Chromatography purification of Rb for accurate isotopic analysis by MC-ICPMS: a comparison between AMP-PAN, cation-exchange, and Sr resins

The isotopic compositions of alkali metal elements are powerful tracers of various geological processes. Coupled K and Rb isotopic studies can potentially yield new clues on the mechanisms responsible for the depletions in moderately volatile elements in planetary objects, global surface geochemical cycles, and mechanistic aspects of water–rock interactions. Rubidium isotopic studies have however been hampered by difficulties in purifying Rb from rocks, notably due to its similar chemical behavior to K. Here we characterize the properties of three different types of resins (AMP-PAN resin; AG50W-X8 and AG50W-X12 cation-exchange resins; Sr resin) for Rb and K purification. We show that AMP-PAN resin and Sr resin can readily separate Rb from K and other matrix elements. However, AMP-PAN resin has a high Rb blank (∼80 ng) and is cumbersome to use, which limits its applicability. For cation resins, we test the effects of column length, acid molarity, temperature, pressure drop (flow rate), and resin cross-linkage on the Rb separation using a Fluoropolymer Pneumatic Liquid Chromatography (FPLC) unit built in our laboratory. Increasing column length or resin cross-linkage has a positive effect on the separation, while increasing acid molarity, temperature, or pressure drop (flow rate) has negative impacts. Gravity-driven cation-exchange resin columns fail to cleanly separate Rb from K, but an AG50W-X12 resin column of 150 cm length and 0.16 cm inner diameter installed on a FPLC unit can cleanly separate Rb from K. We separated Rb from synthetic and natural rock samples using three different purification schemes designed based on the three types of resins, and measured the Rb isotopic compositions of the Rb separates by MC-ICPMS. The three methods yielded consistent results, demonstrating the efficacy of our Rb separation and the accuracy of our Rb isotopic analyses. The Rb isotopic compositions of several geostandards were analyzed (BCR-2, BHVO-2, BE-N, AGV-2, GS-N, G-3, and G-A), which can be used in future studies for ground-truthing methodologies used for studying natural samples. Among the three methods, the Sr resin method is the most straightforward for purifying Rb and K simultaneously, and measuring their isotopic compositions in natural samples.