Characterization of polyoxometalates by electrospray ionization mass spectrometry

The electrospray behaviors of saturated, substituted, and lacunary polyoxometalates (POMs) with classic Keggin and Dawson structures were investigated systematically by electrospray mass spectrometry (ESI-MS). The anions included Keggin [SiW12O40]4−, [SiW11O39]8−, [SiW10O36]8−, [SiW9O34]10−, Dawson [P2W18O62]6−, [P2W17O61]10−, and metal-substituted Keggin derivatives such as [PW11MnO40]7−, [SiW10V2O40]6−, and [GeW9Cu3O37]10−. Common species observed in the mass spectra arose from the protonation or cationization of either intact or dehydrated precursor ions. Compared to saturated and substituted POMs, lacunary POMs exhibited distinguished MS behaviors such as a much higher degree of cationization and dehydration of the bare polyoxoanions present in the mass spectra. In addition, some of these lacunary POMs were found to undergo subtle speciation change in solution. Freshly prepared solutions are suggested for synthetics for which lacunary POMs are starting materials. The advantages of the cation-exchange process prior to MS analysis are illustrated by an example.