Application of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for the investigation of ancient silver coins
Résumé
The main characteristic of ancient silver coins lies in the existence of silver surface enrichment that often covers them. The non-destructive methods used so far to analyze these precious objects do not take into account this phenomenon, whose importance cannot be estimated without destruction of the coins. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), as a spot method particularly convenient for the analysis of solid samples, has been applied to ancient silver coins for the first time. This paper presents the LA-ICP-MS analytical protocol developed to characterize these objects, whose main innovation lies in the realization of concentration depth profiles for the major elements (Ag and Cu) and five main minor elements (Zn, Sn, Au, Pb, Bi). The concentrations determined in this way reflect the composition of the alloy that has not been disturbed by surface enrichment phenomena. Minor and trace elements are determined by average single spot analysis. The optimal experimental conditions have been defined by studying both modern alloys and ancient coins. The reliability of the matrix-matched standards and the reproducibility of the micro-samplings have been checked and the relevance of the protocol developed then verified by a comparison of the results obtained by LA-ICP-MS and fast neutron activation analysis.