Iron isotope ratio measurements by negative thermal ionisation mass spectrometry using FeF4- molecular ions

Based on the formation of molecular FeF4- ions under thermal ion source conditions, a negative thermal ionisation technique for iron was developed. Compared with the widely used iron isotopic analysis by positive thermal ionisation mass spectrometry, a better control of mass-dependent isotopic fractionation is possible by generating molecular FeF4- ions instead of an atomic Fe+ ion beam at comparable ionisation efficiencies. In comparison with the positive thermal ionisation technique, an improvement in the internal (in-run) and external (between-nm) precision by a factor of three to four under equivalent experimental conditions was achieved by this approach. Using a magnetic sector field mass spectrometer and multiple ion beam detection, external precisions well below 0.1% were obtained for the different iron isotope ratios without normalisation of the isotopic data. Using the Fe-54/Fe-56 isotope ratio as an internal standard for normalisation, external precisions of 0.006% for the Fe-57/Fe-58 isotope ratio and 0.02% for the Fe-58/Fe-56 isotope ratio were obtained for an iron sample of natural isotopic composition. (C) 1997 Elsevier Science B.V.