Determination of trace gold in rock samples by a combination of two-stage solvent extraction and graphite furnace atomic absorption spectrometry: The problem of iron interference and its solution

For the accurate determination of trace gold in rocks and iron minerals by graphite furnace atomic absorption spectrometry, a two-stage solvent extraction method (diethyl ether and MIBK) was designed to prevent strong interference from iron and to effectively concentrate gold. The interference was perfectly condoled by adjusting some operating conditions in the solvent extraction. In the analytical method developed, the recovery of gold was >90%, and the precision (C.V.) was 11.7% (1 μg/l). When 0.5 ml of MIBK was used in the second extraction, the detection limit for gold was 0.13 μg/kg (concentration in rock) at S/N = 2. The gold concentration of geological standard rocks (JB-2 and JB-3) measured by the method proposed in this work was in close agreement with the recommended values. The gold concentration of sandstone and shale collected from the Shimanto belt, southwestern Japan, was in the range 1.0-2.5 and 1.1-1.8 μg/kg, respectively, while the gold concentration of pyrite contained in the above rocks was about ten times higher than that of the corresponding rocks.