Negative thermal ion mass spectrometry of oxygen in phosphates

Abstract

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_3^- ion beams. Since P is monoisotopic, the mass spectrum for PO_3^− at 79, 80, and 81 corresponds to ^(16)O, ^(17)O, and ^(18)O. Natural and synthetic phosphates are converted and loaded on the mass spectrometer filament as Ag_3PO_4 precipitated directly from ammoniacal solution. To lower the work function of the filament, BaCI_2, is added in a 1:1 molar ratio of PO_4:Ba. Using these procedures, Br^− mass interference (at 79 and 81 amu) is eliminated for typical analyses. Experiments with ^(18)O-enriched water show less than 1% O-exchange between sample PO_4 and adsorbed water, and there is no O-exchange with trace OZ present in the mass spectrometer source chamber. The ionization efficiency of PO_4, as PO_3^− 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 sub-microgram samples of PO_4, (<21 nmoles equivalent CO_2 gas) without need for lengthy chemical pre-treatment reproducibility of the sample. Overall external precision is ±1%c (2σ) for ^(18)O/^(16)O and ^(17)O/^(16)O with reproducibility of instrumental isotope fractionation (calculated from ^(18)O/^(16)O of ±0.5%c amu^(−1). Small phosphate samples including single mineral grains from meteorites, or apatite microfossils, can be analyzed by this technique.

Additional details