A new method for determining U/U isotope ratios in environmental samples by means OF ICP-MS/MS.

The U/U isotope ratio is a widely used tracer, which provides information on source identification for safeguard purposes, nuclear forensic studies and environmental monitoring. This paper describes an original approach to determine U/U ratios, below 10, in environmental samples by combination of ICP-MS/MS for U/U ratio and multiple collector ICPMS measurements for U/U and U/U isotope ratios. Since the hydride form of UO (UOH) is less prone to occur than UH, we were focused on the oxidised forms of uranium in order to reduce hydride based-interferences in ICP-MS/MS. Then, in-cell ion-molecule reactions with O and CO were assessed to detect the uranium isotopes in mass-shift mode (Q1: U → Q2: UO). The performances in terms of UO sensitivity and minimisation of hydride form of UO were evaluated using five different desolvating systems. The best conditions, using an Apex Ω or an Aridus system, produced uranium oxide hydride rate (UOH/UO) of about 10 with O in the collision cell. The method was validated through measurements of two certified IRMM standards with U/U isotope ratio of 1.245 × 10 and 1.052 × 10, giving results in agreement with certified reference values. The relative standard deviations on seven independent measurements for each standard were respectively of 1.5% and 6.2%. Finally, environmental samples corresponding to sediments from the radioactive contamination plume emitted by the Fukushima Daiichi Nuclear Power Plant accident were analysed after a well-established uranium chemical separation procedure. U/U atomic ratios between 1.5 × 10 and 7 × 10 were obtained with a level accuracy lower than 20%.