Determination of the platinum group elements in geological materials
The available instrumental methods for platinum group element (PGE: Ru, Rh, Pd, Os, Ir, Pt) and gold (Au) determinations are reviewed. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and ICP-mass spectrometry (ICP-MS) enable rapid, multi-elemental analysis, their instrumental and analytical characteristics being discussed here. The suitability of ICP techniques to quantitatively determine the PGEs + Au is demonstrated. The detection limits by ICP-AES range from 6 to 29 ng.mL[sup]-1, while those for ICP-MS range from 0.3 to 0.22 ng.mL[sup]-1, for the individual PGEs + Au. A digestion step is generally required prior to the analysis of geological materials by ICP-AES and/or ICP-MS. Digestion procedures are reviewed, with particular attention to the new method of microwave digestion. A comparative study of three digestion methods was undertaken, a range of well-characterised rock reference materials being used to evaluate open acid digestion, microwave acid digestion and alkali fusion procedures. The precision and accuracy of results obtained by ICP-AES and ICP-MS for 46 elements demonstrates that no single digestion method is universally applicable. It is concluded that the best digestion procedure for PGE-bearing materials is a combination of microwave acid digestion followed by a fusion of the residue. Such a method was developed and its suitability is demonstrated using reference materials containing high levels of the PGEs + Au, which enable their direct determination by solution ICP-MS. Even with the superior sensitivity of ICP-MS, low concentrations of the PGEs + Au in most geological materials preclude the quantitation of unseparated samples. Separation methods which have been used (fire assay, coprecipitation, ion exchange, solvent extraction, distillation) are reviewed. Two ion-exchange methods were developed to separate the PGEs + Au from their associated matrix elements allowing their preconcentration prior to analysis. An anion-exchange method can be used in conjunction with ICP-MS for the separation and determination of ≥1 ng.g[sup]-1 Ir and> 1 [mu]g.g[sup]-1 Ru, Rh, Pd, Pt or Au. A cation-exchange ICP-MS procedure can be used to determine the PGEs at a wider range of concentrations. An evaluation using all the available PGE reference materials showed good agreement with reference values in most instances. Stable isotopes were used to evaluate this method and the results confirm that quantitative results may be obtained. The cation-exchange procedure can be scaled-up to larger samples thus enabling the determination of < 1 ng.mL[sup]-1 of individual PGEs. This is demonstrated using 5 g sub-samples and guidelines are given for further increases in sample size. Slurry nebulisation ICP-MS was developed for the determination of the PGEs + Au in solid samples without a prior digestion stage. An assessment of the method using reference materials demonstrated that quantitative results may be obtained for all seven PGEs + Au at levels above 50 to 200 ng.g[sup]-1 (depending on the element). This method is ideally suited to the routine analysis of mineralised samples or where only small sample sizes are available.