Nickel sulfide fire assay improved for pre-concentration of platinum group elements in geological samples: a practical means of ultra-trace analysis combined with inductively coupled plasma-mass spectrometry

A method was developed for ultra-trace determination of all six platinum group elements (PGE) by inductively coupled plasma-mass spectrometry (ICP-MS). With involvement of Fe to the fusion charge compositions of the conventional NiS fire assay, a whole assay button in water may disintegrate into powder and then be dissolved with HCl. The insoluble residue was collected, and dissolved with HCl + H2O2 in a sealed Teflon beaker. After the HCl + H2O2 treatment the Os signal response is at least 10 times as high as that in HCl medium. With isotopic dilution method for determination of Os, all six PGE were analyzed in one sample preparation. This method was applied to a series of Chinese PGE certified reference materials, GBW 07288 to GBW 07294, and the results were found to be in reasonable agreement with the certified values. Limits of detection were assessed from the whole chemical procedure and they are 0.17 ng g−1 for Ru, 0.01 ng g−1 for Rh, 0.16 ng g−1 for Pd, 0.03 ng g−1 for Os, 0.07 ng g−1 for Ir and 0.11 ng g−1 for Pt. This method of precision (RSD%) was evaluated using two reference materials of GBW 07291 and GBW 07294, and it is within the range from 4.2% to 10.3%.