NEGATIVE THERMAL ION MASS-SPECTROMETRY OF OXYGEN IN PHOSPHATES

A novel technique for the precise measurement of oxygen isotopes by ne gative thermal ion mass spectrometry (NTIMS) is presented. The techniq ue is ideally suited to the analysis of oxygen isotopes in phosphates which form intense PO3- ion beams. Since P is monoisotopic, the mass s pectrum for PO3- at 79, 80, and 81 corresponds to O-16, O-17, and O-18 . Natural and synthetic phosphates are converted and loaded on the mas s spectrometer filament as Ag3PO4 precipitated directly from ammoniaca l solution. To lower the work function of the filament, BaCl2 is added in a 1:1 molar ratio of PO4:Ba. Using these procedures, Br- mass inte rference (at 79 and 81 amu) is eliminated for typical analyses. Experi ments with O-18-enriched water show less than 1% O-exchange between sa mple PO4 and adsorbed water, and there is no O-exchange with trace O-2 present in the mass spectrometer source chamber. The ionization effic iency of PO4 as PO3- is > 10% compared to 0.01% for both conventional dual inlet Gas Isotope Ratio Mass Spectrometry (GIRMS) and secondary i on mass spectrometry (SIMS). Therefore, NTIMS offers exceptional sensi tivity enabling routine and precise oxygen isotope analysis of submicr ogram samples of PO4 (<21 nmoles equivalent CO2 gas) without need for lengthy chemical pretreatment of the sample. Overall external precisio n is +/- 1 parts per thousand (2 sigma) for O-18/O-16 and O-17/O-16 wi th reproducibility of instrumental isotope fractionation (calculated f rom O-18/O-16) of +/- 0.5 parts per thousand amu(-1) . Small phosphate samples including single mineral grains from meteorites, or apatite m icrofossils, can be analyzed by this technique. Copyright (C) 1997 Els evier Science Ltd