Determination of low concentrations of platinum group elements in geological samples by ID-ICP-MS

Abstract

Accurate and precise determination of platinum group elements (PGEs) at ppb concentrations in geological samples is important but problematic. This paper reports an improved analytical method for the determination of Pt, Pd, Ru, Ir and Rh at sub-ppb levels by isotope dilution-inductively coupled plasma mass spectrometry (ID-ICP-MS). Prior to experimentation, all reagents were carfully purified: HBO₃, HCl and SnCl₂ by Te coprecipitation and HNO₃ and HF by subboiling distillation. HF, HNO₃ and HCl were used to decompose 10 gram sample. The fluoride residue from the acid digestion is minimized by using H₃BO₃ for complexation, thus mini-fusion of sodium peroxide can be performed in corundum crucibles instead of bulk Na₂O₂ fusion to lower blank level. The solution of acid digestion and Na₂O₂ fusion are combined and PGEs are then pre-concentrated by Te coprecipitation. Cu, Ni, Zr and Hf are removed using cation exchange resin and P507 extraction chromatography resin combined in the same column to minimize their interference. Pt, Pd, Ru and Ir are determined using ID-ICP-MS, whereas the mono-isotopic element, Rh, is determined by external calibration using highly enriched ¹⁹⁴Pt as the internal standard. The enriched ¹⁹⁴Pt spike behaves similar to Rh during the Te precipitation procedure and acts as an ideal internal standard. The determination limits for PGE range from 0.01–0.19 ng g⁻¹. The results obtained using this new method for the CCRMP (CANMET, Ottawa, Canada) certified reference materials WGB-1 (gabbro), TDB-1 (diabase) and UMT-1 (ultramafic ore tailings), show good agreement with the reported values, but have discrepancies compared with the certified values when the concentration of Ru, Rh and Ir below 0.5 ng g⁻¹.