Can Hydroxyl Radicals Be Spontaneously Formed at the Air–Water Interface of Microdroplets? A Spray Ionization Mass Spectrometry Study

Recent studies have proposed that hydroxyl radicals (OH) could be formed at the air-water interface (AWI) of aqueous microdroplets. We performed spray ionization mass spectrometry investigations using benzoate (BA^-) as an OH scavenger to determine whether OH was spontaneously formed at the AWI of microdroplets. The mass spectrometric signal for hydroxy-BA^-, a major product of BA^- + OH, was confirmed in the presence of OH(g). However, that signal was undetected in water microdroplets containing BA^- in the absence of OH(g). We separately confirmed the formation of hydroxy-BA^- by detection of its signal from an aqueous solution of BA^-/FeCl₂/H₂O₂, but no detection was observed from an aqueous solution of BA^-/FeCl₂. Experiments using DMSO as an alternative OH-scavenger also showed no evidence for spontaneously formed OH at the AWI of microdroplets. We derived an upper limit to the concentration of spontaneously formed OH to be ∼0.2 μM under our experimental conditions.