Kinetically labile equilibrium shifts induced by the electrospray process

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.