New technique for the determination of trace noble metal content in geological and process materials

A new two-step sample preparation technique is proposed for the instrumental determination of trace quantities of noble metals (NM) in refractory geological and process materials. The decomposition procedure is based on the oxidizing fluorination of samples with subsequent sulfatization (OFS) of the sample melt or cake. Fluorination of samples is accomplished using a mixture of KHF2+KBrF4 or KHF2+BrF3 depending on the ratio of sample mass to oxidizing mixture. Both cakes and melts can result using this procedure. Sulfatization of resulting fluorides is completed using concentrated sulfuric acid heated to 550 °C. Validation studies using certified geostandard reference materials (GSO VP-2, ZH-3, Matte RTP, HO-1, SARM-7) have shown that the proposed method is fast, convenient and most often produces non-hygroscopic homogeneous residues suitable for analysis by atomic absorption spectrometry (AAS) and atomic emission spectrometry (AES). Results obtained for NM concentrations in reference materials agreed with certified concentration ranges and results obtained using other methods of analysis. The OFS procedure combined with direct current plasma d.c. plasma AES achieved the following limits of detection (LOD) for the noble metals: Ag, Au, Pd, 1–2×10−6; Pt, 5×10−6; and Ru, Rh, Ir, Os, 1–3×10−7 wt.%. Using graphite furnace AAS (GFAAS) combined extraction pre-concentration the following LODs for NMs were achieved: Pt, Ru, 1×10−6; Pd, Rh, 1×10−7; and Au, Ag, 1−2×10−8 wt.%. The relative standard deviation for NM determinations (Sr) was dependent on NM concentration and sample type, but commonly was in the range of 3–15% for d.c. plasma AES and 5–30% for GFAAS.