Precise Os isotope ratio and Re-Os isotope dilution measurements down to the picogram level using multicollector inductively coupled plasma mass spectrometry

This article describes analytical procedures to measure Os isotopic composition and to determine isotope dilution Re and Os concentrations accurately, precisely, and rapidly on a multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS). For Os measurements, a custom made apparatus to allow efficient direct OsO₄ distillation into the plasma source is described. Static multi-Faraday cup measurements on 50 ng commercially available Os standard yield 2σ external reproducibilities of 0.016% for ¹⁸⁷Os/¹⁸⁸Os and 0.017% for ¹⁸⁶Os/¹⁸⁸Os ratios (n = 5). The mean values are indistinguishable within analytical uncertainty from those determined by negative thermal ionization mass spectrometry (N-TIMS). Further, amounts of 25-250 pg of spiked and unspiked analytes of the same Os standard have been determined by ion counting, which reproduced the ¹⁸⁷Os/¹⁸⁸Os ratio of the Faraday cup measurements within analytical uncertainty. Thereby, the in-run precision increases from 1.2% (25 pg, 2σ) to 0.14% (200 pg) with an overall external reproducibility of 0.32%. Measuring procedures for Re include Ir doping of the analytes for in-run mass fractionation correction, whereby Ir ion beams are detected on Faraday collectors and Re ion beams are simultaneously measured on ion counting electron multiplier collectors. The potential of the method was tested using Re spike calibration and Re-blank measurements, which showed that 0.2 pg Re can be accurately measured to a precision better than 1%, whereas larger samples (>10 pg) allow precisions that are significantly better than any other analytical uncertainty such as weighing errors. Thus, the feasibility of high precision Re-Os analyses at the pg level with multicollector ICP-MS is demonstrated.