Tungsten isotope ratio determinations by negative thermal ionization mass spectrometry

Abstract

A precise determination of the isotopic abundances of tungsten with natural isotopic composition is presented. WO − 3 ions are generated by negative thermal ionization (NTI) in a double-filament ion source. La 2O 3 is used as a chemical substance to reduce the electron work function of the rhenium filament material. An ionization efficiency of 1% is obtained for sample loadings of 100 ng. The isotopic abundances are measured with relative standard deviations of 0.2% for the least abundant 180W isotope and 0.02–0.004% for the other tungsten isotopes. These improved isotopic data are used to recalculate the atomic weight of tungsten as 183.8417 ± 0.0001. The new NTI technique is an ideal tool for the application of isotope dilution mass spectrometry to analyse tungsten traces and for the measurement of isotopic shifts of this element in meteorites produced by the decay of 182Hf.