Negative thermal ion mass spectrometry of oxygen in phosphates (original) (raw)

1997, Geochimica et Cosmochimica Acta

A novel technique for the precise measurement of oxygen isotopes by negative thermal ion mass spectrometry (NTIMS) is presented. The technique is ideally suited to the analysis of oxygen isotopes in phosphates which form intense PO; ion beams. Since P is monoisotopic, the mass spectrum for PO, at 79, 80, and 81 corresponds to 160, "0, and l8O. Natural and synthetic phosphates are converted and loaded on the mass spectrometer filament as Ag,P04 precipitated directly from ammoniacal solution. To lower the work function of the filament, BaCl, is added in a 1: 1 molar ratio of P04:Ba. Using these procedures, Br-mass interference (at 79 and 81 amu) is eliminated for typical analyses. Experiments with '*O-enriched water show less than 1% O-exchange between sample PO, and adsorbed water, and there is no O-exchange with trace O2 present in the mass spectrometer source chamber. The ionization efficiency of PO, as PO