Review of methods to determine gold, platinum and palladium in production-oriented geochemical laboratories, with application of a statistical procedure to test for bias

Abstract

Current methods for determining Au, Pt and Pd in geological materials in Canadian commercial laboratories are reviewed. The relative merits of fire assay (Pb and NiS) and wet-chemical attack as methods of decomposition of rocks, soils and sediments are discussed. Amongst various analytical techniques, neutron activation is compared to atomic absorption spectrometry (AAS), and to three types of analysis employing the inductively coupled plasma as a source-emission spectrometry, mass spectrometry and atomic fluorescence. Clearly, the sensitivity and flexibility of ICP-mass spectrometry, introduced commercially in 1983, ensure a dominant role for this technique in the determination of Au and the platinum-group elements. A new statistical method for detecting bias, applied to the determination of Au in 157 rock samples, shows that instrumental neutron activation analysis (INAA) yields values which are 30% greater than those by graphite furnace AAS with an aqua regia attack; this same bias occurs in a second data set. Although INAA is an excellent technique for the determination of Au in vegetation, the low background levels of Pt and Pd and poor sensitivity of INAA for these elements make analysis by GF-AAS or ICP-MS more attractive. Further research is required in wet-chemical dissolution procedures for Pt and Pd in dried and ashed vegetation. Fire assay is an alternative choice of decomposition for ashed vegetation but, currently, low-level detection (to 1 ppb for 1 g samples) is hampered by contamination during fusion and by contributions to blank levels from flux constituents. Gold, Pt and Pd in waters can now be determined to levels of 1, 4 and 2 ppt, respectively, in a 1-L water sample using ICP-MS or graphite furnace AAS following preconcentration by adsorption on to activated charcoal. The detection limits for Pt and Pd must be lowered further by a factor of ten for application to exploration goechemistry.