High-Precision Determination of 186Os/188Os and 187Os/188Os Isotope Ratios via an Antimony Fire Assay and Multi-collector ICP–MS

The ¹⁹⁰Pt–¹⁸⁶Os and ¹⁸⁷Re–¹⁸⁷Os isotopic systems are powerful tools for researching the evolution of the Earth and the Solar System. However, high-precision ¹⁸⁶Os/¹⁸⁸Os analyses remain challenging. In this study, the preconcentration of osmium (Os) has been achieved by the adoption and decomposition of large amounts of geological materials (over 20 g) with the novel use of the antimony (Sb) fire assay. This enables high-precision ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratio measurements. The total procedural blanks of Os have been reduced to less than 10 pg for samples of 20 g through the purification procedures for fire assay collector Sb₂O₃ and the use of quartz crucibles. This study also applies multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) for the Os isotope compositions, which is simple and fast and saves the complex polyatomic interferences and oxygen corrections of the traditional negative thermal ionization mass spectrometry (N-TIMS) method. Three Os standard solutions (DROsS, GSB, and HPS) and six rock reference materials were measured with this analytical protocol. The ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios of the DROsS are consistent with previously published values within errors including precision and accuracy. Repeated measurements of the DROsS Os standard solution on MC-ICPMS exhibit fine external reproducibility of better than 53 ppm (2 RSD) for ¹⁸⁶Os/¹⁸⁸Os, which is sufficient to identify subtle ¹⁸⁶Os/¹⁸⁸Os variations of geological samples. This study is the first to report the ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios of GSB and HPS, which are consistent with the N-TIMS values. Furthermore, the ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios of six geological reference materials are in good agreement with N-TIMS values, and the results of WPR-1a and GPt-5 are within measurement uncertainty of the previously published N-TIMS results, confirming the validity of our new analytical procedure for the accurate determination of ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios of geological samples.