Precise and accurate cadmium isotope analysis of carbonate samples by double spike MC-ICP-MS

Abstract

Cadmium (Cd) isotopes have great potential for tracing biogeochemical cycling in past and modern oceans. However, precise and accurate analysis of cadmium isotope compositions in carbonate samples remains challenging due to the extremely low Cd concentrations (<100 ng g⁻¹) and strong matrix interference from high-abundance Ca and Mg in carbonates. This study reports an optimized two-step chromatographic purification protocol employing AG 50W-X12 and AG MP-1M resins to efficiently isolate Cd from matrices in carbonate samples and minimize major element interference. Experiments with synthetic carbonates demonstrate that optimal yields (∼80%) are achieved at carbonate loadings equivalent to ∼10 mmol of total Ca + Mg. This method incorporates a ¹¹¹Cd–¹¹³Cd double spike for mass bias correction, enabling high-precision δ^114/110Cd ratio measurements with external precision of ±0.05‰ (2SD) using MC-ICP-MS. Validation with geological reference materials confirms the accuracy of this method, with procedural blanks below 0.09 ng (Cd_blank/Cd_sample < 5 × 10⁻³) and matrix interference below the critical thresholds. This refined analytical protocol enhances the applicability of Cd isotope analyses in paleoceanographic studies by improving analytical efficiency, reducing acid consumption and processing time, and enabling reliable separation of Cd from complex carbonate matrices. Its robustness for low-Cd-content carbonate samples (∼20 ng g⁻¹) highlights its potential for broad applications in paleoproductivity reconstructions and marine biogeochemistry research.