Significantly Enhanced Robustness of K Isotope Analysis by Collision Cell MC-ICP-MS and Its Application to the Returned Lunar Samples by China’s Chang’e-5 Project

Abstract

Stable K isotope ratios, an emerging research tool for a wide range of problems, can be measured precisely with high sensitivity by using collision cell multicollector ICP mass spectrometers (CC-MC-ICP-MS). However, it has been shown that the accuracy of K isotope analysis by CC-MC-ICP-MS could be compromised severely by trace-level Ca contaminants, although the cause of such an effect remains poorly understood. Here, we report that the influence of Ca on K isotope analysis by CC-MC-ICP-MS can be dramatically reduced if D2 rather than H2 (the default gas) is used as the reaction gas that goes into the collision cell. This indicates the generation of positively charged calcium-hydride molecules in the collision cell. Usage of D2 as reaction gas circumvents the Ca-induced inaccuracy issues during K isotope analysis because 40CaD+ does not interfere with 41K+ as 40CaH+ does; as such, the robustness of K isotope analysis by CC-MC-ICP-MS is significantly enhanced. This improved method is verified by K isotope analysis of seven geostandards, and applied to China's Chang'e-5 lunar return samples at submicrogram K consumption, revealing significant K isotope variability within a 17 mg lunar basalt fragment.