Atmospheric Sampling Mass Spectrometers Activate and Ionize Neutral Water Microdroplets to MeV Energies and up to 200,000+ Charges: Implications for Water Stability and Unusual Chemistry in Microdroplets

Abstract

Neutral water microdroplets formed by condensation were introduced into a mass spectrometer with a standard stainless-steel capillary atmospheric interface. Placing volatile samples near the mass spectrometer inlet led to spectra comparable to those of electrospray ionization of the same compounds. Neutral microdroplets introduced into a charge detection instrument through a nearly identical atmospheric sampling interface resulted in charged microdroplets with diameters between 165 nm (∼3,000 charge detection threshold) and 2.8 μm (200,000+ charges). The vast majority of 100,000+ aqueous microdroplets analyzed were positively charged. Aerodynamic acceleration led to average velocities of ∼270 m/s and ∼210 m/s for the smallest and largest microdroplets, respectively, with corresponding energies ranging from a few MeV to 300+ MeV for the largest microdroplets. Current measured on the capillary and other results indicate that interactions between the neutral microdroplets and the capillary can strip off hundreds of thousands of electrons to form positively charged droplets. Negatively charged microdroplets indicate a minor process in which neutral microdroplets are broken up into smaller microdroplets of both polarities. The MeV+ energy involved in these interactions is sufficient to drive many chemical reactions and may lead to unusual and unexpected chemistry when analysis is done using mass spectrometers with atmospheric sampling interfaces.