Electrochemistry of Uranyl Peroxide Solutions during Electrospray Ionization.

The ionization of uranyl triperoxide monomer, [(UO)(O)] (UT), and uranyl peroxide cage cluster, [(UO)(O)(OH)] (U), 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 DO as solvent, and use of N and SF as nebulizer gases, provide insight into the mechanisms of ionization. The U nanocluster under MS/CID/MS with collision energies ranging from 0 to 25 eV produced the monomeric units UO ( = 3-8) and UOH ( = 4-8, = 1, 2). UT under ESI conditions yielded the gas-phase ions UO ( = 4-6) and UOH ( = 4-8, = 1-3). Mechanisms that produce the observed anions in the UT and U systems are: (a) gas-phase combinations of uranyl monomers in the collision cell upon fragmentation of U, (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 ( = 6-8) were investigated using density functional theory (DFT).