EPR analysis of radicals produced by a flowing atmospheric-pressure afterglow

Abstract

Electrical discharges are often used as ionization sources for Ambient Desorption/Ionization – Mass Spectrometry (ADI/MS). One such ADI/MS source, the Flowing Atmospheric-Pressure Afterglow (FAPA), has been characterized with a variety of techniques. Excited radical and neutral species produced by the FAPA have been measured using Optical Emission Spectroscopy (OES); however, the afterglow region of the FAPA is difficult to characterize due to how few photons it emits. In order to overcome the limitations of OES, Electron Paramagnetic Resonance (EPR) and a variety of spin-trapping reagents were used to analyze radical and reactive species in the FAPA afterglow. Solutions containing spin-trapping reagents were treated with FAPA over a range of gas flow rates, discharge currents, FAPA-to-sample distances, and plasma treatment times. Several species were detected: O, 1O2, ●OH, H●, a CO2●-. Furthermore, different configurations of FAPA were compared to see which produced a greater abundance of radicals. Ultimately, EPR and spin-trapping reagents were found to be an effective method for the characterization of the afterglow region of FAPA.