Evaluation of a combination of isotope dilution and single standard addition as an alternative calibration method for the determination of precious metals in lead fire assay buttons by laser ablation-inductively coupled plasma-mass spectrometry

This paper reports on an evaluation of the application of isotope dilution (for Pt, Pd and Ag) and single standard addition and internal standardization (for the mono-isotopic elements Au and Rh) in the analysis of lead buttons obtained by fire assay using laser ablation-inductively coupled plasma-mass spectrometry as an attempt to improve and evaluate the ultimate accuracy and precision of the analytical method. For this purpose, first, a spike lead button, containing the elements of interest in an altered isotopic composition, was prepared. Subsequently, the spike button thus obtained was checked for its homogeneity in terms of element contents and isotope ratios. Additional inductive melting was shown to further improve its homogeneity. In a next step, appropriate portions of this spike button were melted together with an adequate amount of every sample (lead button) to be analyzed and the resulting 'blend' lead buttons were then analyzed using either isotope dilution or single standard addition/internal standardization for quantification. Also external calibration versus matrix-matched lead standards was performed to determine the precious metal concentrations in the same samples, thus allowing comparison of the figures of merit with those of the combined isotope dilution and standard addition/internal standardization approach. Isotope dilution was shown to provide results for the determination of Pt, Pd, and Ag in lead buttons that are more accurate (average deviation between ID result and reference value of <2%) than those obtained by external calibration (average deviation between experimental result and reference value approximate to 8%). For the mono-isotopic elements Au and Rh, determined via single standard addition and internal standardization, no significant difference was observed between the results provided by the three methods investigated. (C) 2012 Elsevier B.V. All rights reserved.