电化学-微米电喷雾-质谱技术用于间甲苯胺电化学氧化机理研究

本实验开发了电化学-微米电喷雾-质谱技术(EC-mESI-MS)，采用微米毛细管作为电化学电解池和电喷雾离子源，内置两根铂金丝分别作为工作电极和辅助电极，通过连接自制的电化学工作站，同时向样品溶液中施加电化学反应电压和电喷雾电压，实现电化学反应的实时、在线质谱分析。基于该技术研究了间甲苯胺（m-toluidine）的电化学氧化机理。在0.39 V恒电位条件下，检测到间甲苯胺在pH 7的乙腈-水溶液中生成的一系列氧化产物，包括阳离子自由基和4种低聚物（从二聚体到五聚体），并得到了各产物在反应过程中随时间变化的趋势。实验结果表明，同类低聚物存在醌环和苯环两种结构。另外，发现溶液pH值对氧化效率有较大影响，在酸性环境下会抑制反应的进行，酸性越强，抑制效果越明显。

Real-Time Detection of m-Toluidine Electrooxidation Using Electrochemistry-Micro Electrospray Ionization-Mass Spectrometry

An electrochemistry-micro electrospray ionization-mass spectrometry (EC-mESI-MS) device was developed for in situ and on-line monitoring of the electrooxidation of m-toluidine in acetonitrile-water solution. A micro capillary was employed as electrochemical cell, which was also functioned as the electrospray ionization source for mass spectrometer. Two platinum wires with different diameters were built in the micro capillary and used as the working electrode (WE) and the counter electrode(CE), respectively. A customized AutoLab connected to the WE and CE in order to achieve the electrochemical reaction of m-toluidine and meanwhile to generate the electric field for spraying out the products from the tip of micro capillary. The applied voltage was 2.0 kV+ΔE for WE and was 2.0 kV for CE, and 2.0 kV could be regarded as the spray volage while ΔE was electrooxidation potential of m-toluidine. Based on the EC-mESI-MS technique, the electrooxidation process of m-toluidine was monitored by mass spectrometer in real-time. Prior to the MS study, the oxidation potentials of 0.1 mmol/L m-toluidine in different pH environments including pH 2, 4, 7 and 10 were determined by cyclic voltammetry. The lowest oxidation potential was observed in pH 7 at 0.17 V while the highest one was found in pH 10 at 0.84 V. Based on the obtained oxidation potentials, the corresponding reaction products were determined by EC-mESI-MS subsequently. Under the constant potential of 0.39 V, a series of oxidation products of 0.1 mmol/L m-toluidine in acetonitrile-water solution (pH 7) were detected by mass spectrometer, including radical cation, dimer, trimer, tetramer and pentamer. Since the reaction was carried out in a certain volume of solution, the polymer chain growth of the oligomer had been recorded as well. The tandem mass spectrometry (MS2) experiments indicated that one kind of oligomer had two different structures, i.e, quinone ring and benzene ring. The formation of quinone ring was due to that the benzene ring lost two protons during the reaction. By collecting the above results, the initial electrooxidation mechanism of m-toluidine was deduced. In addition, it was found that the pH of the solution had a great effect on the electrooxidation efficiency of m-toluidine. The acidic environment will inhibit the polymer chain propagation, and the stronger the acid, the more obvious the inhibition effect. Under alkaline condition, it led to a large number of dimers with quinone structure formed, and thus hinder the generation of trimer. The suggested environment is pH 7, where the large oligomer of m-toluidine can be easily formed at a low oxidation potential.