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Ion Mobility Spectrometry - Page 1
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Introduction and Motivation
Highly charged, volatile droplets evaporate to a point where they become unstable and break apart This creates smaller droplets (electrostatic atomization) and eventually gas phase ions (electrospray ionization). Presently, any mechanism is highly speculative regarding the breakup and analyte ion formation from these charged droplets. Little direct information is available, and many aspects remain unanswered.
Evaporation brings charged droplets to the point at which the force of the surface tension is no longer greater than the force of Columbic repulsion. At this point how are the ions formed?
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Charged Residue Model: This model suggests that at the fission event, the droplet breaks in half either into two identical droplets (even fissioning) or into two disproportionate droplets (uneven fissioning). The charge makeup of these daughter droplets would be comparable with respect to their relative size.
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Ion Desorption Model: Here it is suggested that the force of columbic repulsion is great enough to push or desorb ions directly out of the droplet. Characteristally, the fission event correspond to an almost negligable loss in droplet mass, but a significant drop in charge.
Charge Residue Model
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Ion Desorption Model
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Experimental Approach: Ion Mobility Spectrometry
Charged droplets injected into the Ion Mobility Spectrometer (IMS) are subjected to an electric field which counteracts the force of gravity. Drifting ions separate according to droplet mobility.
Fissioning processes are observed by elastically scattered light from droplets. Fluorescent dye in solution provides enhanced sensitivity for droplet detection, including droplets with subwavelength dimensions.
Experimental Approach: Phase Doppler Anemometry
The Phase Doppler Anemometer measures the size and velocity (mobility) of a particle, based on the received diffraction pattern it creates as it passes through a recombined laser beam.
The phase difference of the two beams is determined by the size of the molecule, and the fringe separation is determined by the particleÕs velocity.
Because the two forces on a particle are from gravity and the electric field, the velocity of the particle can be used to determine the charge on the droplet.
Experimental Approach: Droplet Breakup
At one position in the IMS, a particleÕs size, velocity, and number of charges can be determined and compared to the Rayleigh Limit for a droplet of that size. Rayleigh limit equation:
As the droplet evaporates, the electrical tension and Coloumb repulsion increase. Therefore, as charge density increases, the tendency to fission (to create more surface area) counteracts the surface tension of the particle. The Rayleigh Limit predicts the droplet size at which fissioning occurs. When the charged droplets pass through the 488 nm excitation beam, the distribution of particles is observed. Here, with the aid of the fluorescence markers, the products of fission events can be observed. The residence time of the droplet in the instrument can be varied by changing the strength of the electric field, and the flow of a countercurrent gas.
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