Ir, Au, Ag, and Re is more sensitive by NAA. In contrast, Pt, Pd and Ru are more sensitive determined by USN-ICP-MS. Analysis of the same actual rocks by three different analytical methods allows us to compare the analytical results obtained by these different techniques, i.e. (a) instrumental neutron activation analysis (INAA) without any pretreatment, and (b) NAA and (c) USN-ICP-MS, both with anion exchange preconcentration. Good agreement is found for Ag, Re and Ir data by NAA and USN-ICP-MS measurement, and for most Au data by INAA and NAA determination after anion exchange pretreatment. Some differences between these values may be caused by sample heterogeneity. However, much higher Ru values were obtained by NAA than USN-ICP-MS, probably due to the interfering reaction 235 U(n, f ) 103 Ru in the NAA determination. For more detailed discussions of this interference, see Gijbels [19] and Crocket et al. [6]. In most Bosumtwi crater samples, uranium was found (by INAA) to be

7 X. Dai et al. / Analytica Chimica Acta 436 (2001) present at low ppm levels; this effect can be corrected, but leads to lower precision. Therefore, for Ru, the USN-ICP-MS data are superior. The relatively high and variable Pt and Au data that were determinated by NAA and ICP-MS, using the pretreatment method, likely result from the high procedural blanks (Table 3). Hence, these data should not be used for the determination of the meteorite type that might be responsible for the high PGE abundances in the breccias. We observe that, the INAA Au data seem to be more reliable than the NAA data after anion exchange preconcentration. The Ir data between INAA, and NAA and USN-ICP-MS after preconcentration also show some differences. However, the INAA data are close to the detection limit and, thus, not reliable. Therefore, only for samples with high Ir abundances, such as LB-35 and LB-31A-F, a good agreement was observed. We note that, the concentrations of Os, Ir, Ru and Pd, as well as Ag, in impact glass samples are higher than those in target rocks. This might imply a possible meteoritic contribution in impact glasses (see [20]). The very high and variable abundance of Au determined by INAA is not surprising considering that major Au mineralization (Obuom range) occurs close to the Bosumtwi crater [21]. 4. Conclusions An anion exchange procedure has been optimized for the simultaneous determination of the PGEs (Ru, Pd, Os, Ir, Pt) and Au, Ag, and Re in geological samples. Combined with NAA and USN-ICP-MS measurement, this procedure has been successfully applied to the analyses of most of these elements in Bosumtwi crater country rocks and impact breccias. In addition, a comparison is made of analytical results obtained by INAA, and by NAA and USN-ICP-MS after anion exchange pretreatment. The results indicate (a) that real geological samples pose some analytical difficulties due to the presence of interferences that are absent in standards, and (b) that data obtained by the three different techniques on the same samples show some considerable variation, some of which is probably due to sample heterogeneity (considering the small sample quantities used), and some of which results from analytical effects. This comparison allows to identify the method that yields the best results for each element. Acknowledgements We are grateful to Heinz Huber (University of Vienna) and Dr. Max Bichler (Atominstitute, Vienna) for technical assistance. The authors thank the Geological Survey of Ghana for logistical support. This research was supported by the Austrian FWF, project Y-58 (to C.K.). The support of the Austrian Academic Exchange Service (scholarship to X. Dai) is also appreciated. The comments from two anonymous reviewers are also gratefully acknowledged. References [1] L.W. Alvarez, W. Alvarez, F. Asaro, H.V. Michel, Science 208 (1980) [2] J.W. Morgan, H. Higuchi, R. Ganapathy, E. Anders, in: Proceedings of the 6th Lunar Science Conference, 1975, p [3] H. Palme, M.J. Janssens, H. Takahasi, E. Anders, J. Hertogen, Geochim. Cosmochim. Acta 42 (1978) 313. [4] N.J. Evans, D.C. Gregoire, R.A.F. Grieve, W.D. Goodfellow, J. Veizer, Geochim. Cosmochim. Acta 57 (1993) [5] C. Koeberl, In: M.M. Grady, R. Hutchison, G.J.H. McCall, D.A. Rothery (Eds.), Meteorites: Flux with Time and Impact Effects, Special Publication 140, Geological Society of London, London, 1998, p [6] J.H. Crocket, R.R. Keays, S. Hsieh, J. Radioanal. Chem. 1 (1968) 487. [7] D.C. Colodner, E.A. Boyle, J.M. Edmond, Anal. Chem. 65 (1993) [8] I. Jarvis, M.M. Toland, K.E. Jarvis, Analyst 122 (1997) 19. [9] X. Dai, Z. Chai, X. Mao, J. Wang, S. Dong, K. Li, Anal. Chim. Acta 403 (2000) 243. [10] W.U. Reimold, D. Brandt, C. Koeberl, Geology 26 (1998) 543. [11] CANMET (Energy, Mines and Resources Canada) Report, CCRMP (Canadian Certified Reference Materials Project), 94-1E, p. 68. [12] M. Tredoux, I. McDonald, Geostand. Newslett. 20 (1996) 267. [13] J.W. Morgan, D.W. Golightly, A.F. Dorrzapf, Talanta 38 (1991) 259. [14] Y.V. Yi, A. Masuda, Anal. Chem. 68 (1996) [15] C. Koeberl, J. Radioanal, Nucl. Chem. 168 (1993) 47. [16] M.D. Glascock, Neutron Activation Analysis Tables, Research Reactor Facility, University of Missouri, USA, [17] J.C. Ely, C.R. Neal Jr., J.A. O Neill, J.C. Jain, Chem. Geol. 157 (1999) 219. [18] D.G. Pearson, S.J. Woodland, Chem. Geol. 165 (2000) 87. [19] R. Gijbels, Talanta 18 (1971) 587. [20] C. Koeberl, I. McDonald, W.U. Reimold, Meteoritics Planet. Sci. 34 (1999) A66. [21] C. Koeberl, W.U. Reimold, J.D. Blum, C.P. Chamberlain, Geochim. Cosmochim. Acta 62 (1998) 2179.