Evaluation of electrospray mass spectrometry as a technique for quantitative analysis of kinetically labile solution species.

The extent that a kinetically labile equilibrium reaction was perturbed by passage through the electrospray ion source has been measured. The reaction studied was strontium ion chelation by EDTA (Sr(2+) + Y(4)(-) ⇌ SrY(2)(-)) in 100% aqueous solutions. The forward reaction rate is very fast (10(9) M(-)(1) s(-)(1)) and the reverse rate is very slow (10(0) s(-)(1)) relative to the time scale of the ES process (∼10(-2) s). The SrY(2-) detected with the mass spectrometer were expected to be representative of thermodynamic equilibrium within the droplets, but the position of the equilibrium shifted to the right relative to the solution equilibrium position. Given the current status of understanding of the ion generation process in the electrospray ion source, the degree that the [Sr(2+)] changed due to passage through this ion source was smaller than expected, which is fortuitous with respect to the quantitation of such species. The pH of each calibration set determined the fraction of strontium that was uncomplexed in solution. The equilibrium shift induced by passage through the ion source was constant for solutions at constant pH but differed for solutions at different pH. Decreasing the solution pH generated smaller equilibrium shifts as measured by the change in the [Sr(2+)]. In solutions with free Sr(2+) and excess EDTA at equilibrium, the free [Sr(2+)] decreased by ∼100 and ∼10% in solutions at pH 5.85 and 4.98, respectively. Quantitation of kinetically labile species in complex, multicomponent systems will be straightforward with ES-MS, provided all species involved in the equilibria can be identified and monitored.