Determination of platinum group elements in geological samples by isotope dilution inductively coupled plasma mass spectrometry using hydrogen in collision reaction interface

The determination of platinum group elements (PGE) in mafic and ultramafic rocks including chromites and sulfides has acquired importance in view of its application as a tool in understanding petrogenetic processes and mantle geochemistry, apart from PGE exploration studies. The distribution of PGE in different rock types varies from a few ng/g to several mg/g challenging its precise determination by normal inductively coupled plasma mass spectrometry (ICP-MS) methods, which is marred by polyatomic interferences, mass fractionation, sensitivity, reproducibility, etc. Sample dissolution and solution preparation also suffer from critical issues related to homogeneity and mineralogical composition of the rocks/ores, contributing sufficient matrix to distort precise PGE determinations. An attempt is made here to develop a new method for determination of Ru, Pd, Ir, and Pt, wherein isotope dilution technique involving cation-exchange pretreatment is adopted with ICP-MS using hydrogen in the collision reaction interface (CRI) for selective removal of the interfering argon ions and other plasma matrix ions. Sample preparation consists of its digestion along with PGE spikes (99Ru, 105Pd, 191Ir, and 195Pt obtained from ORNL, Tennessee), in a high pressure asher (HPA-S) with conc. HNO3 and HCl (5:2) at 300°C and 125° bar pressure for 15 h, followed by an off-line separation of major interfering species (e.g., Ni, Cu, Zn) using cation-exchange column (AG50W × 8, 200 mesh, hydrogen form). The eluated solutions were analyzed by ICP-MS with hydrogen as CRI gas for the determination of PGEs. Under optimal ICP-MS settings, the signal of PGE in hydrogen mode is reduced by 2-3 order magnitude (comparedwith normal mode) with a nominal increase in the resolution of analyte mass spectra, thus possibly allowing greater analyteion transmission efficiency and reduced space-charge effects. The method has been applied on two international geochemical reference material, namely, WMS-1 and Su-1b.