Electrospray Behavior of Lacunary Keggin-type Polyoxotungstates [XW11039]p-, (X=Si, P). Mass Spectrometric Evidence for a Concentration Dependent Incorporation of a MOn+ (M=W(VI), Mo(VI), V(V)) unit into the Polyoxometalate vacancy

ESI-MS analysis provides a straightforward tool for the solution characterization of highly charged mono-lacunary Keggin-type polyoxotungstates Ą-[XW11O39]p- (X = Si, P; p = 8, 7). Double and triple charged quasi-molecular ion peaks were identified on the bases of both the theoretical simulation of the isotopic cluster distributions and the fragmentation modes observed with MS/MS technique. This approach allows to characterize the inorganic polyanions transferred into the gas-phase with associated alkali metal cations (Li+, Na+).