Field induced droplet ionization: Investigations of complex chemical reactions at the air-​water interface employing mass spectrometry

Abstract

Every new method for sampling complex mols. from liqs. and solids into the gas phase has fueled the expanding application of mass spectrometry to mol. biol. and other fields of science. Recently developed, field induced droplet ionization (FIDI), combined with mass spectrometric (MS) sampling, holds promise for a broad spectrum of applications. FIDI-MS is based on the physics of nanoliter droplets in time dependent elec. fields, leading to formation of dual Taylor cones from which streams of pos. and neg. charged submicron droplets are emitted in opposite directions, forming what is essentially a dual electrospray ion source. In multi-component systems ionic species with high surface activity are detected with high sensitivity, along with mol. clusters indicative of specific ion pair interactions and mol. aggregation at the air-water interface. The air-water interface represents one of the most ubiquitous chem. environments in nature, present on the surface of oceans and lakes, on atm. aerosols, and even in the human respiratory system. Yet in spite of their prevalence, chem. reactions occurring at these interfaces remain poorly characterized. FIDI-MS has proven itself to be ideally suited for such studies. Several examples of these investigations will be presented, including org. reaction mechanisms involving acid and base catalysis, atm. aerosol chem., and processes that involve biol. interfaces such as the pulmonary surfactant layer, where we have unraveled the myriad chem. and phys. processes assocd. with oxidants such as ozone interacting with lipids and proteins in this complex interfacial environment. Theor. studies involving state of the art mol. dynamics and quantum chem. calcns. play an important role in in these investigations. Several new approaches demonstrating selective sampling of complex mol. species from planar liq. surfaces will also be discussed.