High pressure asher digestion and an isotope dilution-ICP-MS method for the determination of platinum-group element concentrations in chromitite reference materials CHR-Bkg, GAN Pt-1 and HHH

New concentration data for Ru, Rh, Pd, Re, Os, Ir and Pt are presented for three chromitite reference materials. A simple and very effective procedure was applied for the measurements. Samples were spiked with enriched isotopes and digested in a HNO3/HCl (5+2) acid mixture at 300°C and 125 bar (1.25 × 107 Pa) pressure in a high pressure asher (HPA-S, Anton Paar). The programme settings were changed as a function of mass (0.5, 1, 2 and 4 g) and time (5 and 15 hours). Complete chromitite dissolutions for three digestions at each setting were monitored using XRD analyses of the amorphous residue after digestion. The osmium concentration was determined by sparging the OsO4 that was formed during digestion into a quadrupole ICP-MS. After drying and re-dissolution of the remaining residue, the other PGEs were separated on-line from their matrix in a simple cation-exchange column that was coupled to the ICP-MS. The concentrations were determined through isotope dilution and external calibration (Rh). By using the on-line separation, we were able to control interference effects (isobaric and molecular), which resulted in highly reproducible data. Replicate measurements of the reference material CHR-Bkg (SARM CRPG-CNRS) with sample masses ranging from 0.5 to 4 g showed very small standard deviations compared to the results from the initial collaborative trials and published data (e.g., 3.2% RSD vs. 32% RSD for Ru). Results for platinum showed the largest scatter, which is currently attributed to the small size of the test portion. In addition to CHR-Bkg, the first results for two chromitite reference materials "platinum ore" GAN Pt-1 and "chromium ore" HHH issued by the Central Geological Laboratory of Mongolia are presented.