Isotopic Measurements of Re, Os, Ir, and Ru by Quadrupole Negative Thermal Ionization Mass Spectrometry

A negative thermal ionization technique using a small and cost-efficient quadrupole mass spectrometer is presented. Intense ion beams of negatively charged oxides of Re, Os, Ir, and Ru were observed, and the principal molecular species formed in this technique are ReO4-, OsO3-, IrO2-, and RuO3-, respectively. The production of these ion species in the distinct mass ranges allows the isotopic analyses without any chemical separation of these elements each other. For 5 ng Os, all Os isotopic ratios were determined with the precision of better than ±0.5% except 184Os/192Os because of very low isotopic abundance of 184Os. For 30 ng Re, 10 ng Ir, and 30 ng Ru, isotopic measurements were also made with the precision of better than ±0.5%. The precision of the isotopic ratio measurements for Re, Os, Ir, and Ru is sufficient for the isotope dilution technique to determine the concentration of these elements in geological samples easily and cost-efficiently. In addition, an iridosmine from Hokkaido was analyzed for Os and Ir by direct loading technique, and 187Os/188Os ratio of 0.1229±0.0029 was obtained.