Effects of Li Isotopic Fractionation during Ion Exchange on the Measurement Accuracy of Li Isotopes

Abstract

BACKGROUND Lithium isotopes are widely used in various fields of earth and planetary science. Accurate determination of the lithium isotopic ratio is the premise of tracing various natural processes. However, there are large deviations in the measured values of lithium isotopic reference materials reported by international laboratories. For example, the reported seawater δ7Li test value differs by 5‰. To avoid this fractionation, laboratories employ their own empirical recovery values obtained from recovery tests rather than a theoretical one. OBJECTIVES To investigate the reasons of this huge discrepancy and get an instructively theoretical value, so as to appraise Li fractionation during ion exchange and purification processes. METHODS Based on the theory of ion exchange, this study uses the normal distribution function to fit the leaching curve, and theoretically calculates the theoretical value of lithium isotope fractionation caused by the ion exchange purification process, which is independent of MC-ICP-MS detection. However, it has a direct impact on the accuracy of lithium isotope testing. On this basis, the relative recovery is defined to monitor lithium isotope fractionation. RESULTS The fitting calculations show that only when recovery is higher than 99.8%, there would be no observable Li isotope fractionation caused by ion exchange and purification processes. At present, laboratories in the world mainly judge whether or not isotope fractionation occurs in the separation process by absolute recovery or relative recovery. Due to the space charge effect of the test, the absolute recovery is easily overestimated, and > 99% of relative recovery does not reach the theoretically calculated one, indicating that the deviation of the test results of the same reference material in each laboratory is most likely due to ion exchange. CONCLUSIONS It is proposed that for each sample, it is only necessary to separately measure the lithium content in the receiving interval and the intervals before and after the ion exchange processes, and the obtained relative recovery value is compared with the theoretical value to determine whether observable lithium isotopic fractionation occurs during the separation and purification processes.

Keywords

• li isotopes
• relative recovery
• isotopic fractionation
• mc-icp-ms
• theoretical calculation