An Alternative Explanation for Ions Put Forth as Evidence for Abundant Hydroxyl Radicals Formed Due to the Intrinsic Electric Field at the Surface of Water Droplets

Abstract

Ions at m/z 36 and +17 Da higher in mass than protonated caffeine and melatonin have been put forth as evidence for spontaneous formation of abundant OH^• in water droplets due to the high electric field at the surface. Nanoelectrospray ionization was used to produce ions from aqueous caffeine and melatonin solutions under soft mass spectrometer sampling conditions. When the source housing was purged with N₂ gas, abundant protonated caffeine was produced along with a trace abundance of an ion that is +17 Da higher in mass. Accurate mass measurements are consistent with an elemental composition of (caffeine + H + NH₃)⁺ and are sufficiently accurate to rule out the previous identification of this ion as (caffeine + H + OH)^+•. Exhalation of a single breath near the source interface led to between ∼13 and 22-fold increase in the abundance of this ion. The abundance of (melatonin + H + 17 Da)⁺ increases with increasing electrospray potential. It was suggested that the +17 Da ions are fragile and may not survive many mass spectrometry conditions, yet the most abundant collision-induced dissociation product previously reported for these ions is essentially absent in our spectra. These results indicate that ammonia adduction to protonated caffeine and melatonin, along with the possibility of trace sample contamination, is an alternative explanation for the high reported abundances of ions at these nominal masses under some conditions, and these ions are unrelated to the production of hydroxyl radicals that may occur on the surface of water droplets.