Determination of Trace Silver in Carbonate Rock Samples by Graphite Furnace Atomic Absorption Spectrometry with Ammonium Oxalate Matrix Modifier

Direct determination of trace silver in all kinds of geological samples by Graphite Furnace Atomic Absorption Spectrometry has been widely reported. In all of them, the samples were dissolved by acids. Thiourea and diammonium hydrogen phosphate were usually used as the matrix modifiers. However, the matrix elements of calcium and magnesium in the sample interfered with the measurement results of the silver, and the service life of the graphite tube was reduced, when the method was used to determine trace silver in carbonate rocks. For the study reported in this paper, the samples were dissolved by mixed acids of HCl-HNO₃-HF-HClO₄. 45.5 g/L ammonium oxalate was used as matrix modifier. The matrix elements of calcium and magnesium were separated by rapid reaction to form white precipitate. At the same time, a volatile ammonium salt formed, which can be removed by increasing the ashing temperature. The method has been evaluated by determining silver in the National Standard Reference Materials, and the relative uncertainty between the measured values and standard values is less than 10%. The detection limit of this method for silver is 0.010 μg/g, and the precision (n=12) is less than 1.0%. Compared with the previous Emission Spectrometry of one-meter grating (detection limit of 0.04 μg/g), this method not only separates out the interference matrix but also reduces the detection limit.