Sk. Sahoo et A. Masuda, PRECISE DETERMINATION OF LITHIUM ISOTOPIC COMPOSITION BY THERMAL IONIZATION MASS-SPECTROMETRY IN NATURAL SAMPLES SUCH AS SEAWATER, Analytica chimica acta, 370(2-3), 1998, pp. 215-220
The isotopic composition of lithium in an NIST SRM 924 Li2CO3, isotopi
cally 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 minimi
ze isotopic fractionation, the Li-6(+)/Li-7(+) ion ratio was measured
by using a triple filament technique. The method produces a stable, hi
gh intensity Li+ ion beam that allows measurement of ng quantities of
lithium for several hours. Lithium was separated from sample matrix an
d further converted to LiOH by employing a two-column ion exchange pro
cess. The mass ratio of LiOH to phosphoric acid was nearly stoichiomet
ric in relation to Li3PO4. Lithium isotopes in a reference material su
pplied by NIST (L-SVEC Li2CO3) was measured to check the reproducibili
ty of the method. A comparison was made between two TIMS units equippe
d with different types of detectors (a Faraday cup and a secondary ele
ctron multiplier). This highly sensitive technique can be applied to d
etermine isotopic composition of Li in enriched isotopes as well as in
the examination of low concentration Li reservoirs. (C) 1998 Elsevier
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