Development of a Novel Single Droplet Mass Spectrometry Approach to Investigate Interfacial Photochemistry in Aerosol Droplets

Aerosols and droplets are central to a wide range of application areas, including atmospheric science, industrial formulations, and human health. Because they have high surface area-to-volume ratios, reactivity at their microscopic interfaces holds outsized importance. Recently, remarkable accelerations by up to six orders of magnitude in the rates of chemical reactions in microdroplets have been reported. These reaction accelerations are in part attributed to the unique environment of the droplet-air interface. For instance, surface chemistry can dominate over bulk chemistry as the droplet's surface area-to-volume ratio increases. However, few experimental approaches exist to selectively probe chemical composition at these microscopic interfaces.

This project aims to develop and create an instrument for the investigation of interfacial interactions within aerosol droplets at small sizes, approximately 40 microns in diameter. By coupling both field-induced droplet ionization (FIDI), linear quadrupole electrodynamic balances (LQ-EDBs), and time-of-flight mass spectrometry, the instrument will enable a high degree of control over the droplets before selectively sampling their surfaces during ionization. A droplet in a strong electric field will elongate along the electric field axis. The droplet shape becomes unstable when the applied electric field is greater than the Taylor limit, leading to a Rayleigh discharge consisting of progeny droplets from the sheared electrical double layer of the droplet. These charged progeny droplets can then be sampled by mass spectrometry, which provides insight into the droplet's surface composition. Leveraging recent advances in LQ-EDBs, picolitre droplets can be trapped and photochemical reactions can be initiated prior to FIDI and sampling into the mass spectrometer. This approach to interfacial analysis of droplets has potential to provide important insights into chemical reactivity at microscopic interfaces.