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

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.