Electrochemistry of Uranyl Peroxide Solutions during Electrospray Ionization

The ionization of uranyl triperoxide monomer, [(UO₂)(O₂)₃]^4- (UT), and uranyl peroxide cage cluster, [(UO₂)₂₈(O₂)_42 - (OH)₂]^28- (U₂₈), was studied with electrospray ionization mass spectrometry (ESI-MS). Experiments including tandem mass spectrometry with collision-induced dissociation (MS/CID/MS), use of natural water and D₂O as solvent, and use of N₂ and SF₆ as nebulizer gases, provide insight into the mechanisms of ionization. The U₂₈ nanocluster under MS/CID/MS with collision energies ranging from 0 to 25 eV produced the monomeric units UO^- (x = 3-8) and UOH^- (x = 4-8, y = 1, 2). UT under ESI conditions yielded the gas-phase ions UO^- (x = 4-6) and UOH^- (x = 4-8, y = 1-3). Mechanisms that produce the observed anions in the UT and U₂₈ systems are: (a) gas-phase combinations of uranyl monomers in the collision cell upon fragmentation of U₂₈, (b) reduction-oxidation resulting from the electrospray process, and (c) ionization of surrounding analytes, creating reactive oxygen species that then coordinate to uranyl ions. The electronic structures of anions UO^- (x = 6-8) were investigated using density functional theory (DFT).