High-efficiency and high-precision analysis of barium isotope ratios achieved through in-tandem column purification and ICP optimization

Abstract

Barium (Ba) isotopes have emerged as powerful tracers in geochemical, environmental, and cosmochemical studies. However, achieving high-precision Ba isotope measurements remains challenging due to matrix removal, procedural blanks, isotopic ratio measurement uncertainties, and accurate mass bias correction. Here, we develop a robust analytical protocol for δ^137/134Ba determination using a ¹³⁰Ba–¹³⁵Ba double spike on a Nu Plasma II MC-ICP-MS. Our method employs an in-tandem micro-column chromatography (AG50-X12 cation-exchange resin followed by Sr-Spec™ resin) to efficiently purify Ba from matrix elements with minimal acid consumption. By eliminating intermediate evaporation and re-dissolution steps, we achieve rapid purification of Ba with a procedural blank of only 278 pg, negligible for most geological samples. Both MATLAB simulations and experimental validation suggested an optimal of ∼20% double-spike proportion in the spike-sample mixture. Additionally, we found that a 200 ppb Ba concentration balances sample consumption, signal intensity and Faraday cup performance. To further refine sampling strategies and minimize isobaric interferences, we mapped the spatial distributions of Ba and Xenon (Xe) ion intensities, and isotope ratios in the ICP both in wet and dry plasma conditions, identifying a stable plasma region where Ba isotope ratios show minimal variability and Xe interference is low. We demonstrate that even trace matrix elements (a few millivolts in intensity) can significantly impact the precision in isotope ratio measurements. The method achieves a long-term external reproducibility better than 0.03‰ (2SD). Analyses of twelve geological reference materials (AGV-2, BCR-2, BHVO-2, BIR-1a, COQ-1, DTS-2B, GSO-2, GSP-2, GSR-8, JF-1, RGM-2, and SCo-1) yield δ^137/134Ba values consistent with published data except for three previously unreported materials (DTS-2B, JF-1, and SCo-1), confirming the reliability of the proposed method. This protocol provides a robust foundation for the mechanism of ion interaction in the ICP and contributes to high-precision Ba isotope applications across diverse geological processes.