Contact Electrification of Individual Water Microdroplets Inside Mass Spectrometers Leads to Extensive Oxidation or Reduction Irrespective of Initial Droplet Charge or Polarity

Abstract

Charged water microdroplets that are aerodynamically accelerated upon introduction into an atmospheric sampling mass spectrometer interface can undergo glancing collisions that lead to surprising electrochemical processes. A custom charge detection instrument with a 24-element detector array is used to determine the outcomes of glancing collisions of individual charged water microdroplets with stainless-steel surfaces over a well-defined and narrow angle of ∼0.15° ± 0.03°. Water droplets entering the high-vacuum region with angles of <0.11° pass through the detector unimpeded and have kinetic energies in the MeV range depending on droplet size due to aerodynamic acceleration alone. Droplets that have glancing surface collisions undergo extensive oxidation or reduction through the removal or addition of up to tens of thousands of electrons per collision, independent of their initial charge or polarity, although some ion transfer may also play a role. The propensity for these two electrochemical processes appears stochastic, although more highly charged droplets appear to undergo charge reduction more readily. These results clearly demonstrate that highly energetic electrochemical processes of any type can occur within charged or uncharged aqueous microdroplets of any polarity that are introduced into mass spectrometers and may explain the observation of some unusual ions that have been reported in some microdroplet reaction studies.