ANALYSIS OF GEOLOGICAL-MATERIALS FOR GOLD, PLATINUM AND PALLADIUM AT LOW PPB LEVELS BY FIRE ASSAY ICP MASS-SPECTROMETRY

The method to analyse rocks, soils, sediments and humus samples for Au, Pt and Pd at low ppb (ng g-1) levels by Pb fire assay and nebulization ICP-MS is described. This simple and cost effective method is capable of achieving practical detection limits of 2, 0.1 and 0.5 ppb for Au, Pt and Pd, respectively, representing significant improvement over conventional methods for Pt and Pd. Detection capability for Au and Pd would be further enhanced by the purification of flux reagents and dedication of assay equipment (furnaces, crucibles) to the processing of low-level samples only. Replicate results are given for six new geological reference materials intended for monitoring accuracy and precision of analysis for Au and platinum-group elements present at both low and high concentration ranges. Precision at low ppb concentrations is dominated by homogeneity of the element in a particular sample rather than by the variability inherent in the method itself. For example, an average relative standard deviation (RSD) for Pt at the 5-ppb level is +/-9% in the diabase, TDB-1, but climbs to +/-42% in the gabbro, WGB-1, at equivalent Pt concentration. Pd is also more heterogeneously distributed in the gabbro but typically the RSD for Pd at concentrations of 20-1300 ppb is in the range +/-3-10%. The nugget effect for Au is evident, with RSD's in the range +/-11-55%, independent of concentration level or nearness to detection limit. Evidence of excellent precision at analyte levels between 1 and 20 ppb is seen in replicate analyses of 16 sediment and humus samples taken in recent surveys. This methodology will facilitate studies of the mobility of Au, Pt and Pd in the surficial environment and improve our understanding of their behaviour, knowledge hitherto impeded by inadequate analytical capabilities.