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 P03 ion beams. Since P is monoisotopic, the mass spectrum for P0 3 − at 79, 80, and 81 corresponds to 1660, 170, and 180. 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, BaCl, 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 180-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 P0 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 0 15 O with 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.