Extension of electrochemical techniques coupled to non-aqueous capillary electrophoresis towards reduction at silver electrodes

While amperometric detection (AD) has been a powerful technique in non-aqueous capillary electrophoresis (NACE) for many years, it has been used only for several oxidative determinations. Since reductive electrochemistry offers many possibilities, this study aims to extend NACE-AD to reductive electrochemical approaches. After electrode material characterization in electrolytes based on acetonitrile (ACN), silver (Ag) emerged as the most suitable electrode material among gold (Au) and platinum (Pt) due to its higher overpotential for hydrogen evolution, and therefore, offers a broader potential range for reductive determinations in conjunction with electrolytes containing ammonium acetate and acetic acid. The applicability of reductive NACE-AD at Ag electrodes was subsequently evaluated using nitrophenolic compounds as model analytes, namely 4-nitrophenol (4NP), 2,4-dinitrophenol (DNP), and 2,4,6-trinitrophenol (TNP). This method enabled the selective detection of each nitrophenol within the migration window for negatively charged species without oxygen removal. It was found that reductive detection in ACN-based NACE was straightforward in the presence of dissolved oxygen, reaching low limits of detection in the µM range (0.5 µM for TNP and 2.0 µM for DNP). Furthermore, mechanistic insight into the electrode reactions was obtained using a newly developed, user-friendly electrochemical injection module for capillary electrophoresis coupled to mass spectrometry (EC CE-MS), to gain a holistic view of NACE-AD and the electrochemical processes involved. Here, 4-aminophenol, 4-amino-2-nitrophenol, and 4-amino-2,4-dinitrophenol could be identified as the electrode reaction products at a detection potential of - 1.3 V for NACE-AD in ACN-based media.