Osmium isotope ratio measurements by negative thermal ionization mass spectrometry (NTI-MS)

Summary

A negative thermal ionization technique using a Faraday cup as detector is presented, which allows precise isotope ratio measurements of osmium. For the major isotopes the relative standard deviation is in the range of 0.004% to 0.07%. Sample amounts down to 1 ng were loaded as hexachloroosmic acid. From the results it is concluded that also sample amounts in the pg range will result in sufficient ion currents of OsO ⁻₃ , avoiding an electron multiplier as detector. H₂OsCl₆ was chosen because this compound is the most suitable chemical form after separation of osmium from geological samples. The introduction of oxygen or freons into the ion source enhances the emission of OsO ⁻₃ ions by a factor of up to more than ten. Ionization efficiencies of more than 30% are obtained by using oxygen. The stability of the ion current is better in the case of oxygen than in the case of freons. However, both compounds are stabilizing the high voltage potentials of the optical ion lenses in the ion source, a permanent problem in NTI-MS. The described isotope ratio measurement is an important improvement for Re/Os dating of terrestrial and extraterrestrial samples.