Accurate determination of stable silver isotopes in zinc-rich samples through effective separation of silver and zinc

Abstract

Silver isotopes have been extensively used in environment, archaeology, and economic geology. Nevertheless, accurate analysis of Ag isotope ratios faces challenges caused by the presence of matrix elements (e.g. Zn, Cu, and Pb) in most sulfide samples due to isobaric and polyatomic interferences during multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) routines. Most of these elements, such as Cu and Pb, can be effectively removed after the first column through a tandem column setup. However, the complete removal of Zn from Ag remains a significant unresolved issue, particularly given their common coexistence in sulfide minerals. Combining previous studies and the chemical principle of ion exchange resin, we optimized a two-stage ion exchange procedure by meticulously adjusting solution volumes and the column separation process. A condensed Zn-doped NIST SRM (silver international standard reference material) 978a was tested for AG50W-X8 resin following our modified procedure, resulting in distinct elution curves of Ag and Zn. Subsequently, three sphalerite samples were selected for the optimized two-stage ion exchange procedure, and ICP-MS results of silver elution fractions demonstrated complete separation of Ag and Zn. The whole procedure achieved an Ag yield exceeding 98.1 ± 2.3% (2SD, n = 4). NIST SRM 978a and IAEA-S-1 (an in-house standard) were tested for long-term reproducibility with −0.001 ± 0.030‰ (2SD, n = 25) and 0.054 ± 0.023‰ (2SD, n = 22) of δ¹⁰⁹Ag, respectively, indicating the high accuracy in MC-ICPMS routines. Zn-doped NIST SRM 978a and sphalerite samples were further subjected to the modified procedure, and they exhibited values of 0.004 ± 0.014‰ (2SD, n = 3) and −0.214 ± 0.033‰ (2SD, n = 12), respectively, for δ¹⁰⁹Ag. Moreover, we selected eight sphalerite samples from several hypogene deposits in China and obtained δ¹⁰⁹Ag values from −0.229‰ to 2.227‰, indicating the significant Ag isotopic fractionation between Ag-bearing and Zn-rich minerals in hypogene deposits. In conclusion, this study shows the feasibility of accurately measuring Ag isotopes in Zn-rich samples and reveals their application potential in related Ag-bearing and Zn-rich deposits.