Quantitative determination of the platinum-group elements and gold using NiS fire assay with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS )

Abstract

Traditional methods for separating the platinum-group elements (ruthenium, rhodium, palladium, osmium, iridium, platinum; PGE's) and gold (Au) from ores and rocks have been adapted for use with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Samples were prepared using NiS fire-assay techniques resulting in the production of a collector button weighing only ∼ 1 g. The button was ground to produce a flat surface and directly ablated using a Nd:YAG laser operating at fundamental wavelength. Quantitative data were obtained by external calibration against both natural geological reference materials (SARM-7, MINTEK 2/77 and CHR-Pt+) and synthetic Nis standard buttons produced by doping high-purity quartz sand with multi-element stock solutions. Analysis times are rapid, 30 s per analysis, and the sampling protocols developed offer the possibility of full automation. Precision of the measurements varied according to concentration and the element concerned, but were typically 10–15% relative standard deviation or better. Accuracy of the data was assessed by comparison of measured and reference values. Accuracy was partly dependent on element. Data for Ru, Rh and Pd in 2/77 and Ru in CHR-Pt+ were in excellent agreement with reference values, while recoveries for Rh in CHR-Pt+ were a little high. Data for Os and Au could only be evaluated in SARM-7 where agreement with reference values was good. Platinum consistently showed poor recovery with values which were typically 30% low. Experimentally determined detection limits were similar for all elements, from 10 to 90 ng g −1 The method provides for the rapid, simultaneous and sensitive determination of the PGE's and Au in rocks and ores.