Substantial Relief From Oxygen Isotope-related Isobaric Interference in Negative Thermal Ionization Mass Spectrometry

Abstract

A negative thermal ionization mass spectrometry (NTIMS) technique using ¹⁶ O-enriched oxygen is reported, which could overcome the main drawback of NTIMS related to the oxide isobaric interference in isotope ratio determination. To remove the natural oxygen residing in the ion source, the ion source and the oxygen reservoir of the oxygen bleeding system were evacuated to an ultimate vacuum of 8 × 10 ^{- 9} mbar before the highly ¹⁶ O-enriched oxygen (99.99%) was introduced. The OsO ₃ ^- ion was measured by NTIMS under the ¹⁶ O ₂ partial pressure of 2 × 10 ^{- 6} mbar to examine oxygen isotope composition in the OsO ₃ ^- ion. It was observed that the intensities of isobaric interference Os ¹⁶ O ₂ ¹⁷ O ^- and Os ¹⁶ O ₂ ¹⁸ O ^- are notably reduced by factors of 6 and 77 times, respectively, which correspond to a maximum 7.2 × 10 ^{- 16} A if the ion intensity of ¹⁹² Os ¹⁶ O ₃ ^- is 1 × 10 ^{- 11} A. The results show that the oxide isobaric interferences caused by ¹⁷ O and ¹⁸ O can be virtually eliminated and the technique seems very promising for isotope ratio measurement of Os and other elements by NTIMS in geochemistry and cosmochemistry research.