Accelerated bimolecular reactions in microdroplets studied by desorption electrospray ionization mass spectrometry

Functional group derivatization reactions occur in the course of microdroplet/surface collisions in the ambient ionization process of desorption electrospray ionization (DESI). The unique environment in the microdroplet causes rate enhancements of as much as several orders of magnitude in typical bimolecular reactions that proceed through either cationic or anionic intermediates. The environment in the evaporating charged microdroplet differs from that of the bulk: (i) the pH of the solution moves towards the extremes, (ii) the concentrations of the reagents increase, (iii) the relative surface area increases and (iv) collision frequencies increase. The rates of acid-catalyzed reactions, such as the reaction of Girard T reagent with ketosteroids, increase with decreasing pH in positively-charged microdroplets compared to the bulk solution rates. Similarly, the increased pH in evaporating negatively-charged microdroplets contributes to an increase in the rates of base-catalyzed Michael reactions over those recorded under bulk solution conditions. The amount of product formed depends on the reaction time and the droplet size. Nanoelectrospray ionization generates larger droplets than the secondary droplets of DESI so it does not show significant product formation in the analysis period and can be used to analyze products of the DESI experiments. When secondary microdroplets (ca. 1 micron diameter) are generated either by spraying a homogeneous solution of both reagents against an inert surface (reactive DESI) or when a solution of Girard T reagent is sprayed against a solid surface bearing the ketosteroid significant amounts of product are generated. In the case of the Michael reaction with cinnamic acid an alternative dehydrogenated reaction product is formed under microdroplet conditions. Some parallels between the phenomenon reported here and the rate acceleration seen in sonochemistry are noted. The potential value of mass spectrometry in establishing conditions that enhance reaction rates is also indicated. It is possible that these observations will assist in the selection of reaction conditions involving the use of charged microdroplets to enhance the rates of ordinary bulk chemical reactions, especially those involving strong steric hindrance.