Positively Charged Water Microdroplets Ionize Surrounding Gas Molecules

In electrospray ionization, a liquid solution is passed through a narrow capillary held at high voltage─typically +1 to +5 kV for generating positively charged microdroplets or -1 to -5 kV for negatively charged ones. It has recently been claimed that only negatively charged microdroplets can ionize surrounding gases, whereas positively charged microdroplets cannot. In contrast, we present multiple lines of evidence showing that positively charged water microdroplets are indeed capable of ionizing neutral molecules in their vicinity. Using stabilized nanoelectrospray of deionized water and high-resolution mass spectrometry, we detected ionization products of various neutral species, including oxygen, toluene, and other organic vapors. These results indicate that like their negatively charged counterparts, positively charged microdroplets can induce gas-phase ionization involving the loss of an electron from a neutral precursor, likely facilitated by the intense electric field at the microdroplet water-gas interface.