A Source of the Mysterious m/z 36 Ions Identified: Implications for the Stability of Water and Unusual Chemistry in Microdroplets

Abstract

Many unusual reactions involving aqueous microdroplets have been reported, including nitrogen fixation at room temperature, production of abundant hydrogen peroxide, and formation of an ion at m/z 36, attributed to (H₂O–OH₂)^+•, (H₃O + OH)^+•, or (H₂O)₂^+•, which was used to support the hypothesis of spontaneous production of hydroxyl radicals. Here, m/z 36 ions and extensive hydrated clusters of this ion are formed using either nanoelectrospray ionization or a vibrating mesh nebulizer that produces water droplets ranging from ∼100 nm to ∼20 μm. Exhalation of a single breath near the droplets leads to a substantial increase in the abundance of this ion series, whereas purging the source with N₂ gas leads to its near complete disappearance. Accurate mass measurements show that m/z 36 ions formed from pure water are NH₄⁺(H₂O) and not (H₂O)₂^+•. Both the high sensitivity to trace levels of gaseous ammonia (unoptimized detection limit of low parts-per-billion) in these experiments and the likely misidentification of the m/z 36 ion in many previous experiments indicate that many results that have been used to support hypotheses about unusual chemistry and the effects of high intrinsic electric fields at microdroplet surfaces may require a more thorough evaluation.

Accurate mass measurements show that a m/z 36 ion that is sometimes formed from aqueous microdroplets is NH₄⁺(H₂O), not (H₂O)₂^+•, and is abundant when droplets are exposed to human breath.