A sample size of 5 to 50 grams can be used but the routine size is 30g for rock pulps, soils or sediments (exploration samples). The sample is mixed with fire assay fluxes (borax, soda ash, silica, litharge) and with Ag added as a collector and the mixture is placed in a fire clay crucible. The mixture is then preheated at 850°C, intermediate 950°C and finish 1060°C with the entire fusion process lasting 60 minutes. The crucibles are then removed from the assay furnace and the molten slag (lighter material) is carefully poured from the crucible into a mould, leaving a lead button at the base of the mould. The lead button is then placed in a preheated cupel which absorbs the lead when cupelled at 950°C to recover the Ag (doré bead) + Au, Pt and Pd.
The Ag doré bead is digested in hot (95°C) HNO 3 + HCl. After cooling for 2 hours the sample solution is analyzed for Au, Pt, Pd by a Perkin Elmer Sciex ELAN 6000, 6100 or 9000 ICP-MS. On each tray of 42 samples there are two method blanks, three sample duplicates, and 2 certified reference materials.
For vegetation ash samples, a lower weight can be used but will result in elevated detection limits. Smaller sample splits are used for high chromite or sulphide samples to ensure proper fluxing and metal recoveries.
If values exceed upper limits, reanalysis by fire assay Au, Pt, Pd (Code 8) is recommended.
Elements and Detection Limits (ppb)
Hoffman, Eric L. and Dunn, Bernie, 2002. Sample Preparation and Bulk Analytical Methods for PGE.CIM Special Volume 54 The Geology, Geochemistry and Mineral Beneficiation of Platinum Group Elements Edited by Louis J. Cabri, pp.1-11.
Hoffman, Eric L., Clark, John R. and Yeager, James R., 1998. Gold Analysis – Fire Assaying and Alternative Methods. Explor. Mining Geology , Volume 7, Nos. 1 and 2, pp. 155-160.