Simultaneous determination of the noble metals in geological material by radiochemical neutron-activation analysis

A simple procedure is described for determining the noble metals in geological samples rich in chromium and copper. Powdered rock samples (0.1 to 0.3 g) are irradiated in an epithermal neutron flux, under a cadmium filter, to reduce interference from 51Cr and 64Cu. After digestion with a peroxide fusion, the noble metals are separated on Srafion NMRR ion-exchange resin. Activity from chromium(VI), retained on the resin, is eliminated by reduction to chromium(III) with iron(II) sulphate before separation. The gamma-ray activity is measured with a lithium-drifted germanium detector for palladium (109Pd), platinum (199Au), iridium (194Ir) and gold (198Au). The separation yield (iridium 40%, palladium 80%, gold 90%) is determined for each sample by irradiating the resin to activate a stable noble metal carrier. Multi-element noble metal standards (0.1 to 8 μg) are prepared from dilute solutions (100 μl) evaporated on resin (1 ml). The practical detection limits are 2.5 ng of palladium and platinum, 0.1 ng of iridium and 0.01 ng of gold. The reliability of the procedure was confirmed by analysing sulphide standards (PTM, PTC) and standard rocks (PCC-1, DTS-1). The method has been applied to the analysis of chrome-spinels. (author)