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1C - Research - Fire Assay - Au, Pd, Pt-ICP/MS

A sample size of 5 to 50 grams can be used but the routine size is 30 g 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) HNO3 + HCl with a special complexing agent to prevent the Au, Pd and Pt from adsorbing onto the test tube.  After cooling for 2 hours the sample solution is analyzed for Au, Pt and Pd using 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. The ICP/MS is recalibrated every 45 samples. Smaller sample splits are used for high chromite or sulphide samples.

 

For vegetation ash samples a lower weight can be used but will result in elevated detection limits.  For a 1 g sample the detection limit will be 3 ppb for Pt and Pd. Vegetation and humus can be analyzed directly by pre-ashing in the fire assay crucible at 475 degrees to eliminate organics which do not go through the fire assay procedure directly very well.

If values exceed the upper limits, reanalysis by fire assay Au, Pt, Pd (Code 8) is recommended.

 

Code 1C-Reserch Elements and Detection Limits (ppb)

Element

Detection

Limit

Upper

Limit

Au

1

30,000

Pt

0.1

30,000

Pd

0.1

30,000

 

 

 

Reference :

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.

 


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