373

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1

Introduction

Platinum group elements (PGEs) and gold may providedirect information on deep mantle processes, because of theirextremely high concentrations in the mantle relative to thecrust. However, reports of PGE abundances in basaltic sam-ples have been limited because of difficulties in PGE analysisand the complicated behavior of PGEs during basalt differenti-ation processes. The following analytical methods have beenused for determining PGEs in geological rock samples.

The NiS fire assay is widely used for collecting PGEs fromrock samples. Neutron activation analysis (NAA) (e.g.,Schmidt and Pernicka, 1994; Hoffman et al., 1978) or induc-tively coupled plasma mass spectrometry (ICP-MS) (e.g.,Gros et al., 2002; Plessen and Erzinger, 1998; Yli et al., 1998)are used for analysis, because of their high sensitivity. Rocksamples are fused with nickel, sulfur and alkaline fluxes. Inthe resulting melts, the metal-loving elements such as PGEsare collected into a NiS bead and can be readily separatedfrom many cations which partition into a glass slag. Mostnickel sulfide compounds are soluble in hydrochloric acid,while PGE sulfides are not. Therefore, PGEs can be simplyseparated from the nickel matrix by dissolving the NiS beadsin HCl, and filtered off the matrix. To improve chemicalrecovery of the PGEs and Au, tellurium coprecipitation hasoften been carried out before the filteration of PGE sulfides(Shazali et al., 1987; Jackson et al., 1990). The fire assay-Tecoprecipitation technique has the advantage that it can treatrelatively large amounts of sample for analysis, leading to areduction in heterogeneous sampling. In addition, the methodis simple and achieves complete sample decomposition. How-ever, contamination from the reagents used is significant, largeamount of fusion charge are needed for this method.

Since fire assay often suffers from blank problems, an aciddigestion is sometimes preferred. To attain complete digestionof PGE-bearing refractory minerals and isotopic equilibriumbetween isotopes in the spike solution and those in the sam-ples, acid digestion in a sealed, thick-walled Pyrex tube (Car-ius tube) at high temperatures (240ºC) is used (Shirey andWalker, 1995). Rehkämper et al. (1998) used quartz lined Car-ius tubes and obtained procedural blanks of less than 20pg/gfor all PGEs. Because isotope dilution analysis needs at leasttwo isotopes free from isobaric interference, anion exchangeseparation is typically employed for separating the PGEs(Rehkämper et al., 1999; Pearson and Woodland, 2000). Thistechnique combined with multicollector (MC)- or quadropole(Q)-ICP-MS provides very precise analytical results, becausechemical yields do not significantly affect the obtained results,and the isotopic ratio measurement is usually achieved with a

high precision, even by Q-ICP-MS (RSD; ~0.5%). However,the ion-exchange separation and measurement are complicatedand time-consuming because the sample solutions are obtainedas separate fractions for each PGE.

While the major trends for PGE analysis are describedabove, other methods, such as standard acid digestion fol-lowed by cation exchange separation (Jarvis et al., 1997; Elyet al., 1999) or isotope dilution analysis with Te coprecipita-tion (Enzweiler et al., 1995) have also been reported. In thisstudy, we first tried the method of nickel sulfide fire assay fol-lowed by Te coprecipitation reported by Oguri et al. (1999)and examined whether this method is routinely applicable forthe determination of trace PGEs in basalt samples, using twogeological standard rocks (JP-1, peridotite, GSJ, split 10, posi-tion 72 and BHVO-2, Hawaiian basalt, USGS).

Experimental

Reagent and samples

Reagent grades used for the preparation of the sample solu-tions are listed in Table 1. The listed reagents were chosen tominimize the procedural blanks for PGEs. In the present study,an external calibration method was used for quantitativeanalysis. Standard solutions were prepared by mixing com-mercially available standard solutions (SPEX, 1000µg/ml). ANi solution (5ppm), and a mixed Cu (100ppb) and Ta (20ppb)solution were prepared by diluting commercially availablestandard solutions for atomic absorption analysis (Wako purechemicals, 1000ppm). All of these solutions have a similarmatrix composition to that of the sample solution (1% HNO3

and 3% HCl).

Chemical separation by nickel sulfide fire assayfollowed by tellurium coprecipitation

The analytical procedure used in this study is basically sim-ilar to that described in Oguri et al. (1999), which involves adouble fire assay, followed by double Te coprecipitation.Although we first tried to reproduce their method, the analyti-cal results obtained for standard rocks were somewhat erratic.Therefore, some modifications were made to increase chemi-cal recovery and simplify the procedure. The fusion chargecomposition was changed so as to make larger NiS beads,according to a detailed study of fusion conditions by Paukertand Rube ka (1993). The double Te coprecipitation wasreduced to single coprecipitation step.

The analytical procedure is given in Fig.1. The powderedsample (5g for JP-1, 10g for BHVO-2) was mixed with

Determination of platinum group elements using preconcentrationby nickel sulfide fire assay, followed by tellurium coprecipitation

Kazunori Shinotsuka1, Katsuhiko Suzuki1,2 and Yoshiyuki Tatsumi1,3

1 Center for Data and Sample Analyses, Institute for Frontier Research on Earth Evolution (IFREE)2 Institute for Geothermal Sciences, Kyoto University at Beppu3 Research Program for Geochemical Evolution, Institute for Frontier Research on Earth Evolution (IFREE)

374

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1

Ni(1g), S(0.8g) and fluxes (12g of Na2CO3 and 24g ofNa2B4O7). The mixture was transferred into a fire clay crucibleand fused in a muffle furnace at 1010ºC for 1hr. After thefusion was completed, the melt was immediately poured into astainless mold and cooled. The resulting NiS bead was sepa-rated from the slag while the residual slag was crushed intochips for a second fusion. The residual slag chips were thenmixed with Ni (1g), S (0.8g) and half the previous amount offluxes, and fused in the same manner as the first fusion. Theresulting two beads were roughly crushed using a hardenedsteel mortar. The chips were transferred to a 100ml tall beakercovered with a watch glass and dissolved in 50ml of concen-trated HCl solution on a hotplate (120ºC) for 24hr. After thedissolution is completed, 20ml of water and 0.6ml of Te solu-tion (1000ppm) were added to the sample solution. 5ml ofSnCl2 solution (10% for Sn) was then slowly added, while stir-ring the solution. Elemental Te was precipitated by the reduc-tion of tellurite by the stannous chloride (Sandell, 1959) andthe beaker was kept on a hotplate (120ºC) for 1hr until the pre-cipitate coagulated. After cooling the solution, 3ml of SnCl2

solution were again added and the solution stirred. The precip-itates were filtered using a Teflon membrane filter (φ; 47mm,pore size; 0.2µm) and washed with 20ml of 1M HCl solution,using a filtration flask. The filter was then transferred into thesame beaker that was used for HCl dissolution of the beads,and the precipitates on the filter are dissolved with 0.6ml ofconc. HNO3 and 1ml of conc. HCl on a hotplate (50ºC). Thissolution was carefully transferred to a polyethylene bottle. Thebeaker and filter were washed by 0.2ml of conc. HNO3 and2ml of conc. HCl solution, and this solution was also com-bined with the extracted solution. Finally, the solution wasdiluted with de-ionized water, up to 30g of total weight and0.1g of the internal standard solution (Cd, Tl; 1ppm) wasadded. This solution was kept as a working solution for PGEmeasurement.

Analysis

Mass spectrometry was carried out using a Plasma Quad 3(Thermo Elemental) that later had its lens system changed to achicane lens system. The background levels are normally20~30cps in this instrumental, though <10cps of backgroundcan be typically achieved by using the chicane lens system. APFA microflow nebulizer (100µl/min, Elemental Scientific)was used for reducing the amount of sample solution con-sumed, as well as reducing memory effects. The instrumentwas optimized to obtain a flat response (1.5~2×105cps) for1ppb of 115In, 140Ce and 238U with the operation of the S-option.Peak jumping mode (5 points) was used for scanning the set-tled mass range, and the dwell time of each channel was set to1ms. Scanning was carried out for 1 min and repeated 5 timesfor each sample. As described later, the isotopes measuredwere selected in order to minimize isobaric interferences (99Ruand 101Ru for Ru, 103Rh for Rh, 105Pd for Pd, 193Ir for Ir, 194Ptand 195Pt for Pt and 197Au for Au). For internal standardization,111Cd and 203Tl were adopted for the light PGEs (Ru, Rh andPd) and the heavy PGEs (Ir, Pt and Au), respectively.

Results and discussion

Spectral interferences

ICP-MS spectra for the JP-1 solutions following chemicalseparation, are shown in Figs. 2a and 2b for the light and heavyPGE mass range, respectively. The reagent blanks are shown bythe gray line. In these figures, it is obvious that most silicate-lov-ing elements such as rare earth elements (REEs) are effectivelyremoved by the NiS fire assay procedure, while the PGEs areconcentrated in the beads. Nickel, Cu, Zr, Mo and W are retainedin the solution and are possible interfering elements for the PGEmass range. Large amounts of Sn and Te are also found in thesolution. Although the precipitates were well washed with 1 MHCl, these impurities may adhere to the Te microprecipitate andbe recovered together with the PGEs. Since no significant signalis observed at the masses of 111Cd and 203Tl, it is confirmed thatCd and Tl can be used as internal standards.

Possible interferences for the ICP-MS analysis of PGEs aresummarized in Table 2. The isotopes used for the measurementsare italicized. They are chosen so as to reduce the spectral inter-ferences to minimum. Because fire assay effectively reduces Zn,Ge, Rb, Y, REEs, and Hf, only the interferences from argidesand chlorides of Ni and Cu are significant for the light PGEs(Ru, Pd and Rh). The contributions from the signal of interferentto that of analyte are evaluated for the sample solutions of JP-1and BHVO-2 in Table 3. In this table, k values were obtained bymeasuring Ni, Cu and Ta in standard solutions, individually. Asclearly shown in Table 3, fairly large interferences are observedfor Rh and Pd for both JP-1 and BHVO-2. Although the interfer-ences can be corrected for by measuring single solutions of Niand Cu in the same run as the sample solutions are measured,instrumental drift in the argide production ratio during a meas-urement sequence may cause significant errors in the obtainedresults because of the fairly large corrections.

Procedural blanks

Contamination from the reagents used is the most seriouslimitation of this method. PGE concentrations in proceduralblanks were determined by substituting a rock sample withpure silica. The results using various crucibles and reagentsare shown in Table 4, where the reference blank values arealso shown. The results for run 2 were normally used for theblank correction. Because the blank values vary significantlybetween the different crucibles used, it is obvious that theproblematic contamination of PGEs is mainly derived fromimpurities in the crucibles. However, our blank levels are stillhigher than those in the reference (Gros et al., 2002). Usinggraphite crucibles and a pure Te solution may reduce the blanklevels. The contribution of the PGEs in the procedural blank tothose in JP-1 and BHVO-2 are calculated in Table 5, alongwith the concentration limits that the 10% blank correctionimposes. It is found that significant blank contributions areincluded in the analytical results, particularly for Pd and Au.Since the present 10% blank correction levels are comparableto the concentration range for the oceanic island basalts(OIBs) in this table, our proposed technique is applicable toOIBs having relatively high concentrations of PGEs.

375

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1

Analytical results

The results of replicate analyses for JP-1 and BHVO-2 areshown in Table 6. Gold in JP-1 could not be determined due tothe contamination problems discussed above. The results forJP-1 agree well with the reference values (<5%), except for Ir(40%). Fairly good reproducibilities (<10%) are obtained forJP-1, however, BHVO-2 can not be analyzed with sufficientprecision. Errors over 10% were observed for Rh, Pd and Auin BHVO-2, which may be explained by large correction forspectral interferences on Rh and Pd, and improper blank sub-traction for Au.

Conclusion

The preconcentration method of NiS fire assay followed byTe coprecipitation was applied to the determination of tracePGEs in JP-1 and BHVO-2. Analytical reproducibility of lessthan 10% was obtained for JP-1. However, the reproducibili-ties for BHVO-2 were insufficient for Rh, Pd and Au. Largespectral interferences and relatively high blanks are the mainreasons for the large uncertainties for these results. It is con-cluded that our proposed method is applicable for the analysisof samples with higher PGE concentrations, such as peri-dotites. Further efforts are required to obtain precise analyticalresults for basaltic samples.

Acknowledgements. G. Shimoda (Inst. Geosci., AIST) is gratefullyacknowledged for providing the fire assay procedure reported in Oguriet al. (1999). M. Honda (IFREE, JAMSTEC) and K. Abe (IFREE,JAMSTEC) are also thanked for helping with the fire assay and withfurther chemical separations. Q. Chang (IFREE, JAMSTEC) is thankedfor helpful discussions and advice for operating our upgraded ICP-MS.This work is supported in part by a Grant-in-Aid (# 14703002) fromJSPS (Japan Society for the Promotion of Science) to Katz.

References

Ely, J. C., C. R. Neal, J. A. Jr. O’Neill, and J. C. Jain, Quantifying theplatinum group elements PGEs and gold in geological samplesusing cation exchange pretreatment and ultrasonic nebulizationinductively coupled plasma mass spectrometry (USN-ICP-MS),Chemical Geology, 157, 219-234, 1999.

Enzweiler, J., P. J. Potts, and K. E. Jarvis, Determination of platinum,palladium, ruthenium and iridium in geological samples by iso-tope dilution inductively coupled plasma mass spectrometry usinga sodium peroxide fusion and tellurium coprecipitation, Analyst,120, 1391-1396, 1995.

Govindaraju, K., 1989 compilation of working values and sampledescription for 272 geostandards, Geostandards Newsletter, 13, 1-113, 1989.

Gros, M., J. P. Lorand, and A. Luguet, Analysis of platinum groupelements and gold in geological materials using NiS fire assayand Te coprecipitation; the NiS dissolution step revisited, ChemicalGeology, 185, 179-190 2002.

Hoffman, E. L., A. J. Naldrett, J. C. V. Loon, R. G. V. Hancock, and A.Manson, The determination of All the platinum group elementsand gold in rocks and ore by neutron activation analysis afterpreconcentration by a nickel sulphide fire assay technique onlarge samples, Anal. Chim. Acta, 102, 157-166, 1978.

Jackson, S. E., B. J. Fryer, W. Gosse, D. C. Healey, H. P. Longerich,and D. F. Strong, Determination of the precious metals in geolog-ical materials by inductively coupled plasma mass spectrometry(ICP-MS) with nickel sulphide fire assay collection and telluriumcoprecipitation, Chemical Geology, 83, 119-132, 1990.

Jarvis, I., M. M. Totland, and K. E. Jarvis, Determination of platinumgroup elements in geological materials by ICP-MS using microwavedigestion, alkali fusion and cation exchange chromatography,Chemical Geology, 143, 27-42, 1997.

Oguri, K., G. Shimoda, and Y. Tatsumi, Quantitative determination ofgold and the platinum-group elements in geological samples usingimproved NiS fire-assay and tellurium coprecipitation with induc-tively coupled plasma mass spectrometry (ICP-MS), ChemicalGeology, 157, 189-197, 1999.

Paukert, T., and I. Rube ka, Effects of fusion charge composition onthe determination of platinum group elements using collectioninto a minimized nickel sulphide button, Anal. Chim. Acta, 278,125-136, 1993.

Pearson, D. G., and S. J. Woodland, Solvent extraction/anionexchange separation and determination of PGEs (Os, Ir, Pt, Pd,Ru) and Re-Os isotopes in geological samples by isotope dilutionICP-MS, Chemical Geology, 165, 87-107, 2000.

Plessen, H. G., and J. Erzinger, Determination of the platinum groupelements and gold in twenty rock reference materials by induc-tively coupled plasma mass spectrometry (ICP-MS) after precon-centration by nickel sulfide fire assay, Geostandards Newsletter,22, 187-194, 1998.

Rehkämper, M., A. N. Halliday, and R. F. Wentz, Low-blank diges-tion of geological samples for platinum group element analysisusing a modified Carius Tube design, Fresenius J. Anal. Chem.,361, 217-219, 1998.

Rehkämper, M., A. N. Halliday, J. G. Fitton, D. C. Lee, M. Wieneke,and N. T. Arndt, Ir, Ru, Pt and Pd in basalts and komatiites: newconstraints for the geochemical behavior of the platinum groupelements in the mantle, Geochim. Cosmochim. Acta, 63, 3915-3934, 1999.

Sandell, E. B., Colorimetric determination of traces of metals, A seriesof monographs on analytical chemistry and its applications, 3rded., John Wiley & Sons, New York, 711, 1959.

Schmidt, G., and E. Pernicka, The determination of platinum groupelements (PGE) in target rocks and fall-back material of theNördlinger Ries impact crater, Germany, Geochim. Cosmochim.Acta, 58, 5083-5090, 1994.

Shazali, I., L. V. Dack, and R. Gijbels, Determination of Preciousmetals in ores and rocks by thermal neutron activation/spectrome-try after preconcentration by nickel sulphide fire assay and copre-cipitation with tellurium, Anal. Chim. Acta., 196, 49-58, 1987.

Shirey, S. B., and R. J. Walker, Carius tube digestion for low blankrhenium-osmium analysis, Anal. Chem., 67, 2136-2141, 1995.

Yli, S., G. Xiyun, and D. Andao, Determination of platinum groupelements by inductively coupled plasma mass spectrometry com-bined with nickel sulfide fire assay and tellurium coprecipitation,Spectrochim. Acta, B53, 1463-1467, 1998.

376

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1

Figure 2. ICP-MS spectra of the JP-1 solution after NiS fire assay followed by Te coprecipitation.

Figure 1. Chemical separation procedure for PGEs in rock samples.

377

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1

378

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1