Thanks to the reaction cell, the inductively coupled plasma mass spectrometry (ICP-MS) enables the study of the reactivity of actinides mono-cations in the gas phase and the analysis of the actinide trace levels by resolving in situ isobaric and poly-atomic interferences. An experimental study was conducted along the actinides series from thorium to curium (Th, U, Np, Pu, Am, Cm) with four reaction gases (O2, CO2, CH4, NH3) to highlight reactivity differences. For instance, Th+, U+, Np+ and Cm+ react completely with NH3 to form AnNH+, whereas Pu+ and Am+ react only slightly or not at all with NH3.

From an analytical perspective, the reactivity differences with NH3 and O2 help to resolve isobaric interferences such as 238U/238Pu, 241Pu/241Am or 243Am/243Cm, which represent the principal source of bias in the analysis of actinides using ICP-MS. After optimizing the gas parameters, the isotopic analysis of U, Pu and Am was performed on a nuclear sample. Measurement biases of around 5% were obtained for the U and Pu isotope ratios using the « standard bracketing » method to overcome mass bias. Unlike the chemical separations currently used, the reaction cell resolves interferences « quickly » and without producing radiological waste.

Furthermore, quantum chemical computations were performed using density functional theory (DFT) to understand the reaction mechanism between actinides mono-cations (Ac+, Th+, Pa+ ,U+, Np+, Pu+, Am+ and Cm+) and NH3. For Ac+, Th+, Pa+ ,U+, Np+ and Cm+, after the initial formation of the An+-NH3 complex, the actinide mono-cation inserts itself in N-H bond, leading to the elimination of H2 via two transition states and intermediate species HAnNH2+ to form the AnNH+ product. For Pu+ and Am+, the limiting step of the reaction is the formation of the first intermediate species HAnNH2+. The calculated reaction energy between An+ and NH3 reproduces the experimental trends in reactivity with Th+ > Pa+ > U+ > Np+ > Ac+ > Cm+ > Pu+ > Am+.