Changes in bulk solution pH caused by the inherent controlled-current electrolytic process of an electrospray ion source

Abstract

The redox reactions that take place in the metal capillary of an electrospray ion source to maintain charge balance alter the composition of the initial solution entering the capillary. Data presented here demonstrate that under certain ES conditions, solution pH may be decreased significantly (by at least 4 pH units) as a result of the electrolytic oxidation of water in positive ion mode electrospray-mass spectrometry (ES-MS). Furthermore, it is shown that this pH change can have an affect on the appearance of the ES mass spectrum of an of analyte. An ES ion source in which the pH of an indicator solution exiting the capillary is monitored optically, before the spraying process, is used to demonstrate that electrolytic reactions in positive ion mode ES can decrease the pH of the initial solution. ES-MS studies using bovine heart cytochrome c are used to illustrate the influence of the electrolytically-induced pH change on the gas-phase ion signals for the multiply protonated protein. The magnitude of electrolytically-induced changes in solution pH will be most significant in non-buffered solutions near neutral pH when using metal spray capillaries or metal contacts to solution comprised of difficult to oxidize material (e.g. platinum or gold). Any pH changes will increase in magnitude as the flow rate decreases and/or ES current increases (all other ES parameters constant). Therefore, the potential impact of these redox reactions in ES-MS, if any, will probably be most important in the very low flow rate ES-MS systems (≤ 1.0 μL min −1).