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

Excimer nano-second laser ablation inductively coupled plasma mass spectrometry (ns-LA-ICP-MS) is an important tool in chemical characterization of Cu alloys. However, variable matrix-induced elemental fractionation in Cu alloys poses a significant challenge. We systematically investigate this issue by analysing the Cultural Heritage Alloy Reference Material Set (CHARM) of Cu alloy targets. The extent to which silicate glass reference materials can be used when analysing Cu alloy targets is determined, as is the optimal internal standard, for a wide range of Cu alloy compositions. Analyses were further optimized by quantitative assessments of the use of CHARM end-member materials as external standards for other CHARM Cu alloy targets. The variable magnitude of the observed matrix effects is most readily explained by variations in Cu/Zn ratios and resulting differences in melting and boiling points, compared with external reference materials, making the Zn-rich Cu 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 with the use of glass external reference materials.