SUBSTANTIAL RELIEF FROM OXYGEN ISOTOPE-RELATED ISOBARIC INTERFERENCE IN NEGATIVE THERMAL IONIZATION MASS-SPECTROMETRY

A negative thermal ionization mass spectrometry (NTIMS) technique usin g O-16-enriched oxygen is reported, which could overcome the main draw back of NTIMS related to the oxide isobaric interference in isotope ra tio determination. To remove the natural oxygen residing in the ion so urce, the ion source and the oxygen reservoir of the oxygen bleeding s ystem were evacuated to an ultimate vacuum of 8 x 10(-9) mbar before t he highly O-16-enriched oxygen (99.99%) was introduced. The OsO3- ion was measured by NTIMS under the O-16(2) partial pressure of 2 x 10(-6) mbar to examine oxygen isotope composition in the OsO3- ion. It was o bserved that the intensities of isobaric interference (OsO2O-)-O-16-O- 17 and (OsO2O-)-O-16-O-18 are notably reduced by factors of 6 and 77 t imes, respectively, which correspond to a maximum 7.2 x 10(-16) A if t he ion intensity of (OsO3-)-Os-192-O-16 is 1 x 10(-11) A. The results show that the oxide isobaric interferences caused by O-17 and O-18 can be virtually eliminated and the technique seems very promising for is otope ratio measurement of Os and other elements by NTIMS in geochemis try and cosmochemistry research.