RAPID DETERMINATION OF OS ISOTOPIC COMPOSITION BY SPARGING OSO4 INTO A MAGNETIC-SECTOR ICP-MS

Abstract

We present a method for the rapid determination of Os isotopic compositions and platinum group element (PGE) concentrations using ICP-MS on the same sample split. The method makes use of transfer of volatile OsO4 by an Ar gas stream into the torch of a magnetic sector ICP-MS for analysis. A variety of partial or complete sample dissolution methods can be used, namely (1) microwave oven digestion of samples in pressurized Teflon vessels under oxidizing acidic conditions, (2) NiS fire assay for complete sample dissolution and PGE preconcentration, (3) acid leaching techniques to extract labile PGE fractions, and (4) Carius tube digestion. Microwave oven sample digestion with HF-HNO3-HCl in an all-Teflon system as well as Carius tube digestion also allows the determination of Os isotopic composition and Os and Re concentrations on the same sample split. Spike-sample equilibration can be monitored on-line during sparging into the ICP-MS. The elimination of a nebulizer for Os isotope ratio measurements minimizes memory problems that are often associated with the determination of Os isotope ratios in liquid samples by ICP-MS. The external reproducibility of 187Os/188Os measurements of an in-house Os standard solution using a single-collector, magnetic sector ICP-MS (Finnigan ELEMENT) is 0.78% (1 S.D., n=13) for analyte amounts ranging from 81 pg to 1.22 ng total Os. The ion yield for 7-min data acquisitions is ~5x10-5. Compared to negative thermal ionization mass spectrometry (N-TIMS), sample throughput is about on an order of magnitude higher. If NiS fire assay is used for PGE preconcentration, complementary PGE can be determined in the liquid residue by ICP-MS using a microconcentric nebulizer after the extraction of OsO4.