Investigation of the Electrooxidation of Thymine on Screen-Printed Carbon Electrodes by Hyphenation of Electrochemistry and Mass Spectrometry

Abstract

The electrooxidation of thymine on screen-printed carbon electrodes was investigated utilizing different complementary instrumental approaches. The potential-dependent product profile was obtained by recording real-time mass voltammograms. Electrochemical flow cells with integrated disposable electrodes were directly coupled with mass spectrometry to facilitate a very fast detection of electrogenerated species. Thymine dimers were found at a potential of about 1.1 V in ammonium acetate (pH 7.0) and 1.25 V in ammonium hydrogencarbonate electrolyte (pH 8.0). Electrochemistry-capillary electrophoresis-mass spectrometry measurements revealed that two isobaric isomers of the dimeric oxidation product were formed. Separations at different time inter-vals between end of oxidation and start of separation showed that these were hydrated over time. An investigation of the pKa values by changing the separation conditions in electrochemistry-capillary electrophoresis-ultraviolet-visible spectroscopy measurements allowed for further characterization of the primary oxidation products. The results showed that both isomers exhibited two deproto-nation steps. The oxidation products were further characterized by high-performance liquid chromatography-tandem mass spec-trometry. Based on the obtained data, the main oxidation products of thymine in aqueous solution could most likely be identified as N(1)-C(5’) and N(1)-C(6’) linked dimer species evolving into the corresponding dimer hydrates over time. The presented methods for online characterization of electrochemically pretreated samples showed that not only mass spectrometric data can be obtained by electrochemistry-mass spectrometry but also further characterizations such as the investigation of product stability and the pH dependent protonation or deprotonation behavior are possible. This is not only valid for stable oxidation products but also for in-termediates as analysis can be carried out within a short timescale. Thus, a vast amount of valuable experimental data can be ac-quired, which can help in understanding oxidation processes.