Development of gold reference materials for in situ Ag isotopic analysis

Abstract

Microanalysis of silver isotopes in gold (Au) by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) has been an important tool to reveal the source and enrichment processes of Au. However, preparing an Au-matrix reference material with homogeneous Ag isotope compositions at a micron scale remains a major challenge. This study presents a novel method to synthesize a homogeneous gold reference material, named NWU-Au, via flux-free fusion. Moreover, a natural gold reference material, DD440, is prepared for in situ Ag isotope measurement by LA-MC-ICP-MS. Elemental mapping obtained by electron probe microanalysis indicates that both gold reference materials have homogeneous Au and Ag contents, while LA-MC-ICP-MS measurements further demonstrate their homogeneous distribution of Ag isotopes. The intermediate precision of δ¹⁰⁹Ag was better than 0.06‰ (2s) for NWU-Au and DD440, which were suitable to serve as matrix-matched calibration materials. Silver isotopic compositions of six gold samples were also determined using LA-MC-ICP-MS with matrix-matched calibration, and the δ¹⁰⁹Ag_NIST978a values were consistent with those obtained by SN-MC-ICP-MS, suggesting that LA-MC-ICP-MS can accurately determine Ag isotopic ratios in gold samples. Thus, this synthesis method, along with an analytical approach, enables potential determination of spatially resolved Ag isotope compositions at the mineral scale to reveal the source and enrichment processes of Au in the Au deposit.