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Field Induced Droplet Ionization - Mass Spectrometry
What the heck is field induced droplet ionization?
Succinctly, Field-Induced Droplet Ionization (FIDI) uses high electric fields to elongate and induce a disruption in neutral
droplets. Disruptions result in the ejection of two oppositely charged jets from the droplet which are ultimately a source
of intact gas-phase ions.
Motivation
Original work on neutral droplets in high electric fields dates back to the 1920's. W.A. Macky, C.T.R. Wilson and G.I. Taylor, among others, were interested in understanding how suspended cloud droplets responded to the electric fields present in those clouds.
Macky's apparatus, shown to the right, allowed millimeter-sized droplets to fall through an electric field region (defined by plates "A" and "B") where they would elongate parallel to the field. At a sufficiently high electric field, or the critical field, the droplets ejected jets of progeny droplets.
In the 1960's Geoffery Taylor began rigorous mathematical treatment of the phenomenon. He accurately related the applied electric field to the resulting prolate deformation to the droplet size and surface tension. At a critical electric field which we refer to as the Taylor limit the prolate droplet becomes unstable and emits fine jets.
In reviewing this work, we posit that the ejected jets are electrically charged and that these charged jets represent a viable source of gas-phase ions for mass spectrometry.
Initial Experiments
The figure to the right shows the experimental arrangement. A vibrating orifice aerosol generator (VOAG) generates ~170 micron methanol droplets. The droplets pass between two plates that define and electric field. In this arrangement, droplets elongate horizontally and eject jets towards either plate. When the left-hand plate is grounded and the right-hand plate is held at a positive high voltage, the jet directed to the left is positively charged and the jet directed to the right is negatively charged. If the right hand plate was held at negative high voltage, the jets are oppositely charged. We call the phenomenon "Field-Induced Droplet Ionization" or FIDI.
Droplets were visually characterized. A xenon flashlamp was pulsed at the same frequency as the VOAG giving the illusion of suspended droplets. Droplets were magnified with a microscope and imaged using a high-resolution consumer digital camera. The picture below shows a single elongated methanol droplet with jets formed at conical tips. In the picture the droplet is moving downwards in the low-drag VOAG stream while the jets are subject to higher aerodynamic drag resulting in the slight upward curvature. The bar represents 100 microns.
FIDI Mass Spectrometry
Mass spectrometry verifies the claim that the ejected jets are oppositely charged. The left-hand plate contains a small aperture through which the jets pass into a desolvation capillary of a mass spectrometer. Positive and negative species may be simultaneously mass analyzed from the same droplets with multiple mass spectrometers, but a single instrument must sequentially analyze positive and negative species. Grimm, Ronald L.; Beauchamp J. L. Journal of Physical Chemistry - B 2003, 107(51), 14161-14163 reports on this work.
Next, Page 2: Dynamics of FIDI
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