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Published January 1991 | public
Journal Article

Negative thermal ion mass spectrometry of osmium, rhenium, and iridium


We report on a technique for obtaining intense ion beams of negatively charged oxides of Os, Re and Ir by thermal ionization, in a conventional surface ionization mass spectrometer. It was found that the principal ion species of Os, Re and Ir produced are OsO_3^−, ReO_4^− and IrO_2^−. The sharp distinction in the masses of the dominant molecular species produced by this technique permits the measurement of isotopic compositions of each element from mixtures of platinum-group elements without significant isobaric interferences. For ^(187)Re-^(187)Os isotope studies, this technique offers the advantage of isotopic analyses without prior chemical separation of Re from Os, as no isobaric interference between the oxides of ^(187)Os and ^(187)Re exists under these conditions. For 4 ng Os, stable ion currents of 3 × 10^(−12) A can be maintained for over one hour, which allows determination of isotopic ratios with a Faraday collector to a precision of better than ±2‰ (2 σ_m. For 70 pg Os, isotopic ratios can be measured with a precision of better than ±5‰ using a secondary electron multiplier. The detection limit for Os is estimated to be below 10^(−14) g. Osmium isotopic ratios have also been determined by direct loading of natural iridosmine with a precision of ±0.5‰ or better. We have obtained ionization efficiencies of 2–6% for Os and >20% for Re; these are superior to those reported for other techniques available to date and demonstrate that negative thermal ion mass spectrometry will have widespread application to ^(187)Re-^(187)Os chronometry and to studies of the geochemistry and environmental chemistry of the platinum-group elements.

Additional Information

© 1991 Pergamon Press. Received November 13, 1990; accepted in revised form December 13, 1990. We thank Professor K. G. Heumann for the suggestion that Os may form negative thermal ions. The iridosmine specimen was kindly provided by E. Olsen, of the Chicago Field Museum of Natural History. This research was supported by NASA grant NAG9-43 and NSF grant EAR 8816936. Division contribution #4947 (719). Editorial handling: G. Faure

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