Analysis of the CHARM Cu-alloy reference materials using excimer ns-LA-ICP-MS: assessment of matrix effects and applicability to artefact provenancing

Steenstra ES, Berndt J, Klemme S, Heginbotham A, Davies GR

Abstract

Excimer nano-second laser-ablation inductively coupled plasma mass spectrometry (ns-LA-ICP-MS) is an important tool in chemical characterization of copper alloys. However, variable matrix induced elemental fractionation in copper alloys pose a significant challenge. Here, we systematically investigate this issue by analyzing the CHARM (Cultural Heritage Alloy Reference Material Set) copper alloy targets. The extent to which silicate glass reference materials can be used in analyzing copper alloy targets is determined, as is the optimal internal standard, for a wide range of copper alloy compositions. Analyses were further optimized by quantitative assessments of the use of CHARM end-member materials as external standards for other CHARM copper alloy targets. The variable magnitude of observed matrix effects is most readily explained by variations in Cu/Zn ratios and resulting differences in melting- and boiling points, compared to external reference materials, making the Zn-rich copper alloy targets most prone to matrix effects. With the correct choice of an external matrix matched standard (Pb-rich CHARM reference material 32X LB14F) and internal element standardization, an accuracy of <20% can be achieved for virtually all elements of interest in brass and bronze artefacts, which is a significant improvement compared to the use of glass external reference materials