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

Abstract

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-(1H)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.

Structural analysis of organometallic compounds with soft ionization mass spectrometry

“…In contrast to main-group metals, transition-metals are mostly in the form of metal complexes (Henderson & Evans, 1999;Di Marco et al, 2007) and metallocenes; not so many real organometallics that contain the covalent bond between carbon and metal is described.…”

“…These complexes are often formed by the metal complexation with amino acids, sugars, and drugs (Alvarez, Vartanian, & Brodbelt, 1997;Vaisar et al, 2005;Fournier et al, 2008). Their gas-phase ion-molecule reactions are often studied (Gatlin & Tureček, 1997;Lemr, Holčapek, & Jandera, 2000;Asmis & Sauer, 2007;Di Marco et al, 2007;MacAleese & Maitre, 2007;Mo et al, 2007;Tureček, 2007;Burgert & Schnockel, 2008;Jirásko et al, 2008;Polfer & Oomens, 2009). The most famous metal complexes are even naturally occurring, such as chlorophyll (Mg), hemoglobin (Fe), and myoglobin (Fe), and their presence in living organisms is indispensable (Wöhrle & Pomogailo, 2003).…”

Determination of binding constants by affinity capillary electrophoresis, electrospray ionization mass spectrometry and phase-distribution methods

“…However, different molecular species may undergo different vaporization and ionization efficiencies. Response factors may also change with pH, further complicating quantitation [89]. This represents one of the biggest disadvantages of the ESI-MS method for binding-constant determination [90].…”

“…Recent studies on biological [94] and inorganic [89] systems concluded that the ESI-MS approach was useful for quantitative measurements of binding constants. However, a close reading shows that confidence in a quantitative result is warranted only if there is already available a considerable amount of information on the system.…”

Ispitivanje reakcija hidrolize i kompleksiranja u rastvorima aluminijum(III)-jona i nekih fluorohinolona metodom elektrosprej-tandem masene spektrometije

“…Карактеристике комплекса метал лиганд и специјација зависе од pH, [141] јер комплекси мењају своје стање протоновања и наелектрисања са променом pH (показују или кисела или базна својства).…”