Electrochemistry of Uranyl Peroxide Solutions during Electrospray Ionization.

The ionization of uranyl triperoxide monomer, [(UO 2 )(O 2 ) 3 ] 4- (UT), and uranyl peroxide cage cluster, [(UO 2 ) 28 (O 2 ) 42 -  x (OH) 2 x ] 28- (U 28 ), 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 D 2 O as solvent, and use of N 2 and SF 6 as nebulizer gases, provide insight into the mechanisms of ionization. The U 28 nanocluster under MS/CID/MS with collision energies ranging from 0 to 25 eV produced the monomeric units UO x - ( x = 3-8) and UO x H y - ( x = 4-8, y = 1, 2). UT under ESI conditions yielded the gas-phase ions UO x - ( x = 4-6) and UO x H y - ( x = 4-8, y = 1-3). Mechanisms that produce the observed anions in the UT and U 28 systems are: (a) gas-phase combinations of uranyl monomers in the collision cell upon fragmentation of U 28 , (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 - ( x = 6-8) were investigated using density functional theory (DFT).