Electrospray ionization of uranyl-citrate complexes: Adduct formation and ion-molecule reactions in 3D ion trap and ion cyclotron resonance trapping instruments

Abstract

Results presented here demonstrate the usefulness of electrospray ionization and gas-phase ion-molecule reactions to predict structural and electronic differences in complex inorganic ions. Electrospray ionization of uranyl citrate solutions generates positively and negatively charged ions that participate in further ion-molecule reactions in 3D ion trap and FT-ICR mass analyzers. Most ions observed are derived from the major solution uranyl-citrate complexes and involve species of {(UO 2) 2Cit 2} 2−, (UO 2) 3Cit 2, and {(UO 2) 3Cit 3} 3−, where Cit indicates the citrate trianion, C 6H 5O 7 3−. In a 3D ion trap operated at relatively high pressure, complex adducts containing solvent molecules, alkali and ammonium cations, and nitrate or chloride anions are dominant, and proton/alkali cation (Na +, K +) exchange is observed for up to six exchangeable protons in an excess of alkali cations. Adduct formation in a FT-ICR cell that is operated at lower pressures is less dominant, and direct detection of positive and negative ions of the major solution complexes is possible. Multiply charged ions are also detected, suggesting the presence of uranium in different oxidation states. Changes in uranium oxidation state are detected by He-CID and SORI-CID fragmentation, and certain fragments undergo association reactions in trapping analyzers, forming “exotic” species such as [(UO 2) 4O 3] −, [(UO 2) 4O 4] −, and [(UO 2) 4O 5] −. Ion-molecule reactions with D 2O in the FT-ICR cell indicate substantial differences in H/D exchange rate and D 2O accommodation for different ion structures and charge states. Most notably, the positively charged ions [H 2(UO 2) 2Cit 2(H)] + and [(UO 2) 2(Cit)] + accommodate two and three D 2O molecules, respectively, which reflects well the structural differences, i.e., tighter uranyl-citrate coordination in the former ion than in the latter. The corresponding negatively charged ions accommodate zero or two D 2O molecules, which can be rationalized using suggested solution phase structures and charge state distributions.