A Modified Acid Digestion Method for Analysis of Gold in Geological Samples: A Comparative Study

Quantification of gold in geological samples using fire-assay technique is a well-established and recommended method worldwide. However, the method is laborious, complicated, sluggish, and health hazardous. Only a well-trained chemist can perform gold analysis through fire assay routinely. Although classical aqua regia digestion for determination of gold yields high throughput, this method too has disadvantages of sample nature dependency, thus brings down the recovery levels. Herein, we report a modified aqua regia digestion method, followed by solvent extraction using methyl isobutyl ketone for the accurate and precise estimation of Au in variety of geological samples of different genre in broad concentration range. Roasting of sample was avoided and excess of aqua regia (6 times or higher than of sample weight) was used to bring all gold in aqueous phase. This method was applied to soil and rock samples and the results obtained were compared with conventional fire-assay technique. The analytical results for international reference materials of gold (SRM/CRM) measured by the proposed method were in close agreement with those obtained by fire-assay method and recommended values. Very low detection limits were achieved in the proposed method (1 ng/g by GF-AAS and 100 ng/g in F-AAS). This method also works satisfactorily at moderately higher concentration of gold in rock samples, at least up to 15 ppm. Thus fire-assay technique may be substituted by the proposed method for detection of Au in 10 ppb–15 ppm concentration range in the geological samples mentioned under present study.