Uranyl peroxide nanoclusters (UPCs) are observed by electrospray ionization mass spectrometry (ESI-MS) to verify the presence of clusters in solution, yet they have not been extensively studied in the gas phase. Mechanisms of cluster transfer into the gas phase under negative ion mode have not been explored. The removal of cations from the cluster in the electrospray process is proposed to be the primary ionization method and transfer to mass spectrum detection. The uranyl triperoxide monomer should also transfer by similar mechanisms, although it has not been observed despite attempts under the same conditions in negative ion mode. We hypothesize that peroxide moieties undergo electrochemical changes due to the high voltage applied to the capillary tip (~4000 V to break the surface tension of water), which could contribute to an ionization mechanism distinct from cation removal in which uranyl peroxide compounds transfer into the gas phase. We tested this hypothesis by 1) studying the low mass gas-phase species of LiUT Li4[(UO2)(O2)3] under the same instrument methods applied to cluster Li-U28 Li28[(UO2)28(O2)42]; 2) studying the low mass ions produced from Li-U28 under collision-induced dissociation (CID) from kinetic energies of 0-25 eV; 3) applying CID of the LiUT monomer gas-phase species to verify assignments; 4) and increasing capillary voltage from 2500-5500 V for Li-U28 and the LiUT monomer. ESI-MS/CID/MS studies of Li-U28 show similar low mass gas-phase anions produced for monomeric unit LiUT. In general, ESI-MS/CID/MS of Li-U28 indicate that the cluster fragmented to yield the gas phase uranium oxide anions UOx– (x= 3-8) and UOxHy– (x= 4-8, y=1-2). LiUT under ESI-MS conditions yields the gas phase ions UOx– (x= 3-6) and UOxHy– (x= 4-8, y=1-2). The electronic structure of many of these low-mass anions, such as UOx– (x= 6-8), is unknown and will be investigated using density functional theory (DFT). These calculations will help elucidate the relative stabilities of potential structures, characterize moieties, and help determine the redox chemistry of cluster and monomer ionization.

Electrospray ionization mass spectrometry with collision induced dissociation of aqueous uranyl peroxide solutions

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