Precise determination of lithium isotopic composition by thermal ionization mass spectrometry in natural samples such as seawater

The isotopic composition of lithium in an NIST SRM 924 Li2CO3, isotopically enriched (Li2CO3)-Li-6 supplied by ORNL and in seawater has been determined by using thermal ionization mass spectrometry (TIMS) based on the use of lithium phosphate as the ion source. In order to minimize isotopic fractionation, the Li-6(+)/Li-7(+) ion ratio was measured by using a triple filament technique. The method produces a stable, high intensity Li+ ion beam that allows measurement of ng quantities of lithium for several hours. Lithium was separated from sample matrix and further converted to LiOH by employing a two-column ion exchange process. The mass ratio of LiOH to phosphoric acid was nearly stoichiometric in relation to Li3PO4. Lithium isotopes in a reference material supplied by NIST (L-SVEC Li2CO3) was measured to check the reproducibility of the method. A comparison was made between two TIMS units equipped with different types of detectors (a Faraday cup and a secondary electron multiplier). This highly sensitive technique can be applied to determine isotopic composition of Li in enriched isotopes as well as in the examination of low concentration Li reservoirs. (C) 1998 Elsevier Science B.V. All rights reserved.