Reassessment of the antioxidative mixture for the challenging electrochemical determination of dopamine, noradrenaline and serotonin in microdialysis samples

In recent years, the simultaneous monitoring of the monoamine neurotransmitters dopamine, noradrenaline and serotonin in vivo is advancing due to innovations in miniaturized and fast chromatographic techniques. However, the determination of the most hydrophilic compound, noradrenaline, in microdialysis samples by (ultra-)high performance liquid chromatography ((U)HPLC) with electrochemical detection (ECD) is impeded by a broad solvent front, caused by the addition of antioxidative agents. Hence, an elaborate reassessment of currently used antioxidative mixtures is necessary for further analytical improvements. The proposed mixture, containing 100mM acetic acid, 0.27mM Na2EDTA and 12.5μM ascorbic acid (pH 3.2), is less complex than previously described mixtures and shows minimal ECD interference. It stabilizes the three monoamines in standard solutions and in microdialysis samples, considering both autosampler stability at 4°C for 48h and long term stability at -20°C for a duration of six months. An in vivo microdialysis experiment demonstrates the possibility to monitor changes in extracellular levels of the three monoamines simultaneously in the rat hippocampus with UHPLC-ECD using the optimized antioxidative mixture.