Determination of 236U/238U ratios in environmental samples using ICP-MS/MS.

International audienceOn account of the virtual absence of 236U in nature and its abundance closely related to its provenance, the 236U/238U ratio has been commonly employed as a tracer to provide information in source identification for safeguard purposes, nuclear forensic studies and environmental monitoring. Due to their high performance capabilities, ICP-MS techniques are currently the most widely employed to perform radionuclide determination. However, in the case of 236U and due to its very low relative abundance, its determination is limited by the formation of 235U1H+ in the plasma and by the influence of the tail of the major isotope 238U (abundance sensitivity).In this work, we present an approach based on the ICP-MS/MS technology to determine 236U/238U ratios in environmental samples. The combination of two quadrupole-based mass filters, before (Q1) and after (Q2) the cell, in the MS/MS configuration provides abundance sensitivity values lower than 10-10, avoiding the influence of the 238U tail. In order to reduce the influence of the hydridebased interferences, different desolvation systems have been evaluated in relation to the sensitivity and the hydride rate obtained. In-cell ion-molecule reactions with O2 and CO2 have also been assessed to detect the uranium isotopes in mass-shift mode (Q1 U+ et61664; Q2 UO+). Best conditions provided a hydride rate value (235U16O1H+/235U16O+) in the order of 10-7. Finally, the methodology was validated with IRMM certified standards, and successfully applied to the determination of 236U/238U ratios in U samples from soils originating from the radioactive contamination plume caused by the Fukushima Daiichi Nuclear Power Plant accident. Ratios down to 10-9 have been obtained with precisions lower than 20