Free radicals play an important role in many chemical and biological processes, but due to their highly reactive and short-lived nature, they evade most analytical techniques, limiting our understanding of their formation and reactivity. Spin trapping molecules can react with free radicals to form radical adducts with lifetimes long enough for analysis. Mass spectrometry is an attractive way to identify radical adducts, but due to their radical nature, they form untraditional oxidized [M]+ and reduced [M+2H]+ ions, which complicates the interpretation of mass spectrometry analysis. This work uses simplified mixtures of radicals generated in both water and dimethyl sulfoxide (DMSO) with spin trap 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO), to elucidate the behavior of nitroxide spin traps in electrospray ionization (ESI) mass spectrometry (MS) interfaced with liquid chromatography (LC). This study proposes a disproportionation mechanism to explain the formation of the oxidized and reduced BMPO adducts detected by LC-ESI-MS and explores the formation of di-adducts through radical recombination. We finally present a framework for differentiating between the different types of ions using collision induced fragmentation mass spectra (MS/MS). This work offers a comprehensive investigation into the behavior of radical adducts in ESI-MS to streamline the identification of organic radicals and advance understanding of radical chemistry.

The photochemical aging of biomass-burning organic aerosols (BBOAs) by exposure to sunlight changes the chemical composition over its atmospheric lifetime, affecting the toxicological and climate-relevant properties of BBOA particles. This study used electron paramagnetic resonance (EPR) spectroscopy with a spin-trapping agent, 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO), high-resolution mass spectrometry, and kinetic modeling to study the photosensitized formation of reactive oxygen species (ROS) and free radicals in mixtures of benzoquinone and levoglucosan, known BBOA tracer molecules. EPR analysis of irradiated benzoquinone solutions showed dominant formation of hydroxyl radicals (^•OH), which are known products of reaction of triplet-state benzoquinone with water, also yielding semiquinone radicals. In addition, hydrogen radicals (H^•) were also observed, which were not detected in previous studies. They were most likely generated by photochemical decomposition of semiquinone radicals. The irradiation of mixtures of benzoquinone and levoglucosan led to substantial formation of carbon- and oxygen-centered organic radicals, which became prominent in mixtures with a higher fraction of levoglucosan. High-resolution mass spectrometry permitted direct observation of BMPO-radical adducts and demonstrated the formation of ^•OH, semiquinone radicals, and organic radicals derived from oxidation of benzoquinone and levoglucosan. Mass spectrometry also detected superoxide radical adducts (BMPO-OOH) that did not appear in the EPR spectra. Kinetic modeling of the processes in the irradiated mixtures successfully reproduced the time evolution of the observed formation of the BMPO adducts of ^•OH and H^• observed with EPR. The model was then applied to describe photochemical processes that would occur in mixtures of benzoquinone and levoglucosan in the absence of BMPO, predicting the generation of HO₂^• due to the reaction of H^• with dissolved oxygen. These results imply that photoirradiation of aerosols containing photosensitizers induces ROS formation and secondary radical chemistry to drive photochemical aging of BBOA in the atmosphere.