High precision isotopic measurement of lithium by thermal ionization mass spectrometry

A new procedure for the determination of lithium isotope ratios based on the use of lithium phosphate as the loading material by positive thermal ionization mass spectrometry has been investigated. The Li-6(+)/Li-7(+) ion ratio was measured using a triple filament technique to minimize fractionation. A two-column ion exchange process was employed to separate lithium from sample matrix and convert to LiOH, which further reacts with phosphoric acid to form lithium phosphate. Optimal conditions for measurement were established. Lithium isotopes in reference material supplied by NIST as L-SVEC Li2CO3 were measured with a precision of 0.07%. A comparison was made by using two different types of thermal ionization mass spectrometers employing different detectors such as a Faraday cup assembly and secondary electron multiplier. The composition of lithium in several Li2CO3 reagents and in one meteorite was also determined by this procedure to examine possible variations in isotopic compositions.