Metal-ligand solution equilibria studied by electrospray ionization mass spectrometry: correlation between ion intensity and acid-base equilibria in solution

Electrospray ionization mass spectrometry (ESI-MS) is increasingly used in the study of metal–ligand equilibria in aqueous solutions. However, the correlation between conditions in solution and mass spectra in the gas phase is far from being completely established. In the present work the equation i=kC0f was used to correlate relative ion intensity (i) in an ESI mass spectrum, the stoichiometric concentration (C0) in solution of the complex which produced this ion, and the fraction ( f ) of complex having the same protonation state as that of the ion detected in the spectrum. This equation takes into account that metal–ligand complexes have acid–base properties, and that these properties affect the efficiency by which the ions are brought from the solution to the gas phase. The equation was experimentally checked by electrospraying solutions containing aluminium(III) and any of the four ligands 3,4-dihydroxybenzoic acid, 3-hydroxy-2-(1H)pyridinone, citric acid, and ethylenediaminetetramethylenephosphonic acid at different pH values. ESI-MS experimental i values and C0f values calculated from literature data were plotted versus the solution pH. Values are correlated in the majority of cases, thus confirming the validity of the approach proposed. Correlation is lost, as expected, for low f or C0 values, and when extensive gas-phase reactions occur. The equation i=kC0f can be used to estimate quantitative data for unknown metal–ligand solutions analyzed by ESI-MS.