Uranium isotope abundance ratio determination as UO2 species via mass shifting with O2 in a Sapphire Collision/Reaction Cells

Highlights
• 99.9 % of UO2 formation when using O2 as cell gas with sensitivity > 1500 V/ppm or > 900 V per ng/s
• High fractionation stability over several hours
• Short-term repeatability of 70-100 ppm (2σS SD) with a measurement precision 2σS SE of 50-70 ppm for 235U/238U using 13 ng U and a mass-scaled measurement precision of 2σs SE = 180-252 ppmm per measurement,
• Robustness against concentration mismatching up to at least 25%
• Excellent measurement trueness even when measuring samples with highly different degrees of 235U enrichment, with 235U/238UNBS-U030 = 0.031426 ± 0.000003 (2σS SD; N = 6) and 235U/238UNBS-U050 = 0.052777 ± 0.000005 (2σS SD; N = 6)

Uranium is the heaviest of the naturally occurring elements and comprises two long-lived radioactive isotopes (235U with t1/2 = 0.704 Ga and 238U with t1/2 = 4.47 Ga) and one short-lived radioactive isotope (234U with t1/2 = 246 ka; naturally abundant through continuous decay of 238U; Andresen et al., 2017). The U series decay chains end with stable Pb isotopes. As such, U has found wide spread application in geochronology, from dating of ancient geological and cosmological processes via U-Pb dating (e.g., Allegre et al., 1995; Amelin et al., 2010; Chang et al., 2006; Chew et al., 2011) to dating of very recent processes (< 1 Ma) such as carbonate formation through the U-series disequilibrium series including 234U (Ivanovich, 1994). Uranium is also widely used as nuclear fuel with artificially enriched 235U/238U, and more recently U isotope abundance ratios have been used to trace geochemical processes such as fluid-rock interactions and/or redox processes (Kendall et al., 2013; Lu et al., 2020). For many of these numerous applications, the high accuracy and precision determination of U isotope abundance ratios is vital.

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