Exploring Ionization Mechanisms and Accelerated Chemical Reactivity with Droplet Assisted Ionization-Mass Spectrometry

BRYAN R. BZDEK, Joshua Harrison, Edward Neal, Jim Walker, University of Bristol

     Abstract Number: 175
     Working Group: Instrumentation and Methods

Abstract
Aerosol droplets are unique microcompartments containing microscopic amounts of material and exhibiting surprising reactivity relative to macroscopic systems. Many approaches for online molecular analysis of aerosols and droplets require a separate ionization stage (e.g. extractive electrospray, chemical ionization) before introducing the sample to the mass spectrometer (MS). A separate ionization stage can have drawbacks, as the ionization step can extend over relatively long times (seconds), potentially reducing sensitivity by spreading signal intensity over time or introducing artifacts (e.g. undesirable reactions) that complicate measurement interpretation. Droplet assisted ionization (DAI) is a novel inlet ionization approach that overcomes these drawbacks. In DAI, particles are grown hygroscopically to micron-sized liquid droplets immediately before being sampled into the MS inlet. Within the inlet, rapid (micro- to millisecond timescale) aerodynamic or thermal droplet breakup generates charge that ionizes the analyte molecules within the droplet. In this presentation, the role of the solvent matrix in controlling ionization in DAI will be described. Ion yields are highly correlated with particle phase and relative humidity (which controls droplet water content). These effects are easily controlled by standardizing the relative humidity around the aerosol plume immediately before sampling into the MS. The DAI ionization efficiency for plumes of submicron aerosol will be compared against complementary measurements made on individual levitated droplets in an electrodynamic balance, enabling a comprehensive understanding of the ionization mechanism across multiple orders-of-magnitude in droplet size and water content. Experiments on biomolecules suggest that molecular structure is preserved upon MS analysis due to the charge-limited nature of ionization. Lastly, an esterification reaction observed to occur orders-of-magnitude faster in aerosol than in macroscopic solution is studied using DAI, elucidating the underlying mechanism for reaction acceleration. Combined, these results demonstrate the unique capabilities of this novel approach to aerosol chemical analysis.