Optimisation of ¹⁸⁶Os/¹⁸⁸Os Measurements by N‐TIMS Using Amplifiers Equipped with 10¹³ Ω Resistors

Abstract

Analysis of 186Os/188Os variations in most natural samples is hampered by their low Os mass fractions (usually < 100 pg g−1), requiring Os separation from up to a kilogram of powder to obtain high precision measurement results. Recently developed amplifiers equipped with 1013 Ω feedback resistors improve the signal to noise ratio by an order of magnitude, allowing acquisition of high precision data (2SE < 50 ppm) from 186Os16O3− ion beams of ~ 40 fA and thus analysis of less material (~ 10–500 g). Factors controlling the uncertainty of 186Os/188Os N‐TIMS measurement results, using a mixed array of 1013 and 1011 Ω amplifiers, include isobaric interferences, baseline variability, gain calibration, decay time corrections and oxygen isotope composition variations. Isobaric interferences present the greatest challenges, but these can be largely overcome by using double Pt filaments and periodically monitoring interferences during each measurement. The optimised procedures produce 186Os/188Os ratios with precisions close to the theoretical limits imposed by counting statistics and Johnson–Nyquist electronic noise for ion beams of 10–70 fA. The intermediate measurement precision (85 ppm; 2s) approaches theoretically expected values (~ 50–80 ppm). This precision is sufficient to allow 186Os/188Os analyses of crustal rocks, for which relatively large variations are expected.

A new gain calibration protocol for Faraday amplifiers equipped with a 1013 Ω resistor

“…Depending on the version, the amplifiers show different responses to any instrumental trouble in the constant current generator. This idea does not contradict with previous studies 17–19 that reported no systematic error in the isotopic data obtained by TIMS using the 3.3 pA constant current calibration.…”

“…A new calibration technique for the 10 13 U Faraday amplier was recently reported to achieve better precision in gain measurement. [16][17][18][19] This technique employs a fully sowarecontrolled procedure that entails the use of a 3.3 pA constant current generator, while the automation of the calibration protocol tends to obscure technical or instrumental problems. In practice, our preliminary isotopic data yielded a systematic error of >0.1&, suggesting a need for the validation of the calibration protocol prior to an isotopic analysis.…”

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

“…1 1 0 signal filtering. Though our results fail to improve on the precision of previous procedures for determining Os isotopes, they indicate a role for amplifier gain factor uncertainty (Reisberg and Zimmermann 2021) and potentially Faraday Cup deterioration (Fukai et al 2017, Di et al 2021 as important contributors to Os isotope imprecision, and highlight the need for multi-dynamic Os isotope measurements.…”

“…However, it is not possible to use multi-dynamic methods that measure 186 Os and 188 Os on the same cup with 10 12 Ω or 10 13 Ω amplifiers, as appropriate 186 Os signal intensities would likely saturate the 10 12 Ω amplifier during analysis of the second jump. The only case in which 10 12 Ω or 10 13 Ω amplifiers may be suitable for 186 Os determination in samples with low Os mass fractions (Reisberg and Zimmermann 2021), though the large associated uncertainties ($ 85 ppm) means their utility may be restricted to samples with high Pt/Os.…”

“…A weakness of using mixed amplifier arrays: A corollary of the improved measurement of 10 11 Ω amplifier gain factors in our study compared with those of Reisberg and Zimmermann (2021), is that measurements undertaken using mixed arrays of amplifiers (e.g., 10 11 Ω amplifiers mixed with 10 12 Ω or 10 13 Ω amplifiers) will lead to larger uncertainties in the measured 10 11 Ω amplifier gain factors than measurements using only 10 11 Ω amplifiers. The calibration current in our N-TIMS appears to be limited by the size of the largest resistor in the amplifier array, meaning that increased precision in the analysis of low abundance isotopes with 10 12 Ω or 10 13 Ω amplifiers may be offset by greater uncertainties in the isotopes used for normalisation and fractionation correction.…”