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

Abstract

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 obtained. 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.