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A new method for determining $^{236}$U/$^{238}$U isotope ratios in environmental samples by means of ICP-MS/MS

Abstract : The $^{236}$U/$^{238}$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 $^{236}$U/$^{238}$U ratios, below 10$^{−8}$, in environmental samples by combination of ICP-MS/MS for $^{236}$U/$^{238}$U ratio and multiple collector ICPMS measurements for $^{235}$U/$^{238}$U and $^{234}$U/$^{235}$U isotope ratios. Since the hydride form of U$^+$ (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$_2$ and CO$_2$ 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 ($^{235}$U$^{16}$O1H$^+$/$^{235}$U$^{16}$O+) of about 10$^{-7}$ with O$^2$ in the collision cell. The method was validated through measurements of two certified IRMM standards with $^{236}$U/$^{238}$U isotope ratio of 1.245×10$^{17}$ 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. $^{236}$U/$^{238}$U atomic ratios between 1.5×10$^{-8}$ and 7×10$^{-9}$ were obtained with a level accuracy lower than 20%.
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Silvia Diez-Fernández, Hugo Jaegler, Carole Bresson, Frédéric Chartier, O. Evrard, et al.. A new method for determining $^{236}$U/$^{238}$U isotope ratios in environmental samples by means of ICP-MS/MS. Talanta, Elsevier, 2020, 206, pp.120221. ⟨10.1016/j.talanta.2019.120221⟩. ⟨cea-02610582⟩

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