Shinichi Enami
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 – /FeCl 2 /H 2 O 2, but no detection was observed from an aqueous solution of BA – /FeCl 2 . 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.