Contamination-free preparation of geological samples for ultra-trace gold and platinum-group element analysis

Abstract

In order to determine the most contaminant-free sample processing procedures for ultra-trace platinum-group element (PGE) and gold analysis, we have crushed and milled pure quartz in a variety of media and then performed analysis by inductively coupled plasma mass spectrometry (ICP-MS) using isotope dilution (Pt, Pd, Ru and Ir) and external calibration (Au and Rh). Initial sample desegregation was best achieved in a Mn steel jaw crusher which contributed <0.1 ppb Pt and Au, <0.05 ppb Pd and Ru, <0.01 ppb Rh and <0.006 ppb Ir. Final milling must be performed in a ceramic mill (or presumably agate, although this mill composition was not investigated). Up to 0.008 ppb Ir and 0.08 ppb Au can be contributed from the ceramic mill (α-alumina in our case), but other elements were at or below the limits of detection. Tungsten carbide appears to be a problem, both in terms of Au contribution (up to 0.19 ppb from the tungsten carbide press) and an isobaric interference of tungsten oxide on 198Pt. The latter problem can be avoided by choosing alternate Pt isotopes for spike and reference (e.g., 194Pt and 195Pt, respectively). Based on these results, we have determined that the optimum low-contaminant processing for whole rock analysis is initial crushing in the Mn steel jaw crusher followed by milling in α-alumina ceramic. This results in below detection limit blanks for Pt, Pd and Ru and a total contribution of up to 0.008 ppb Ir, 0.01 ppb Rh and up to 0.18 ppb Au. For mineral separates such as magnetite, sulphide or chromite, a combination of steel crushers and mills can be used, provided suitable acid washing is performed prior to sample dissolution and analysis.