Fundamental Studies of the Mechanisms and Applications of Field-Induced Droplet Ionization Mass Spectrometry and Electrospray Mass Spectrometry

Citation

Grimm, Ronald L. (2006) Fundamental Studies of the Mechanisms and Applications of Field-Induced Droplet Ionization Mass Spectrometry and Electrospray Mass Spectrometry. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/E8EV-W425. https://resolver.caltech.edu/CaltechETD:etd-10092005-222651

Abstract

This thesis explores the evaporation and Rayleigh discharge dynamics of highly charged micron-sized droplets and explores new methodologies for extracting ions for mass analysis from neutral droplets using strong electric fields in a technique termed field-induced droplet ionization.

A phase Doppler anemometer characterizes individual highly charged droplets moving through a uniform, mild electric field within an ion mobility cell according to size, velocity, and charge. Repeated reversals of the electric field allow multiple characterizations on selected droplets. This "ping-pong" technique provides droplet histories that determine the solvent evaporation and Rayleigh discharge behavior. The ping-pong experiment characterizes volatile droplets of the hydrocarbon solvents n-heptane, n-octane, and p-xylene as well as two-component droplets of either 2-methoxyethanol, tert-butanol, or m-nitrobenzyl alcohol with methanol. On average, hydrocarbon droplets eject 18% of their net charge into progeny droplets with an undetectable loss in mass. Rayleigh discharge events in the polar, binary droplets release between 20 and 35% of the net charge with a correspondingly undetectable loss in mass.

In other experiments, strong electric fields elongate neutral droplets along the field axis. Field-induced droplet ionization (FIDI) occurs at sufficient field strengths as the droplets eject opposing jets of positively and negatively charged progeny droplets. Images of droplets from a vibrating orifice aerosol generator illustrate this phenomenon, and mass spectrometric sampling of the progeny droplets demonstrates that they are a viable source of desolvated gas-phase ions. Switched electric field experiments relate the timescale of droplet elongation and progeny droplet formation in FIDI to the timescale of oscillations of droplets in sub-critical field strengths. FIDI mass spectra are presented for several species, including tetraheptyl ammonium cation, deprotonated benzene tetracarboxylic acid, and multiply protonated cytochrome c.

Droplets may serve as reactors before being sampled by FIDI-MS. FIDI-MS probes the products of heterogeneous reactions between solution-phase oleic acid or a lysophosphatidic acid and gas-phase ozone.

Thesis Files