Electrospray ionization mass spectrometry in studies of aluminium(III)-ligand solution equilibria

The application of electrospray ionization mass spectrometry (ESI-MS) in the study of aluminium(III)-ligand solution equilibria was examined. The advantages and the possible limitations were reviewed. As an example, ESI-MS was used to study the complex formation between Al(III) and the two ligands 4-hydroxy-3-pyridinecarboxylic acid and 3-hydroxy-4-pyridinecarboxylic acid. The results were compared with the speciation data obtained from potentiometric, UV-vis, and NMR measurements. A very good agreement was obtained for the number and for the stoichiometry of the main species in solution, while their relative concentrations were found to be significantly different. This suggests ES-MS to be a powerful qualitative technique for the study of any aluminium(III)-ligand systems, even for very complicated ones, but on the other hand the accuracy of ESI-MS quantitative data is still questionable.