Fractional determination of gold in geological reference samples by sequential extraction/graphite furnace AAS

A five-step sequential chemical-extraction scheme is described for the analysis of Au in geological reference samples by solvent extraction/graphite furnace AAS. Each chemical fraction is operationally defined as follows: 1) 0.1 M ammonium chloride for the fraction of exchangeable form; 2) 0.1 M HCl - 0.1 M KI for the fraction of amorphous form; 3) 0.1 M HCl-0.2 M KI at about 100 degrees C for the fraction of metal form: 4) a mixture of HNO3 and HCl for the fraction of aqua regia soluble form; and 5) a mixture of HNO3, HCl and HF for the fraction of residual form. The relative standard deviations for Au at levels of 0.6 and 50 ng, with a few exceptions, were 33 and 6%, respectively. The detection limit was approximately 0.2 ppb for a 0.5 g sample. Satisfactory agreement was observed between the sum of the Au values from exchangeable to residual fractions and the reported total An values. The analytical results of Au for 11 geological reference samples, such as igneous rocks, sediments, soils and ores were tabulated, and the geological and mineralogical features are discussed.