Investigation Of Redox Reaction In Electrospray Ionization Mass Spectrometry

Electrospray ionization (ESI) could be viewed as a two-electrode controlled-current electrochemical flow cell. When positive high voltage is applied on the spray emitter, the emitter serves as the anode, and the mass spectrometer acts as the cathode. During the electrospray process, a circuit is formed during the two electrodes, and an electrolytic cell is constructed. Compared with conditional electrolytic cell, the voltage applied on ESI electrolysis is on the scale of kilovolt rather than volt. The electrochemical reaction in ESI is a double-edged sword. On one hand, it would affect the spectrum interpretation and decrease the detection sensitivity. On the other hand, it could be applied to protein labeling and investigation of life metabolism. Other than electrochemical reaction, discharge-induced reaction, a kind of radical reaction, is another reaction in ESI. With application of high voltage, abundant radical was produced at the tip of the Taylor cone, inducing the occurrence of redox reaction during molecule collision process. In the same way, discharge-induced reaction has both advantage and disadvantage to spectrometry analysis as well. It not only suppresses sample signal but also could applied to proteomics.The first task in our work is to investigate the effect of three kinds of electrochemical oxidation in ESI on protein analysis, and to try to alleviate them with induced ESI (IESI). The reasons we chose proteins as analysis target are as follows:1) ESI is the most commonly used interface for LC-MS, and LC-MS is generally applied to separation and detection of proteins and peptides.2) Investigation of electrochemical oxidation in ESI is mostly focused on small molecules, and biomolecules oxidation is ignored. Since peptides and proteins are commonly analyzed in positive mode, we mainly investigate the oxidation reaction occurring in this mode. For electrochemical oxidation reactions (oxidation of electrode, solvent, and analyte) in ESI, continual release of cations in positive mode is the essential reason. To sustain spray current and keep the solution electroneutral, oxidation reaction will occur at the solution/electrode interface to eliminate remaining anions. On the basis of this principle, we tried to use IESI to alleviate the electrochemical oxidation in ESI since the spray voltage in IESI is induced inside the spray emitter and there is no solution/electrode contact. Moreover, cations and anions are alternatively released during the spray process.The second task in our work is to distinguish electrochemistry-from discharge-induced oxidation in ESI. Since these two kinds of oxidation coexist during the spray process, they are hard to be distinguished. Whereas electrochemical-induced oxidation occurs at the solution/electrode interface, and discharge-induced oxidation occurs in the gas phase, we proposed a time-resolved method to distinguish them in this work. For electrochemical oxidation, the oxidation efficiency is in proportion to solution/electrode contact time; for discharge oxidation, the reaction finishes instantaneously, and the oxidation efficiency is irreverent to spray time. Therefore, on the basis of time difference, the differentiation of discharge-from electrochemistry-induced oxidation was achieved.The third task of this work is to explore the reasons resulting in the unexpected reduction of quinones in positive ESI mode. In principle, electrochemistry-induced reduction should occur in negative mode. However, we observed the reduction phenomenon of quinones in positive mode. To illustrate that the reduction was induced by discharge rather than electrochemistry, we applied the above time-resolved method to differentiate them, and also explored the effect of such parameters as solvent, voltage, spray emitter material, and sheath gas on quinone reduction.At last, we developed a new ambient ionization source, thermal bursting ionization (PATI) source. Since PATI avoids the application of high voltage, electrochemistry-and discharge-induced oxidations were excluded. Moreover, when PATI was applied to monitor organic reaction, the reactive intermediates were successfully captured.