C₂H₄ as a Novel Reaction Gas for Eliminating ²³⁸U Isobaric Interference in Ultratrace Plutonium Isotope (²³⁸Pu,...

Accurate determination of ultratrace plutonium isotopes (²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Pu) generally requires the utilization of multiple techniques. Among them, alpha spectrometry is the only way to measure ultratrace ²³⁸Pu (<1 mBq/1.6 fg) but with a very long counting time (hours to days). The accurate and rapid analysis of ultratrace plutonium isotopes is still a challenge. In this work, a novel analytical method for the accurate determination of ultratrace plutonium isotopes (²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Pu) in the same run was developed by ICP-MS/MS in He-C₂H₄ mode combined with a TK200 resin column. A high decontamination factor (DF) of ²³⁸U (1.81 × 10⁹) was achieved by only a single TK200 resin column. ²³⁸U and its polyatomic ions (²³⁸U¹H⁺ and ²³⁸U¹H₂⁺) can be further eliminated using C₂H₄ as a reaction gas with a DF(²³⁸U) of 6.63 × 10⁵, and the overall removal efficiency of ²³⁸U reached 1.21 × 10¹⁵. The excellent measurement sensitivity of plutonium (5602 cps/(pg/g)) was achieved. Meanwhile, the investigation on the mechanism of the C₂H₄ reaction by DFT calculations could provide a novel method for the better understanding of the improvement of Pu sensitivity and the elimination of ²³⁸U interference. The detection limits for all plutonium isotopes were subfemtogram level, demonstrating the feasibility of ICP-MS/MS using C₂H₄ for the accurate determination of ultralow-level plutonium isotopes containing subgram level ²³⁸U. This method could be expected to replace alpha spectrometry for ultratrace ²³⁸Pu routine analysis.