Addressing\nthe Challenge of a Simultaneous Destructive\nAssay of Plutonium and Uranium: Highly Precise, Robust, Universal,\nand Reagent-Free Differential Pulse Voltametric Method Development\nin Biodegradable Methanesulfonic Acid Medium

The destructive assay of bulk uranium and plutonium,\na cornerstone\nfor chemical quality control and nuclear material accounting of fuel\nmatrices, mandates robust and precise methodologies. Despite ongoing\nresearch, simultaneous, matrix independent determination of U and\nPu has eluded solution owing to inherent limitations in aqueous acid\nmedium, viz., coexistence of multiple oxidation states, coupled electrochemical\nreactions, smaller potential window, and requirement for multistep\nsample preconditioning and tedious electrode modification. The present\nstudy addresses this challenge wherein non-aqueous methanesulfonic\nacid (MSA) served the dual role of solvent and analyte media with\na bare glassy carbon (GC) electrode. Fuel matrices, viz., (U, Pu)C,\n(U, Pu)O<sub>2</sub>, PuO<sub>2</sub>, UO<sub>3</sub>, UO<sub>2</sub>, and U<sub>3</sub>O<sub>8</sub>, were quantitatively dissolved in\nbiodegradable MSA, without using any additives. Redox speciation of\nthe analytes, U and Pu, in MSA was probed by ultraviolet–visible\nspectrophotometry and electrometry, revealing the absence of electrocatalytic\nregeneration and stabilization of single oxidation state, viz., U(VI)\nand Pu(IV), along with relevant redox-energetic (electron transfer\nand reversibility) and kinetic data. Bidentate coordination of MSA\nwith the U analyte was indicated by X-ray absorption spectroscopy\nstudies and was corroborated by density functional theory-based investigations,\nproviding the optimized structure, viz., [UO<sub>2</sub>(MSA)<sub>2</sub>] and [Pu(MSA)<sub>4</sub>], binding modes and energy, partial\ncharges, and molecular orbital diagrams. Based on these insights,\nthe feasibility of differential pulse voltammetry (DPV)-based assay\nmethod development for U and Pu separately and in different U/Pu ratios,\nrepresenting assorted fuel matrices, was probed. Analytical figures\nof merit for both U and Pu (detection limit of ∼10<sup>–5</sup> M, precision of ∼0.2%, accuracy of ∼0.2%, high sensitivity,\nrepeatability, and non-influence of relevant interferences) were determined,\nmethod validated employing actual fuel samples, and compared with\nthe established, multi-step biamperometry method. Hence, a universal,\nsimultaneous U and Pu destructive assay method in non-aqueous MSA\nmedia based on DPV with a commercial GC electrode was demonstrated.