A new method for determining ²³⁶U/²³⁸U isotope ratios in environmental samples by means OF ICP-MS/MS

The ²³⁶U/²³⁸U 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 ²³⁶U/²³⁸U ratios, below 10^-8, in environmental samples by combination of ICP-MS/MS for ²³⁶U/²³⁸U ratio and multiple collector ICPMS measurements for ²³⁵U/²³⁸U and ²³⁴U/²³⁵U 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 O₂ and CO₂ 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 (²³⁵U¹⁶O¹H⁺/²³⁵U¹⁶O⁺) of about 10^-7 with O₂ in the collision cell. The method was validated through measurements of two certified IRMM standards with ²³⁶U/²³⁸U isotope ratio of 1.245 × 10^-7 and 1.052 × 10^-8, 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. ²³⁶U/²³⁸U atomic ratios between 1.5 × 10^-8 and 7 × 10^-9 were obtained with a level accuracy lower than 20%.