Differences between metal-ligand speciation results of electrospray ionization mass spectrometry (ESI-MS) and those of traditional techniques

Electrospray ionization mass spectrometry (ESI-MS) is being increasingly applied to study the solution equilibria of metal-ligand systems,[1] together with or in alternative to the techniques traditionally employed to this aim, e.g. potentiometry, UV-Vis, NMR. ESI-MS results are usually different from those obtained by traditional techniques, both in the qualitative (number and stoichiometry of the species observed in solution) and quantitative data (relative concentrations of all species at equilibrium). To our knowledge, a systematic investigation of the nature of these differences and of their causes is lacking in the literature. In this work, selected aluminium(III)-ligand systems were examined by ESI-MS. The ligands considered were 3-hydroxy-2(1H)-pyridinone, 3,4-dihydroxybenzoic acid, citric acid, and ethylenediaminetetramethylenephosphonic acid. Spectra (in positive and/or in negative ion mode) were obtained at various metal-to-ligand ratios and pH values, and results were compared with those obtained previously [2-5] by traditional techniques. For the qualitative results, the main differences between ESI-MS and traditional techniques regard the acidic proton content of the metal-ligand species, and the presence of clusters at ES conditions. The former difference is observed when the complexes in solution have acid-base properties and a high charge (> ± 1), and it is due to the easy of the proton displacement at ES conditions. The formation of clusters is an ES artifact which regards polycarboxylic and polyphosphonic ligands, but it can be minimized by strictly optimizing the instrumental parameters. Other minor ES artifacts are the formation of solvent and cation (or anion) adducts, which usually don’t impair the qualitative analysis. For the quantitative results, the differences between ESI-MS and traditional techniques are more evident for the systems which also show significant qualitative differences, and they are due to different solvents and ionic strengths of the solutions analysed, to a different pH definition, and to the presence of ion response factors in ESI-MS. References (1) Di Marco V.B.; Bombi G.G. Mass Spectrometry Reviews 2005, submitted for publication (2) Di Marco V.B.; Bombi G.G; Tapparo A.; Powell A.K.; Anson C.E. J. Chem. Soc. Dalton Trans. 1999, 2427 (3) Kiss T.; Atkári K.; Jezowska-Bojczuk M.; Decock P. J. Coord. Chem. 1993, 29, 81 (4) Harris W.R.; Wang Z.P.; Hamada Y.Z. Inorg. Chem. 2003, 42, 3262 (5) Di Marco V.; Kilyen M.; Jakusch T.; Forgó P.; Dombi G.; Kiss T. Eur. J. Inorg. Chem. 2004, 2524