Evaluation of pulsed radiofrequency glow discharge time-of-flight mass spectrometry for precious metal determination in lead fire assay buttons

Abstract

In this paper, an exploration of the capabilities and limitations of pulsed radiofrequency glow discharge time-of-flight mass spectrometry (GD-TOFMS) for the determination of the precious metals Ag, Au, Pd, Pt and Rh in lead buttons obtained by Pb fire assay is reported on. Since the matrix consists almost entirely of lead (>99%), the occurrence of doubly charged Pb (Pb2+) ions can hinder accurate determination of Rh. This problem was counteracted by relying on the time-resolved formation of different ion types over the pulse period of the glow discharge, which allows discrimination against the Pb2+ ions. The formation of ArCu+ ions as a result of the use of a copper anode was assessed to pose no threat to the accuracy of the results obtained for the set of samples analyzed as its contribution to the total signal at m/z = 103 could be adequately corrected for. The method developed was evaluated in terms of accuracy and precision using a set of Pb button standards with analyte concentrations between 5 and 100 μg g−1. For the purpose of validation, a 10 μg g−1 standard was considered as a sample. Overall, an acceptable accuracy was obtained with a bias of <5% between the experimental results and the corresponding reference values, except for Au, for which a larger deviation occurred. Precision values (repeatability) of typically <5% relative standard deviation (RSD) (for N = 3) were obtained and the limits of detection (LODs) vary from ∼0.020 μg g−1 for Ag to ∼0.080 μg g−1 for Pt.