Potential Misinterpretation of Reaction Intermediates Due to In-Source Fragmentation and Rearrangement in Mass Spectrometry

Abstract

Mass spectrometry has become an invaluable tool for real-time reaction monitoring and capturing highly reactive intermediates, significantly contributing to the understanding of various chemical reactions mechanisms. Recent studies involving the capture of elusive carbocations from different reactions within water microdroplets have drawn considerable attention. Inspired by George Olah's pioneering works on carbocations and superacid chemistry, they attributed the stabilization of transient carbocations to the acidic environment spontaneously formed on the surface of microdroplets, as per microdroplets' intriguing feature. In this work, the electrospray ionization mass spectrometry (ESI-MS) was used to determine a variety of carbocation precursor molecules mentioned. It's important to note that commercial ESI sources are typically unaffected by microdroplet effects due to the short lifespan of the droplets, resulting from an extremely short flight distance and immediate evaporation facilitated by the drying gas. Surprisingly, even in the absence of the ultra-acidic environment considered necessary for the stabilization of carbocations, the carbocation signals are able to be detected under specific analytical conditions. By studying the mass spectra of the same analytes when changing the inlet capillary temperature and switching tube lens voltage, it was revealed that the precursor species might undergo in-source fragmentation due to the combined effects of the inlet capillary's high temperature and the high tube lens voltage. Tube lenses, designed to improve the ion transport efficiency, were located at the entrance from the first to the second pumping stage, where the remaining pressure could be around 133 Pa. The voltage was applied here right after the ions flow out of the inlet capillary. Therefore, the temperature and tube lens voltage might interact with each other, and together with the remaining pressure in this rough vacuum region, the ions could potentially collide with gaseous molecules, leading to the production of carbocation fragments. These fragments, which were in consistent with those generated from collision-induced disassociation (CID) of the precursor species, could possibly be misinterpreted as intermediates of some certain reactions, thereby misleading our understanding of the reaction mechanism. Through the detailed investigation of instrument parameters, and a closer look at the structure of the atmospheric pressure interface was achieved, this work highlights the importance of carefully considering various instrument parameters when mass spectrometry is used for reaction monitoring and intermediate capture studies, to ensure the accuracy of the results, and avoid such pitfalls.

Keywords

• • electrospray ionization mass spectrometry (esi-ms)
• carbocations
• in-source ragmentation