Probing the 186Os/188Os Precision Barrier: New Recommended Values for the DROsS Reference Material and an Assessment of Mixed 1011 and 1012 Ω Amplifier Arrays

We present high precision negative ion thermal ionisation mass spectrometry (N-TIMS) Os isotope measurement results for the DROsS isotope reference material (iCRM), to investigate the limits on the precision of TIMS-based ¹⁸⁶Os/¹⁸⁸Os results. We used analytical conditions previously highlighted to optimise precision, present a new flexible data processing protocol, and measured ¹⁸⁴Os intensities on a Faraday Cup equipped with an amplifier using a 10¹² Ω resistor. Despite a measurement procedure that minimised uncertainty contributions from counting statistics and Johnson-Nyquist noise, the intermediate measurement precision of our approach does not significantly improve on previous high precision Os isotope measurements, with the exception of ¹⁸⁴Os/¹⁸⁸Os. This is probably due to uncertainties in measured amplifier gain factors, which are greater when using mixed arrays of 10¹¹ and 10¹² Ω resistors than when using 10¹¹ Ω resistors alone, though Faraday Cup deterioration could also contribute. We propose that multi-dynamic Os isotope measurements could largely eliminate both of these uncertainties. Our ¹⁸⁴Os/¹⁸⁸Os measurement results are the most precise yet, yielding ¹⁸⁴Os/¹⁸⁸Os = 0.0013036 ± 0.0000007 (2s, n = 38). Additionally, we average our data with published data to recommend the following isotope ratios for DROsS: ¹⁸⁶Os/¹⁸⁸Os = 0.1199319 ± 0.0000024, ¹⁸⁷Os/¹⁸⁸Os = 0.1609227 ± 0.0000022, ¹⁸⁹Os/¹⁸⁸Os = 1.219709 ± 0.000010, ¹⁹⁰Os/¹⁸⁸Os = 1.983793 ± 0.000011.