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

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.

Building upon the previously reported empirical external normalization (EEN) with ⁹²Zr/⁹⁰Zr combined with the SSB method, we adopted ⁸⁸Sr/⁸⁶Sr 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 ⁸⁷Sr on ⁸⁷Rb 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).

The results show that the USTC-Rb standard yielded δ^87/85Rb 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).

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.