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

Abstract Electrospray ionization quadrupole ion trap mass spectrometry (ESI-QIT/MS) of the ammonium cerium(III) phosphomolybdate complex (NH 4 ) 11 [Ce(III)(PMo 11 O 39 ) 2 ] in aqueous media has revealed a concentration-dependent behavior. Under fixed instrumental parameters, the Ce-containing polyoxomolybdate complexes H 2 Ce(III)P 2 Mo 22 O 75 3− and Ce(III)PMo 11 O 38 2− are the primary species present at 11 mM (pH = 4.3); at 0.7 mM (pH = 3.6), Ce(III)PMo 10 O 35 2− is the predominant species, Ce(III)PMo 11 O 38 2− is quite diminished, and H 2 Ce(III)P 2 Mo 22 O 75 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 1 H + and 7 Li + . 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 31 P NMR spectroscopy showed speciation dependent on solution concentration, not on pH.