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Metal–ligand solution equilibria studied by electrospray ionization mass spectrometry: correlation between ion intensity and acid–base equilibria in solution
Author(s) -
Di Marco Valerio B.,
Bombi G. Giorgio,
Ranaldo Martina,
Traldi Pietro
Publication year - 2007
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.3292
Subject(s) - chemistry , electrospray ionization , base (topology) , electrospray , mass spectrometry , ion , metal , analytical chemistry (journal) , metal ions in aqueous solution , ligand (biochemistry) , inorganic chemistry , chromatography , organic chemistry , mathematical analysis , biochemistry , receptor , mathematics
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 = kC 0 f was used to correlate relative ion intensity ( i ) in an ESI mass spectrum, the stoichiometric concentration ( C 0 ) 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‐(1 H )pyridinone, citric acid, and ethylenediaminetetramethylenephosphonic acid at different pH values. ESI‐MS experimental i values and C 0 f 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 C 0 values, and when extensive gas‐phase reactions occur. The equation i = kC 0 f can be used to estimate quantitative data for unknown metal–ligand solutions analyzed by ESI‐MS. Copyright © 2007 John Wiley & Sons, Ltd.