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 tech nique for determining the platinum-group elements (PGE's) and Au, 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 PGE's and Au by ICP-MS. In one method, 0.5-g samples are dissolved in sealed all-PF A microwave digestion vessels using HNO3-HCl-HF-HClO4 acids. Samples a re transferred to open PTFE beakers and evaporated to incipient drynes s, final solutions being taken up in 1 M HCl prior to analysis. The me thod has been evaluated using a suite of well-characterised internatio nal reference materials (RM's). In some cases, the method resulted in a complete digestion and quantitative data were obtained for Ph, Pd, I r, Pt and Au, although insufficiently low limits of determination prec luded the determination of all five elements in all materials. In othe r cases, an insoluble residue remained and recoveries of the PGE's and Au were dependent on the element concerned and the mineralogy of each sample. A second method employs 1-g samples and microwave digestion w ith aqua regia-HF, in higher-pressure Ultem(R) jacketed Teflon(R) PFA sealed-vessels. Samples are subsequently evaporated to near dryness, d igested in 0.5 M HCl, filtered, and the insoluble residues are fused w ith small quantities of 1:1 Na2O2+Na2CO3 or Na2O2, before being dissol ved in 0.5 M HCl. The combined solutions are analysed by ICP-MS. Data obtained for a wide range of RM's showed good agreement with reference values. Both methods provide viable means of quantifying Ru, Ph, Pd, Ir, Pt and Au in mineralised samples, but both are limited by modest l ower limits of determination in samples of 0.2-1 mu g g(-1). Only the combined microwave digestion-minifusion technique yields fully quantit ative data for samples containing refractory minerals.