Ions generated from uranyl nitrate solutions by electrospray ionization (ESI) and detected with fourier transform ion-cyclotron resonance (FT-ICR) mass spectrometry

Electrospray ionization (ESI) of uranyl nitrate solutions generates a wide variety of positively and negatively charged ions, including complex adducts of uranyl ions with methoxy, hydroxy, and nitrate ligands. In the positive ion mode, ions detected by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry are sensitive to instrumental tuning parameters such as quadrupole operating frequency and trapping time. Positive ions correspond to oligomeric uranyl nitrate species that can be characterized as having a general formula of [(UO₂)_n(A)_m(CH₃OH)_s]⁺ or [(UO₂)_n(O)(A)_m(CH₃OH)_s]⁺ with n=1–4, m=1–7, s=0 or 1, and A = OH, NO₃, CH₃O or a combination of these, although the formation of NO₃-containing species is preferred. In the negative ion mode, complexes of the form [(UO₂)(NO₃)_m]⁻ (m=1–3) are detected, although the formation of the oxo-containing ions [(UO₂)(O)_n(NO₃)_m]⁻ (n=1–2, m=1–2) and the hydroxy-containing ions [(UO₂)(OH)_n(NO₃)_m]⁻ (n=1–2, m=0–1) are also observed. The extent of coordinative unsaturation of both positive and negative ions can be determined by ligand association/exchange and H/D exchange experiments using D₂O and CD₃OD as neutral reaction partners in the gas-phase. Positive ions are of varying stability and reactivity and may fragment extensively upon collision with D₂O, CD₃OD and N₂ in sustained off-resonance irradiation/collision-induced dissociation (SORI-CID) experiments. Electron-transfer reactions, presumably occurring during electrospray ionization but also in SORI-CID, can result in reduction of U(VI) to U(V) and perhaps even U(IV).