Stopped-flow electrospray ionization mass spectrometry: a new method for studying chemical reaction kinetics in solution

In this work a new mass spectrometry based method for monitoring the kinetics of chemical reactions in solution is described. A stopped-flow mixing instrument is coupled to an electrospray ionization (ESI) mass spectrometer via a novel type of interface. Chemical reactions are initiated by rapid mixing of two reactant solutions. The mixture is instantaneously transferred to a reaction tube where the kinetics can be monitored in real-time by ESI mass spectrometry. With the current setup, a time window from 2.5 to 36 seconds after mixing of the reactants can be monitored. The experimental setup is used to study the kinetics of acetylcholine hydrolysis under alkaline conditions as a function of pH. The intensities of reactant (acetylcholine) and product (choline) ions are monitored simultaneously as a function of time. The reaction is carried out under pseudo-first-order conditions and the intensity-time curves are well described by single exponentials. The rate constants determined from these fits compare favorably with previous data from the literature.