The N,N-dimethylaniline (DMA) radical cation DMAC + ,along-sought transient intermediate,w as detected by mass spectrometry (MS) during the electrochemical oxidation of DMA. This was accomplished by coupling desorption electrospray ionization (DESI) MS with awaterwheel working electrode setup to sample the surface of the working electrode during electrochemical analysis.T his study clearly shows that DESI-based electrochemical MS is capable of capturing electrochemically generated intermediates with half-lives on the order of microseconds,w hich is 4-5 orders of magnitude faster than previously reported electrochemical mass spectrometry techniques.Determining the first step in electrooxidation poses an experimental challenge.Often, aone-electron loss occurs,but the resulting radical cation escapes detection because it reacts so rapidly.O ne such example that has been extensively studied is the radical cations of aromatic amines. [1][2][3][4][5][6][7][8][9] Efforts to identify the radical cation species of aromatic amines with MS have been successful for some of the more stable radical cations. [2,3,8] However,i dentification of the radical cation of N,N-dimethylaniline (DMA) has not been achieved by thermospray MS, [10] spectroelectrochemistry,a nd electron paramagnetic resonance, [11] or fast scanning cyclic voltammetry. [12][13][14] By using fast scanning cyclic voltammetry,t he second-order rate constant for the dimerization of DMAC + , as well as other aromatic amines,was determined to be on the order of 10 5 m À1 s À1 . [12,14] In 2014, however,Cao,Kim, and Bard succeeded in observing what is proposed to be DMAC + by using scanning electrochemical microscopy (SECM) with ultramicroelectrodes.[1] DMAC + is generated at the tip of aPt wire (ca. 500 nm radius) and is collected on aP ts ubstrate (5 mmr adius) separated from the Pt tip by af ew hundred nanometers.B yv arying the separation, the dimerization process was shown to follow second-order kinetics with arate constant of 2.5 10 8 m À1 s À1 .S ECM experiments could be performed at DMA concentrations as high as 0.4 mm in acetonitrile,u nder which conditions the half-life is 10 ms. At greater concentrations,the dimerization is reported to be too fast to obtain good collection efficiency. [1] We report herein MS detection of the electrochemical formation of DMAC + at ac oncentration of 4.7 mm DMA in acetonitrile by using an ambient ionization platform described recently [6] .U sing the measured rate constant of 2.5 10 8 m À1 s À1 , [1] the calculated half-life of DMAC + is less than 1 ms. Our approach is general, easy to implement, and is compatible with aw ider range of operational concentrations relative to SECM. [1] This study suggests that many other short-lived electrochemical intermediates could be identified and studied by this means since MS is ageneral detector with high chemical specificity,unlike electrochemical detectors.Theexperimental design in Figure 1greatly resembles the waterwheel setup previously reported, [6] which employs ar ound rotating ...