Automated analysis of ultra-trace 236U and natural uranium isotopes via integrated sample preparation and ICP-MS/MS detection

Highly efficient and accurate analysis of ultra-trace anthropogenic 236 U in the presence of natural uranium isotopes is essential for nuclear forensics and environmental monitoring. In this study, we developed an integrated analytical method for the determination of 234 U, 235 U, 236 U, and 238 U by combining highly automated sample preparation with highly sensitive ICP-MS/MS detection. Automated total dissolution was employed to ensure complete extraction of both endogenous and exogenous uranium from solid matrices, while reproducible chemical separation was achieved using an automated platform equipped with regenerable UTEVA resin. This optimized procedure yielded exceptionally low operational blanks for 236 U at femtogram level. Moreover, the final eluent volume was minimized to 1.5 mL, allowing direct introduction into the ICP-MS/MS. By utilizing a membrane desolvation sample introduction system and a novel mass-shift mode (targeting UO 2 + species), the ICP-MS/MS sensitivity exceeded 4.1×10 6 cps/ppb, while the 235 UH + interference formation rate for 236 U was suppressed to 7.1×10 -10 . Consequently, the achieved detection limits for 236 U and the 236 U/ 238 U atom ratio were as low as 1.85 fg/g and 5×10 -12 , respectively. This method was successfully applied to determine the uranium isotopic composition in sediments from the adjacent sea area of the Daya Bay Nuclear Power Plant. These results demonstrate that the proposed method provides a robust and high-throughput solution for accurately quantifying of ultra-trace 236 U in environmental samples, such as those impacted by the global fallout.