IRON ISOTOPE RATIO MEASUREMENTS BY NEGATIVE THERMAL IONIZATION MASS-SPECTROMETRY USING FEF4- MOLECULAR-IONS

Based on the formation of molecular FeF4- ions under thermal ion sourc e conditions, a negative thermal ionisation technique for iron was dev eloped. Compared with the widely used iron isotopic analysis by positi ve thermal ionisation mass spectrometry, a better control of mass-depe ndent isotopic fractionation is possible by generating molecular FeF4- ions instead of an atomic Fe+ ion beam at comparable ionisation effic iencies. In comparison with the positive thermal ionisation technique, an improvement in the internal (in-run) and external (between-nm) pre cision by a factor of three to four under equivalent experimental cond itions was achieved by this approach. Using a magnetic sector field ma ss spectrometer and multiple ion beam detection, external precisions w ell below 0.1% were obtained for the different iron isotope ratios wit hout 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.