The oxygen stable isotope composition of CRM 125-A UO2 standard reference material

While there is a clear need for standardized reference materials for analytical calibrations and for inter -laboratory comparisons, there are not currently any for the oxygen stable isotopic composition of uranium ox-ides. In this paper we summarize the results from four laboratories by seven different methods of oxygen stable isotope analyses using fluorination techniques of CRM 125-A UO2 Standard Reference Material. We synthesize these data and methods to arrive at a consensus oxygen stable isotope composition for CRM 125-A 818O = -9.63%o (+/- 0.29%o) VSMOW. We discuss methodological differences between analytical approaches, including furnace vs laser heating, fluorination using BrF5 or ClF3, as well as calibration strategies. We highlight the po-tential effects of calibration scale compression from single-point calibrations using reference material with 818O values having a large relative difference from the sample being analyzed. We demonstrate how calibration scale compression can yield differences in calibrated 818O values up to-2%o for samples with-20%o difference from a single reference material, if the calibration slope of different analytical systems differs by 0.1. We suggest the use of liquid water calibration standards sealed in silver capillary tubes for multi-point calibrations of fluorination analysis systems.