Rapid, precise and accurate Os isotope ratio measurements of nanogram to sub-nanogram amounts using multiple Faraday collectors and amplifiers equipped with 1012 Ω resistors by N-TIMS

Abstract

We present a new approach to obtain rapid and precise (0.1 % or better) Os isotopic compositions for small (nanogram to sub-nanogram) amounts of Os extracted from geological samples using static collection with Faraday cups and amplifiers equipped with 1012 Ω resistors, by negative thermal ionization mass spectrometry (N-TIMS/Triton Plus). The results show that the measurement repeatability of Os isotopic ratios (190Os/188Os, 187Os/188Os and 186Os/188Os) changes as a function of signal intensity that varied from 0.005 to 0.05V for 192OsO3− (here and throughout the text, given intensities are equivalent signals that would be obtained using a 1011 Ω standard amplifier; 0.001V (1mV) is equivalent to ~62500 cps). At 192OsO3− ion beams greater than 0.02V with 50 ratios measured over a 10min acquisition time, the repeatability on 187Os/188Os, as well as 186Os/188Os and 190Os/188Os, is better than that obtained by the conventional peak-hopping electron multiplier (SEM) at 192OsO3−=~200,000 cps with 500–100 ratios measured over a 30–60min acquisition time. At 192OsO3− ion beams of ~0.04V or above, the 187Os/188Os and 186Os/188Os data for loads of 1 and 0.1ng Os reference materials can be measured with a repeatability of <0.1% (2σ) and deviate by <0.1% from the accepted values. Similar results can be achieved for Os load sizes in geological samples as low as~0.025ng. In addition, duplicate Faraday measurements of six spiked peridotitic samples (total Os loaded=0.3–1.2ng) yield results within uncertainty of data obtained by peak-hopping SEM, including both 187Os/188Os and Os concentration. At much higher 192OsO3− ion beams (~0.5–1V) with 300 ratios taken (total acquisition time circa one hour), the Faraday measurements of 1 to 3ng Os loads of reference material solutions can generate high-precision 186Os/188Os data with a repeatability of 30–50ppm and a mean intermediate precision of 10–30ppm. Collectively, our tests demonstrate that the use of Faraday cups equipped with 1012 Ω amplifiers in a static collection mode by N-TIMS can rapidly produce precise and accurate (within 0.1 %) Os isotopic data (both 187Os/188Os and 186Os/188Os) for small amounts of Os (as low as 0.025ng) extracted from geological samples. The optimum 192OsO3− signal is 0.04V or above to achieve the best results. While the smaller signal intensities used in concert with 1012 Ω amplifiers necessarily yield lower precision measurements than the best achievable with larger signals on 1011 Ω amplifiers, the repeatability we have achieved on reference materials can expand the application of the 190Pt–186Os decay system to geological samples containing significantly less Os than those normally measured on 1011 Ω amplifiers, if isotopic variations in excess of 50ppm are present.