An Improved Fe—Ni Sulfide Fire Assay Method for Determination of Re,Platinum Group Elements,and Os Isotopic Ratios by Inductively Coupled Plasma- and Negative Thermal Ionization—Mass Spectrometry

An improved Fe–Ni sulfide fire assay method has been developed for determination of Re, the platinum group elements (PGE), and Os isotopic ratios using inductively coupled plasma–mass spectrometry (ICP-MS) and negative thermal ionization–mass spectrometry (NTI-MS). Recovery of Re using the neoclassical NiS fire assay technique is very low, but recoveries of up to 75% can be achieved by using Fe–Ni sulfide as a collector and Na₂B₄O₇ as a flux. Using isotope dilution for determination of Re, a number of standard reference materials were analyzed for PGE and Re, and the results are consistent with their certified values. Multiple analyses of the reference standard GBW 07290 produced precisions ranging from 2.2% for Os to 5.9% for Ir. The detection limits are 2 pg g⁻¹ for Ru, 1.5 pg g⁻¹ for Rh, 25 pg g⁻¹ for Pd, 23 pg g⁻¹ for Re, 0.7 pg g⁻¹ for Os, 1 pg g⁻¹ for Ir, and 6 pg g⁻¹ for Pt. The low procedural blank of Os (1.3 pg g⁻¹) makes the Fe–Ni sulfide fire assay suitable for analysis of Os isotopic compositions. Using NTI-MS, the ¹⁸⁷Os/¹⁸⁸Os ratios were measured for WPR-1 and they are consistent with literature values.