A Pretreatment Workflow for Cadmium Isotope Analysis of Ultra-Trace Samples Using Large-Mass Digestion and Stepwise Purification

Cadmium (Cd) occurs at ultra-trace levels in geological and environmental materials, and conventional digestion procedures often fail to provide sufficient Cd for high-precision isotopic analysis. Increasing the sample mass can alleviate this limitation; however, it introduces substantial matrix loads that complicate subsequent purification. Here, we develop an integrated pretreatment workflow for low-Cd, matrix-rich solid samples by combining large-mass digestion (up to 5 g), batch resin-based prepurification, and final column purification. This approach enables efficient Cd isolation and quantitative recovery under matrix-rich conditions while maintaining low total procedural blanks (0.04−0.10 ng). In this workflow, the AG-MP-1M resin is applied in a batch resin-based prepurification step prior to column purification to effectively remove major matrix elements. For the soil reference material GSS-3a, the Cd recoveries exceeded 95% across digestion masses of 0.5−5 g, whereas the δ114/110Cd values remained highly consistent (−0.373 ± 0.054‰, 2 SD, n = 14), indicating that no resolvable isotope bias was introduced with the increasing sample load. Binary mixing experiments between GSS-3a and NIST SRM 2711a show excellent agreement with the theoretical predictions (R2 = 0.9933). Further validation using soils derived from different parent materials and a suite of reference materials (soils, sediments, rocks, and plants) demonstrated the robustness and matrix tolerance of the workflow. Overall, this workflow provides a reliable and scalable pretreatment approach for gram-scale, low-Cd, matrix-rich samples, extending Cd isotope analysis to materials with Cd concentrations down to approximately 5 ng g−1.