Measurement of lithium isotopic ratios as lithium tetraborate ions

Determination of Li-6/Li-7 ratios using Li2BO2+ ions analysed by thermal ionisation mass spectrometry with ion counting has been investigated by numerical simulation and experiment. It is possible to solve for one or more of the Li-6/Li-7 ratio, B-10/B-11 ratio and fractionation if one or more of the ratios of the 54, 55, 56, 57, 58 and 59 masses of Li2BO2+ are measured. Monte Carlo simulation of data sets suggests that within-sample precision on measurement of Li2BO2+ mass ratios is largely limited by counting statistics and should result in precision as good as 0.25 parts per thousand (1a) on the Li-6/Li-7 ratio. However repeatability of Li-6/Li-7 and B-10/B-11 ratios gives precisions no better than approximately +/-3 parts per thousand (1 sigma) and the probable cause is fractionation of the Li-6/Li-7 ratio on the filament due to the large Li+ ion beam produced during analysis of Li2BO2+. Plots of variation of sample fractionation with time confirms that fractionation is primarily controlled by varying Li-6/Li-7 ratio rather than varying fractionation of Li2BO2+ or B-10/(11) ratio. (Int J Mass Spectrom 202 (2000) 273-282) (C) 2000 Elsevier Science B.V.