Two New Devices for Identifying Electrochemical Reaction Intermediates with Desorption Electrospray Ionization Mass Spectrometry.

Desorption electrospray ionization mass spectrometry (DESI-MS) previously has been used to capture and identify transient intermediates in electrochemical redox reactions on a platinum-covered rotating waterwheel. We present here two different setups that use a flat surface with porous carbon tape as the working electrode, where analyte-containing microdroplets from the DESI probe contacted with electrolyte supplied onto the surface. One setup had the conducting carbon tape in the form of a grooved inclined plane; the other one was in the form of a flat plane that had the conducting carbon tape as its front surface. Both these setups, which were relatively robust and easy to operate, overcame interference from the electrospray sheath gas that disturbs and dries the flowing electrolyte. By using the inclined-plane device, we observed radical cations and dimer species generated in the electrochemical oxidation of triphenylamine, diimine and imine alcohol in the electrochemical oxidation of uric acid, and the reductive cleavage of disulfide bonds in glutathione disulfide. By using the device with the flat carbon tape, we detected nitrenium ions generated in the electrochemical oxidation of N,N'-dimethyoxydiphenylamine and di-p-tolylamine. Our experience suggests that the flat porous carbon tape surface might be preferable over the inclined plane because of its ease of setup.