Microwave digestion and alkali fusion procedures for the determination of the platinum-group elements and gold in geological materials by ICP-MS

Inductively coupled plasma-mass spectrometry (ICP-MS) is an ideal technique for determining the platinum-group elements (PGEʹs) and An, with rapid data acquisition and low detection limits of 0.03–0.22 ng ml−1. Two methods are described for the preparation of geological samples using microwave digestion prior to determination of the PGEʹs and Au by ICP-MS. In one method, 0.5-g samples are dissolved in sealed all-PFA microwave digestion vessels using HN03HCIHFHC1O4 acids. Samples are transferred to open ʹPTFE beakers and evaporated to incipient dryness, final solutions being taken up in 1 M HCI prior to analysis. The method has been evaluated using a suite of well-characterised international reference materials (RMʹs). In some cases, the method resulted in a complete digestion and quantitative data were obtained for Rh, Pd, Ir, Pt and Au, although insufficiently low limits, of determination precluded the determination of all five elements in all materials. In other cases, an insoluble residue remained and recoveries of the PGEʹs and Au were dependent on the element concerned and the mineralogy of each sample. A second method employs 1-g samples and microwave digestion with aqua regia-HF, in higher-pressure Ultem-jacketed Teflon® PFA sealed-vessels. Samples are subsequently evaporated to near dryness, digested in 0.5 M HCI, filtered, and the insoluble residues are fused with small quantities of 1:1 Na2O2+Na2CO3 or Na2O2, before being dissolved in 0.5 M HCI. The combined solutions are analysed by ICP-MS. Data obtained for a wide range of RMʹs showed good agreement with reference values. Both methods provide viable means of quantifying Ru, Rh, Pd, Ir, Pt and Au in mineralised samples, but both are limited by modest lower limits of determination in samples of 0.2–1 μg g−1 Only the combined microwave digestion-minifusion technique yields fully quantitative data for samples containing refractory minerals.