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%. [Display omitted] •The ICP-MS/MS associated to a desolvating system reduces UOH+/UO+ down to 4.10−7.•O2 and CO2 show similar reaction profiles for U in the cell.•Precise uranium isotope ratios in sediments by combination of ICP-MS/MS and MC-ICP-MS.•Measurements of 236U/238U down to 10−8 in sediments for Fukushima area.•Precision obtained is the better ever achieved for 236U/238U ratios below 10−8.