The determination of platinum group elements (PGEs) in geological samples is difficult because of their low and large rang abundance and heterogeneous distribution. Pre-concentration and separation must be needed for a successful analysis. The techniques of fire assay and Carius tube are usually used for sample digestion. The NiS fire assay technique utilizes relatively large amounts of sample, so the nugget effect of sample heterogeneity is largely reduced, but the procedural blanks for PGEs are relatively high. Because samples are dissolved with aqua regia in a sealed glass tube under high temperatures, the Carius tube technique has the lowest procedural blanks amongst all the techniques and all the PGEs are in their highest oxidation state with complete equilibration between spikes and samples. However, the Carius tube is highly explosive at high temperature.

Professor QI Liang from Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) has improved the technique for PGE determination in geological samples since 2010. In his study, samples are first digested by HF in a custom made 120 ml PTFE beaker on a hot plate to remove silicates. The dried residue is then digested with HF + HNO3 at 190 ℃ for about 48 hours after the beaker is sealed in a stainless steel pressure bomb. Even only HF + HNO3 are used for sample digestion at relatively low temperature, ultramafic rocks with refractory minerals can be effectively decomposed by this method. Analytical results for CCRMP (CANMET, Ottawa, Canada) certified reference materials WGB-1 (gabbro), UMT-1 (ultramafic ore tailings), WPR-1 (peridotite) and WMS-1(Massive Sulphide), agree well with the certified values. The proposed method not only simplifies the previously published analytical techniques, but also leads to a reliable, simple, low cost and suitable way for routine analysis of PGEs in mafic and ultramafic rocks and Ni-Cu-sulphides samples.

The above study was published on the latest issue of Journal of Analytical Atomic Spectrometry (Qi, L., Gao, JF., Huang, XW., Hu, J., Zhou, MF., and Zhong, H. An improved digestion technique for determination of platinum group elements in geological samples. Journal of Analytical Atomic Spectrometry, 2011, 26, 1900-1904.)

Fig.  A sketch diagram showing the design of the custom-made high pressure bomb (Image by IGCAS)

Table.  Analytical results (ng g-1) for reference materials Gpt-3 using different digestion techniques

(By QI Liang)