On-line Mass Spectrometric Characterization of Key Intermediates in Luminol Electrochemiluminescence
Due to the low oxidation potential and high electrochemiluminescence(ECL)quantum yield,Luminol has been widely applied in analytical and detection fields,espe-cially in immunoassay,food safety and environmental monitoring.However,there is a lack of sufficient experimental evidence on the ECL mechanism of Luminol,which affects the further improvement of its ECL efficiency.Herein,an electrochemistry-micro tube-mass spectrometry(EC-MT-MS)device was constructed for on-line monito-ring of Luminol ECL reaction.In this EC-MT-MS device,a high borosilicate glass tube with a nano tip was used as electrochemical cell and electrospray ionization source,a platinum working electrode and a platinum counter electrode were inserted into the tube and connected to a customized power station,which could simultaneously input a low voltage(volt level)for the ECL reaction and a high voltage(kilovolt level)for the formation of electrospray.Prior to the EC-MT-MS experiment,the composition of the reaction solution was optimized to obtain the highest MS signal response,and Luminol was dissolved in acetonitrile-water(4∶1,V/V)solvent with ammonium acetate as the electrolyte.The concentrations of Luminol and ammonium acetate in the prepared solu-tion were both 0.1 mmol/L,and the pH was adjusted to 11 using ammonium aqueous.The oxidation potential of Luminol was subsequently studied using a traditional three-electrode system by cyclic voltammetry in the same solution,and the determined oxida-tion potential for ECL reaction was 0.87 V.During the EC-MT-MS measurements,(2.5 kV+0.87 V)and 2.5 kV were input to the working and counter through the pow-er station,respectively.A series of short-lived intermediates and products such as L1(m/z 198)and the luminescent precursor AP-(m/z 180)in the Luminol ECL reaction process were successfully captured and analyzed by MS.The molecular structures of these products were characterized by tandem mass spectrometry(MSn)technique.There are mainly two fragments of m/z 163,136 present in the MS2 spectrum of lumi-nescent precursor AP-,attributed to the(NH2+H)and COO losses,respectively.In addition,the promotion mechanism of hydrogen peroxide(H2 O2)for the ECL perform-ance of Luminol was also investigated.A key reaction intermediate LO2 H-was found in the MS spectrum at m/z 208.Meanwhile,the peak intensity of luminescent precursor AP-significantly increased from 1.3％to 15.7％,proving that the presence of H2O2 effectively promoted the generation of key luminescent intermediate.Finally,by adding 1,4-benzoquinone to the Luminol-H2 O2 system,the central role of the superoxide anion radical(O2-)derived from H2O2 in the Luminol ECL process was confirmed.The relevant results provide a direct data reference for the study of the Luminol ECL mechanism.
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维普
