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

Abstract

Electrospray ionization (ESI) time-of-flight mass spectrometry has been used to investigate the pH-dependence of phosphomolybdates in solution from pH = 1.7 to 10.2. Mass spectra are reported for the first time for H₂Na₂P₂Mo₅O₂₃^2–, PMo₉O₃₁^3–, PMo₁₀O₃₄^3–, and PMo₁₁O₃₇^3–. The pH dependence of phosphomolybdate species as measured by electrospray is compared with measurements reported for combined potentiometry—³¹P NMR. The pH ranges of most species are similar (±1 pH unit) when measured by these two methods, although some differences are observed. Protonation and solvation of phosphomolybdate species measured by ESI differ from those observed by ³¹P NMR. However, the core phosphomolybdate species are measured correctly by ESI.