High-precision in-situ determination of silver isotopes in argentiferous minerals by fsLA-MC-ICP-MS

Silver (Ag) isotopes have emerged as effective tracers for investigating processes in Earth sciences, planetary evolution and archaeology. Conventional bulk Ag isotope analysis is typically performed using solution nebulizer (SN-) multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), which lacks spatial resolution and may mask heterogeneities within samples. In contrast, in-situ Ag isotope analysis enables precise detection of inter- and intragranular isotope variations, thanks to its high spatial resolution and analytical throughput. However, in-situ methods remain unreported. The scarcity of matrix-matched reference materials and the limited understanding of matrix effects among diverse samples are major constraints on the advancement of in-situ Ag isotope analysis.

This paper presents a robust in-situ Ag isotope analysis method for argentiferous minerals (electrum, tetrahedrite, proustite, native silver, and argentite), which is implemented using femtosecond laser ablation (LA) MC-ICP-MS. Significant matrix effects were observed among these argentiferous minerals during non-matrix-matched calibration under dry plasma conditions; however, these effects were effectively suppressed by introducing water vapor during in-situ analysis. Nonetheless, matrix effects persist when argentite was used to correct electrum, even under wet plasma conditions. Therefore, to ensure reliable in-situ results, the application of matrix-matched reference materials is indispensable. Four potential in-house reference materials were developed for in-situ Ag isotope analysis. Measured δ¹⁰⁹Ag_{NIST 978a} values (2SD, n = 3-4) by solution nebulizer method for native silver, argentite, tetrahedrite, and proustite are 0.00 ± 0.01 ‰, 0.13 ± 0.03 ‰, 0.23 ± 0.01 ‰, and -0.17 ± 0.01 ‰, respectively, recommended as reference values.

For the first time, an in-situ Ag isotope ratios analysis method for argentiferous minerals (electrum, tetrahedrite, proustite, native silver, and argentite) was developed using fsLA-MC-ICP-MS. A systematic evaluation of matrix effects in the five argentiferous minerals was conducted under both dry and wet plasma conditions. Additionally, four potential natural in-house reference materials for in-situ Ag isotope analysis using LA-MC-ICP-MS are proposed. This work is expected to broaden the applications of in-situ Ag isotopes.