No abstract

Determination of uranium isotope ratios is of great expedience for assessing its origin in environmental samples. In particular, the 236 U/ 238 U isotope ratio provides a powerful tool to discriminate between the different sources of uranium (uranium ore, geochemical background and uranium from anthropogenic activities). However, in the environment, this ratio is typically below 10 -8 . This low abundance of 236 U and the presence in large excess of major isotopes (mainly 238 U and 235 U) complicates the accurate detection of 236 U signal by mass spectrometry and thus highly sensitive analytical instruments providing high abundance sensitivity are required. This work pushes the limits of triple quadrupole-based ICP-MS technology for accurate detection of 236 U/ 238 U isotope ratios down to 10 -10 , which is so far mainly achievable by AMS. Coupled with an efficient desolvating module, N 2 O was used as the reaction gas in the collision reaction cell of the ICP-MS/MS. This configuration allows a significant decrease of the uranium polyatomic interferences ( 235 UH + ions) and an accurate determination of low 236 U/ 238 U isotope ratios. This new methodology was successfully validated through measurements of certified reference material from 10 -7 to 10 -9 and then through comparisons with AMS measurement results for ratios down to 10 -10 . This is the first time that 236 U/ 238 U isotope ratios as low as 10 -10 were determined by ICP-MS/MS. The possibility of measuring low 236 U/ 238 U isotope ratios can offer a large variety of geochemical applications in particular for the determination of uranium sources in the environment. ASSOCIATED CONTENT Supporting InformationDemonstration of Eq. 1 and Eq. 2. and a figure on the influence of the 235 UH + interference on the measurement of 236 U/ 238 U isotope ratio for a given hydride formation rate.