High‐Precision Rb Isotope Analysis by MC‐ICP‐MS Using Sr Empirical External Normalization

ABSTRACT Rationale Stable rubidium (Rb) isotopes have received increasing attention as emerging geochemical tracers for investigating planetary evolution, water–silicate interactions, and global Rb cycling. High‐precision Rb isotope measurements by multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS) are commonly performed using the standard‐sample bracketing (SSB) method, but this approach is sensitive to short‐term instrumental drift. Methods Building upon the previously reported empirical external normalization (EEN) with 92 Zr/ 90 Zr combined with the SSB method, we adopted 88 Sr/ 86 Sr for EEN + SSB MC‐ICP‐MS Rb isotope measurement, owing to the closer mass discrimination between Sr and Rb. In this study, we correted the isobaric interference of 87 Sr on 87 Rb using an exponential law, compared Sr‐EEN with Zr‐EEN using the in‐house USTC‐Rb standard, and evaluated elemental doping effects (K, Ca, Na, and Mn). Results The results show that the USTC‐Rb standard yielded δ 87/85 Rb values of −0.15‰ ± 0.04‰ (2SD, n = 33) by Sr‐EEN and −0.16‰ ± 0.04‰ (2SD, n = 33) by Zr‐EEN, indistinguishable within analytical uncertainty. Elemental doping experiments suggest that the measured Rb isotope data were not significantly affected when the K/Rb, Ca/Rb, Na/Rb, and Mn/Rb ratios were up to 100. The long‐term intermediate reproducibility for NIST SRM 984 was 0.00‰ ± 0.04‰ (2SD, n = 121). Conclusions Because Sr can be effectively separated from Rb using Sr‐spec resin, this Sr‐EEN method provides a precise and reliable alternative for Rb isotope determination by MC‐ICP‐MS and exhibits high tolerance to matrix elements.