Investigation of the electrochemical behavior of cysteine by hyphenation of electrochemistry and mass spectrometry

The identification of the reaction product species stands out as one of the main limitations of the classical electrochemical techniques. This can be overcome by the combination of electrochemistry (EC) to mass spectrometry (MS). Moreover, the method can be further enhanced by implementing a separation technique between EC and MS. In the present work, the oxidation behavior of cysteine with coupling EC directly to MS (real-time EC–MS) or by implementing capillary electrophoresis (CE), to separate the analytes before the injection into the mass spectrometer, is investigated. Electrochemical measurements and pre-treatment were applied on screen-printed electrodes (SPEs) based on carbon. Direct EC–MS measurements were carried out with a modified flow cell, while online EC–CE–MS studies of cysteine oxidation were conducted with a custom-made setup. An electrochemical conversion yield for cysteine of up to 69% was found. Cystine, cysteic acid, and cysteine sulfinic acid were found as oxidation products. The identification of these product species was carried out according to their migration behavior in CE, and mass-to-charge ratios in addition to their isotopic patterns shown in the MS spectra