Electrospray ionization mass spectrometry of a cerium(III) phosphomolybdate complex: Condensed and gas-phase cluster chemistry

Electrospray ionization quadrupole ion trap mass spectrometry (ESI-QIT/MS) of the ammonium cerium(III) phosphomolybdate complex (NH₄)₁₁[Ce(III)(PMo₁₁O₃₉)₂] in aqueous media has revealed a concentration-dependent behavior. Under fixed instrumental parameters, the Ce-containing polyoxomolybdate complexes H₂Ce(III)P₂Mo₂₂O₇₅^3- and Ce(III)PMo₁₁O₃₈^2- are the primary species present at 11mM (pH=4.3); at 0.7mM (pH=3.6), Ce(III)PMo₁₀O₃₅^2- is the predominant species, Ce(III)PMo₁₁O₃₈^2- is quite diminished, and H₂Ce(III)P₂Mo₂₂O₇₅^3- is absent. As a result of the complex isotopic fingerprints from multiple molybdenums, compositions of such ions are difficult to assign-successive collision induced dissociation (CID) of large ions produced smaller ions for which calculated and experimental isotopic patterns could be compared. The oxidation state of Ce and the number of counter cations on negative complexes was discerned from spectra of ions containing ¹H⁺ and ⁷Li⁺. The overall result is an ESI method applicable to phosphomolybdate complexes containing redox sensitive f-block metal ions such as Ce(IV) and Pu(III/IV). Dissociation studies also gave insight into favored fragmentation pathways, and generated gas ions with empirical formulae similar to known condensed-phase ions. Deconvolution of concentration- and pH-dependent solution behavior via ESI/MS and ³¹P NMR spectroscopy showed speciation dependent on solution concentration, not on pH.