Kinetically Labile Equilibrium Shifts Induced by the Electrospray Process

Abstract

The complexation reactions between the alkaline earth metal ions and EDTA were studied by electrospray mass spectrometry to measure the change in concentration of the metal ion-EDTA complex (MY(2)(-)) in the gas phase relative to the solution-phase equilibrium concentration. This work focused on the solution pH range from 4 to 7 where there exists free metal ions in solution at equilibrium. The equilibrium shift, measured through quantitation of the increased abundance of the MY(2)(-) species in the gas phase, was largest for barium and smallest for magnesium. The cause of the net equilibrium shift of the MY(2)(-) species is the combined effect of an electrolytic increase in pH within the capillary plus an additional shift within the evaporating droplets. In a thin diffusion-limited layer created by the products of electrolysis mixing with the bulk solution at the ES capillary tip, the labile species reequilibrate at a new, higher pH. In the evaporating droplets, the formation of new labile species due to increased solute concentrations is kinetically controlled because the ion residence time in the droplet prior to desorption is only ∼5 μs. These results are briefly discussed with respect to the potential for utilizing electrospray mass spectrometry for kinetically labile equilibrium studies.

Poly(ethylene glycol) doubly and singly cationized by different alkali metal ions: Relative cation affinities and cation-dependent resolution in a quadrupole ion trap mass spectrometer

“…Ammonium ion, present as an impurity in reagent grade methanol [54] competed with the uptake of Li, and for this reason, the hydroxide salts of Li, Na, and K were used for several of the metal ion uptake experiments, otherwise, the reagent grade chloride salts of Na, K, Rb, and Cs were used. A minimum mole ratio of total metal ion to PEG was set at 10:1 to ensure the starting solution was under thermodynamic conditions [13] to minimize kinetic [55] and/or solute clustering [56,57] contributions to the sampled ion population. The sample preparation procedure was simply to dissolve the salts with a minimum of distilled deionized water, and then diluting with reagent grade methanol.…”

Fe- and Cu-Complex Formation with Artificial Ligands Investigated by Ultra-High Resolution Fourier-Transform ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS): Implications for Natural Metal-Organic Complex Studies

“…It should be noted that the curves of the EDTA ± SA+ titration series acquired by FT-ICR-MS matched those produced by (CLE-)ACSV, although the latter had a higher ionic strength and higher (HEPES buffered) pH (7.8) compared to the former, indicating a resilience of the tested metal-ligand equilibria toward the induced changes of the sample matrix. For studies of kinetically more labile metalorganic complexes, additional measures such as higher sample flow rates, generally low ionic strength, and/or the use of a buffer, may further prevent electrochemical interferences of the equilibria at the ESI source (Wang and Agnes, 1999;Van Berkel and Kertesz, 2007).…”

Evolution of the solvent polarity in an electrospray plume

“…(J Am Soc Mass Spectrom 2010, 21, 378 -385) © 2010 American Society for Mass Spectrometry E lectrospray ionization-mass spectrometry (ESI-MS) is widely used in many important fields, e.g., biochemistry, the food industry, and pharmacy, and is still under fast development. Despite its huge success, only relatively few fundamental studies on the mechanism of ESI have been undertaken: the different modes of the electrospray [1,2], partitioning effects in sprayed droplets [3][4][5], size, charge, and mass changes of the droplets during solvent evaporation [6 -11], chemical equilibria in the electrospray process [12][13][14], and ion formation from the droplets [15][16][17][18][19][20] have been studied. However, more fundamental studies are still needed to make ESI-MS a totally reliable method for wider applications.…”