Observations on sample size for lead isotopic analysis of ancient copper alloys

Abstract

Provenance studies of ancient copper alloys typically involve a combined approach using trace element and lead (Pb) isotopic analysis. While minimum sample masses for elemental analysis have been previously discussed in literature, these have rarely been considered for isotopic analysis. In the context of increasingly small samples employed in archaeometallurgical studies, this paper evaluates the effect of low sample mass for Pb isotopic analysis of different copper alloy and Pb standards. The Pb isotope ratio results obtained using multi-collector ICP-mass spectrometry (MC-ICP-MS) show excellent long-term stability and reproducibility for standards, ensuring comparability of data over time. However, some variability is observed in Pb isotope ratios obtained for different sample aliquots taken from individual samples and standards. This observed variability, often strongly exceeding the accepted precision for MC-ICP-MS Pb isotopic analysis, relates to low sample masses and is further exacerbated for low-lead alloys. These results suggest the existence of micro-scale heterogeneity in the Pb isotope ratios of some copper alloys, which can lead to (strongly) divergent results when taking very small samples – a problem which cannot be recognized a priori nor a posteriori without fully independent replicate analyses (i.e. based on another sample aliquot). In the context of archaeological research and the analysis of museum-curated objects, however, repeated sampling of ancient copper alloys is usually impossible. Thus, these results outline the necessity for sample masses of > 20–30 mg for the Pb isotopic analysis of copper alloys to prevent incompatible datasets: smaller samples may not be representative, even if their isotopic analysis is feasible.