Determination of Au and Pt in Geological Samples by Graphite Furnace Atomic Absorption Spectrometry with Concentrate and Extraction by Foam Plastics and Thiourea

Extraction by foam plastics is a common method to determine platinum group elements (PGEs) for most geological samples, however, the high temperature ashing method conducted to release Au and Pt, has complex processing. According to the insoluble residues for Pt in aqua regia, produced by the high temperature ashing method, the analysis results are unstable and produce low recovery rates. The unsatisfactory duplicate results were yielded by the sole use of 20 g/L thiourea as the elution solution. In this paper, this method was improved by decomposing the samples in 50% aqua regia in a closed system, reduced by SnCl₂ and concentrated by foam plastics then eluted by 20 g/L thiourea solution-20% hydrochloric acid. Au and Pt were continuously determined by Graphite Furnace Atomic Absorption Spectrometry. The adsorption efficiency was stable under the oscillation time and the absorption temperature of 30 min and 20℃, respectively. The recovery rates of Au and Pt were both greater than 95%. The detection limits of the Au and Pt were 0.23 ng/g and 0.39 ng/g with RSD of 1.8%-10.3% and 1.3%-13.3% (n=10), respectively. The method has been verified by determination of national first grade standard material with satisfactory results. The extraction of foam plastics avoided the high-temperature ashing by being extracted several times with aqua regia and one time in 100℃ water bath which greatly simplified the chemical procedure. Compared with high-temperature ashing, the extraction temperature was greatly reduced, the analysis process was simple and the number of samples was significantly increased for each batch with less interferences and low blanks. The improved method met the needs of various testing of geological samples except for the insoluble Pt minerals by aqua regia.