A novel approach to achieve Os isotope equilibration under ambient conditions for negative thermal ionization mass spectrometry (N-TIMS) analysis

Abstract

The rhenium–osmium (Re–Os) isotopic system has emerged as a widely utilized tool in cosmochemistry and high-temperature geochemical studies. A critical prerequisite for high-precision Os isotope analysis via Negative Thermal Ionization Mass Spectrometry (N-TIMS) involves achieving complete isotopic equilibrium between the sample and spike through the isotope dilution technique. In this study, we present an innovative technique to address valence state disparities in Os species within mixed solutions, thereby enabling effective isotopic equilibration through a chemical reducing reaction with a strong reductant under ambient conditions. Experimental results demonstrate that a solution containing 0.05 mol L⁻¹ hydroxylamine hydrochloride (NH₂OH·HCl) coupled with 0.01 mol L⁻¹ HBr effectively stabilizes a nanogram-level Os mass. Systematic optimization of reductant concentration and sample-to-reductant ratio yielded a mean ¹⁹⁰Os/¹⁸⁸Os value of 1.2280 ± 0.0006 (2s, n = 18) for a mixed Os spike solution, closely aligned with the recommended value of 1.2278 ± 0.0003 (2s, n = 12) obtained through the conventional Carius tube method. Controlled comparative experiments revealed critical considerations for method implementation: (1) a freshly prepared reductant must be employed immediately to ensure reagent efficacy, and (2) precise absolute volume control of the reductant solution is essential to mitigate potential losses of volatile OsO₄ linked to extended decomposition times. This novel approach successfully achieves isotopic equilibration between sample–spike and inter-spike components under ambient conditions, offering significant analytical advantages over traditional methods. The validity of the proposed approach was demonstrated by analyzing reference material WMS-1a yielding a ¹⁸⁷Os/¹⁸⁸Os value of 0.1665 ± 0.0006 (2s, n = 5), which agrees within uncertainty with the established value of 0.1664 ± 0.0003 (2s, n = 5) obtained via the Carius tube method. The protocol enhances operational efficiency by reducing equilibration time and explosive risk, and decreases procedural Os blanks via reducing material and reagent consumption. The combined improvements in precision, cost-effectiveness, and methodological flexibility position this technique as a valuable advancement in Os isotope geochemistry.