Osmium isotope ratio determinations by negative thermal ionization mass spectrometry

A new technique for osmium isotope ratio determinations by negative thermal ionization mass spectrometry using the formation of OsO-3 ions is presented. Different filament materials and chemicals to reduce the electron work function are investigated. With Ba(OH)2 and platinum as filament material an ionization efficiency of more than 1% is obtained for nanogram sample amounts. The isotopic abundances of a laboratory standard are measured with relative standard deviations of less than 0.01% for the most abundant isotopes. This significant improvement in the precision of the osmium isotopic measurements, compared with previous investigations, provides a suitable tool to develop a Re---Os age determination method on this basis. The new isotopic data could also be used recalculate the atomic weight of osmium to be 190.2251 ± 0.0001, which is much better in uncertainty than the current IUPAC data.