Rubidium purification via a single chemical column and its isotope measurement on geological standard materials by MC-ICP-MS

Abstract

A chromatographic procedure for Rb, K, Ba and Sr one-by-one separation from geological materials has been developed by using a single column with packing Sr-spec resin, followed by high-precision Rb isotope measurement by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The main matrix elements (Ti, Al, Mg, Fe, Mn, Na and Ca) were removed with 4.4 mL of 3 M HNO₃ first; Rb and K were then sequentially eluted in different volumes with the same acid. After that, Ba was eluted with 8 M HNO₃, and finally Sr was eluted with Milli-Q water. This procedure enables us to collect pure Rb, K, Ba and Sr one by one in a single column with recovery close to 100%. The purified Rb was measured by using a standard-sample bracketing method via MC-ICP-MS. The short-term precision for δ⁸⁷Rb was better than ±0.03‰ (2SD, for 15 hours), and the long-term (more than 12 months) external precision was better than ±0.06‰ (2SD) based on repeated analysis of Rb standard solution NIST SRM 984. Matrix effects on Rb isotope measurement by MC-ICP-MS were tested by doping pure Rb standard with various amounts of matrix elements and were found to be insignificant when Na/Rb < 2, Ca/Rb < 4 and K/Rb < 25. We measured a set of geological reference standards by this method and found significant Rb isotopic variation. Our study suggests that Rb isotope geochemistry may be a promising tracer of various cosmochemical and geological processes.