Negative thermal ion mass spectrometry of oxygen in phosphates

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 PO3- ion beams. Since P is monoisotopic, the mass spectrum 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 mass spectrometer filament as Ag3PO4 precipitated directly from ammoniacal 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 interference (at 79 and 81 amu) is eliminated for typical analyses. Experiments with O-18-enriched water show less than 1% O-exchange between sample PO4 and adsorbed water, and there is no O-exchange with trace O-2 present in the mass spectrometer source chamber. The ionization efficiency of PO4 as PO3- is > 10% compared to 0.01% for both conventional dual inlet Gas Isotope Ratio Mass Spectrometry (GIRMS) and secondary ion mass spectrometry (SIMS). Therefore, NTIMS offers exceptional sensitivity enabling routine and precise oxygen isotope analysis of submicrogram samples of PO4 (<21 nmoles equivalent CO2 gas) without need for lengthy chemical pretreatment of the sample. Overall external precision is +/- 1 parts per thousand (2 sigma) for O-18/O-16 and O-17/O-16 with reproducibility of instrumental isotope fractionation (calculated from O-18/O-16) of +/- 0.5 parts per thousand amu(-1) . Small phosphate samples including single mineral grains from meteorites, or apatite microfossils, can be analyzed by this technique. Copyright (C) 1997 Elsevier Science Ltd