Simultaneous measurement of lithium and boron isotopes as lithium tetraborate ion by thermal ionization mass spectrometry

A method for the measurement of Li2BO2+ ions by thermal ionization mass spectrometry employing a Daly ion-counting system was examined. By solving simultaneous quadratic equations of intensity ratios ranging between m/z 55 and 57, and between m/z 56 and 57, precise and accurate measurement of lithium and boron isotope ratios can be carried out simultaneously. The intensity ratio between m/z 54 and 57 can be used to check the internal consistency of the lithium and boron isotope ratios obtainable. The isotopic fractionation rate with the species Li2BO2+ was low. Prior to measurement, a two-column ion-exchange process was used to separate lithium from the sample matrix and then to convert LiCl into LiOH, which subsequently reacts with H3BO3 to form lithium tetraborate. Lithium and boron isotope ratios were successfully measured simultaneously with precisions (relative standard deviations) of 0.05% for lithium and 0.01% for boron. (author)