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%.
Origin : Files produced by the author(s)
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