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

The U-236/U-238 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-236/U-239 ratios, below 10(-8), in environmental samples by combination of ICP-MS/MS for U-236/U-239 ratio and multiple collector ICPMS measurements for U-233/U-239 and U-234/U-235 isotope ratios. Since the hydride form of UO+ (UGH +) 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-2 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 Omega or an Aridus system, produced uranium oxide hydride rate ((UOH+)-U-235-O-160-H-1/(UO+)-U-235-O-16) of about 10(-7) with O-2 in the collision cell. The method was validated through measurements of two certified IRMM standards with U-236/U-239 isotope ratio of 1.245 x 10(-7) and 1.052 x 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. U-236/U-239 atomic ratios between 1.5 x 10(-8) and 7 x 10(-9) were obtained with a level accuracy lower than 20%.