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