High-precision measurement of Ag isotopes for silicate rocks by MC-ICP-MS

Abstract

Silver (Ag) isotopes can be useful geochemical and environmental tracers, but their applications are hindered by the challenges of Ag isotope analyses, especially for samples with extremely high matrix element and low Ag contents (<100 ng g⁻¹). In this study, we developed a new protocol by using four-step chromatography for high-precision Ag isotope measurements for silicate samples. The δ¹⁰⁹Ag values were measured using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) using dry plasma. The sensitivity was 100 V ppm⁻¹, reducing the consumption of Ag to 5 ng. Moreover, instrumental mass bias was corrected using the Pd-doping and standard-sample bracketing methods. The robustness of this method was assessed by analyzing the NIST SRM978a doped with different matrix element ratios. The average δ¹⁰⁹Ag_SRM978a value of synthetic solutions (0.00 ± 0.05‰; 2SD, n ≥ 4) is consistent with the recommended values (0). The external precision is 0.05‰ (2SD) based on the measurement of two in-house standards. We further analyzed seven USGS rocks, including basalt (BCR-2 and BIR-1), andesite (AGV-2), granite (GSP-2 and G-2), rhyolite (RGM-2), and ferromanganese crusts (NOD-P-1). Their δ¹⁰⁹Ag_SRM978a values vary from −0.24 ± 0.05‰ to 0.20 ± 0.05‰, suggesting that Ag isotopes can be significantly fractionated in silicate rocks. This study provides a great opportunity for the future application of Ag isotopes in geochemical and environmental studies.