Electrospray behavior of lacunary Keggin-type polyoxotungstates [XW11O39](p-) (X = Si, P): Mass spectrometric evidence for a concentration-dependent incorporation of an 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 alpha-[XW11O39](p-) (X = Si, P; p = 8, 7). Doubly and triply charged quasi-molecular ion peaks were identified on the basis of both the theoretical simulation of the isotopic cluster distributions and the fragmentation modes observed with MS/MS technique. This approach allows the characterization of the inorganic polyanions transferred into the gas-phase with associated alkali metal cations (Li+, Na+). Indeed, the ESI-MS spectrum was found to be dependent on the counterion nature and on the initial concentration of the polyoxotungstate. While lithium salts at millimolar concentration represent the best choice for the analytical protocol, upon dilution the inorganic structure rearranges to a saturated Keggin dodecametalate complex. This phenomenon is induced by the electrospray ionization process and was exploited to produce gas-phase ions of monosubstituted species formed. through incorporation of molybdenum and vanadium into the polyoxotungstate framework.