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Abstract Formation of energetic species at the surface of aqueous microdroplets, including abundant hydroxyl radicals, oxidation products, and ionized N₂and O₂gas, has been previously attributed to the high electric field at the droplet surface. Here, evidence for a new mechanism for electronic excitation involving electron emission from negatively charged water droplets is shown. Droplet evaporation can lead to the emission of ions and droplet fission, but unlike positively charged droplets, negatively charged droplets can also shed charge by electron emission. With nanoelectrospray, no anions or negatively charged droplets are produced with a positive electrospray potential. In contrast, abundant O₂^+•and H₃O⁺(H₂O) are formed with negative electrospray. When toluene vapor is introduced with negative electrospray, abundant toluene radical cations and fragments are produced. Both O₂^+•and toluene radical cations are produced with pneumatic nebulization. The electrons produced from evaporating negatively charged droplets can be accelerated by an external electric field in electrospray, or by the field generated between droplets with opposite polarities produced by pneumatic nebulization. This electron emission/ionization mechanism leads to electronic excitation >10 eV, and it may explain some of the surprising chemistries that were previously attributed to the high intrinsic electric field at the surface of aqueous droplets. 

Abstract Spontaneous ionization/breakup of water at the surface of aqueous droplets has been reported with evidence ranging from formation of hydrogen peroxide and hydroxyl radicals, indicated by ions atm/z36 attributed to OH⋅‐H₃O⁺or (H₂O‐OH₂)⁺⋅ as well as oxidation products of radical scavengers in mass spectra of water droplets formed by pneumatic nebulization. Here, aqueous droplets are formed both by nanoelectrospray, which produces highly charged nanodrops with initial diameters ~100 nm, and a vibrating mesh nebulizer, which produces 2–20 μm droplets that are initially less highly charged. The lifetimes of these droplets range from 10s of μs to 560 ms and the surface‐to‐volume ratios span ~100‐fold range. No ions atm/z36 are detected with pure water, nor are significant oxidation products for the two radical scavengers that were previously reported to be formed in high abundance. These and other results indicate that prior conclusions about spontaneous hydroxyl radical formation in unactivated water droplets are not supported by the evidence and that water appears to be stable at droplet surfaces over a wide range of droplet size, charge and lifetime. 

Laser-heated electrospray ionization with mass spectrometry enables melting temperature measurements of aggregation-prone proteins from which thermochemical and mechanistic information about protein unfolding and ligand loss is deduced. 

A recently developed method enables the loss of individual charges from 1 to 10 MDa salt clusters to be resolved using charge detection mass spectrometry. This technique is well suited for investigating the mechanics of late stage ion formation.