Novel HF pre-enrichment approach for high-precision molybdenum isotope analysis in low-Mo carbonates by MC-ICP-MS

Molybdenum (Mo) isotopes in carbonates are powerful tracers for reconstructing paleoenvironmental conditions in ocean-atmosphere systems. However, obtaining high-precision Mo isotope ratio measurements in carbonates is challenging due to their ultralow Mo concentrations (typically 0.01-1 μg g^-1) and significant matrix effects, particularly in marine carbonates that represent key archives of ancient marine environments. Conventional chromatographic methods are hindered by limited resin capacity, which complicates the processing of large sample masses and often involves laborious, time-consuming procedures. In this study, we present a simplified and efficient Mo pre-enrichment protocol using hydrofluoric acid (HF), which effectively removes over 97% of major matrix elements (Ca, Mg, and Al) from carbonate samples. The remaining matrix components are further eliminated using a single-step DGA resin separation. The method consistently achieves low procedural blanks (<100 pg Mo), which are negligible relative to the typical processed sample size (>10 ng Mo). Mo isotope ratios were measured using MC-ICP-MS with a combined double-spike and standard-sample bracketing (SSB) technique for accurate mass bias correction. The method was validated using synthetic solutions and geological reference materials, yielding δ⁹⁸Mo values in excellent agreement with published values. Importantly, we report the first comprehensive Mo isotope data for seven carbonate reference materials, including both whole-rock and carbonate-associated fractions. This protocol provides a robust and streamlined analytical framework for Mo isotope analysis in low-Mo carbonates, enabling more efficient and reliable investigations of paleoenvironmental conditions.