Re-Certification of the IRMM-183-187 Series of Uranium Nitrate Solution Reference Materials

This contribution describes the re-certification of the IRMM-183-187 series of uranium nitrate solution reference materials with isotope amount fractions n(235U)/n(U) ranging from 0.3 % - 4.5 %, certified for the uranium isotopic composition. This certification project was a collaboration between the Joint Research Centre's Unit G.2 in Geel/Belgium, the International Atomic Energy Agency (IAEA), in Seibersdorf Austria, the Oak Ridge National Laboratory (ORNL), in Oak Ridge, USA and the Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA) in France.

The IRMM-183-187 series of reference materials were originally produced and certified in the 1980s to 1990s. The so-called "major" isotope ratios n(235U)/n(238U) were certified using gas source mass spectrometry (GSMS) and the so-called "minor" isotope ratios n(234U)/n(238U) and n(236U)/n(238U) using thermal ionization mass spectrometry (TIMS). The minor ratios were re-certified in 2005, but since several years customers like the IAEA have reported discrepancies of their measurement results from the certified values. Therefore a re-certification project was started by JRC-G2 in 2019, which included a complete re-characterization of the IRMM-183-187 series. For the measurement of the major ratios the double spike method (TIMS/DS) according to ASTM C1871 and for the minor ratios the modified total evaporation method (TIMS/MTE) according to ASTM C1832 were used. New verification measurements were performed using TIMS/MTE by ORNL and CEA, and historical data for TIMS/MTE measurements on three instruments from 2012-2021 were obtained from the IAEA. Based on the good agreement of the verification measurements and historical data with the new measurement results obtained during the re-characterization at JRC-G2, the isotope ratios of the IRMM-183-187 series were re-certified in line with ISO 17034.

For the geochemistry and cosmochemistry communities, the re-certified isotope ratios for IRMM-184 are the most relevant new values from this project: n(235U)/n(238U) = 0.0072631(11), n(234U)/n(238U) = 0.000053196(16), n(236U)/n(238U) = 0.00000012410 (96) (coverage factor k=2).