In situ copper isotope analysis by femtosecond laser ablation multicollector inductively coupled plasma mass spectrometry (fs-LA-MC-ICP-MS) on historical gold coins

Abstract

This study investigates the potential of femtosecond laser ablation coupled with multicollector inductively coupled plasma mass spectrometry (fs-LA-MC-ICP-MS) for copper isotopic analysis in gold matrices applied to cultural heritage. Elemental analyses, which have commonly been used so far, provide information on the circulation of metal stocks based on elemental signatures but fail to pinpoint the precise source of gold. In contrast, isotopic analyses can offer a more accurate means of identifying the source of the metal, yet their application to gold matrices remains a challenge. For the first time, we successfully determined copper isotope ratio in gold matrices and achieved repeatabilities of 0.12‰ to 0.26‰ (2SD) for δ⁶⁵Cu analyses carried out over up to 8 days, demonstrating the feasibility of copper isotopic analyses in gold coins at the micron-scale. This work was conducted using isotopically characterised in-house matrix-matched gold standard with copper concentrations varying from 4.5 wt% to 9.6 wt%. Our results open new avenues of research for provenance studies of precious museum artefacts and archaeological finds, with potential applications in authentication analyses on similar gold materials. The micro-sampling performed by femtosecond laser ablation minimises the damages on such ancient artifacts. However, the Cu concentrations had to be of at least 4 wt% with our analytical set up and a special care must be taken on the laser beam focusing in order to obtain accurate δ⁶⁵Cu measurements in gold matrices.