Iron isotope ratio measurements by negative thermal ionisation mass spectrometry using FeF 4− molecular ions

Abstract

Based on the formation of molecular FeF 4 − 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 FeF 4 − 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-run) 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 54Fe/ 56Fe isotope ratio as an internal standard for normalisation, external precisions of 0.006% for the 57Fe/ 58Fe isotope ratio and 0.02% for the 58Fe/ 56Fe isotope ratio were obtained for an iron sample of natural isotopic composition.