Determination of Ag in Geological Materials by Isotope Dilution ICP-MS Using NH4HF2 Digestion and Interference Removal via Ammonia Coprecipitation

Abstract

Silver is a powerful geochemical tracer in various geological processes (e.g., hydrothermal deposit formation and evolution of the terrestrial and Martian mantles). The measurement of the ultra-trace Ag (< 0.1 μg g^-1) in geological samples by ICP-MS is often hindered due to serious polyatomic interferences arising from Zr, Nb and Y elements presented in samples. In this study, a simple and rapid ammonia coprecipitation method was developed to remove interference elements (Zr, Nb and Y) in NH₄HF₂ digests for the determination of Ag mass fractions in geological materials by isotope dilution ICP-MS (ID-ICP-MS). The removal of interference elements (Zr, Nb and Y) in digestion solutions of geological materials was achieved using ammonia coprecipitation, effectively eliminating their polyatomic interferences. To compensate for the incomplete recovery of Ag during ammonia coprecipitation, isotope dilution calibration was chosen over internal standard calibration to achieve final accurate results. The limit of quantitation for Ag using the proposed method was 1.83 ng g^-1. The results obtained for Ag mass fractionation in nine international geological reference materials were in good agreement with the published values obtained by ID-ICP-MS and ICP-MS/MS. The proposed simple, rapid and practical analytical method for the determination of Ag mass fractions improves our understanding of the behaviour of silver in cosmochemistry and geochemistry studies.