Electrospray-Ionization Time-of-Flight Mass Spectrometry: pH-Dependence of Phosphotungstate Species

Electrospray-ionization (ESI) time-of-flight mass spectrometry has been used to investigate the pH-dependent behavior of phosphotungstates in solution from pH = 2.7 to 8.0. Mass spectra are reported for the first time for H3PW11O394–, H4PW11O393–, and H5PW11O392–. The pH dependence of phosphotungstate species as measured by electrospray is compared with that measured by 31P NMR. The pH ranges of species are similar (±1 pH unit) when measured by these two methods. Protonation and solvation of phosphotungstate species measured by ESI differ from those observed by 31P NMR. However, the core phosphotungstate species are measured correctly by ESI.