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A. Computational chemistry applied to the analysis of air pollution reaction mechanisms ; B. Fundamental studies of droplet evaporation and discharge dynamics in electrospray ionization

Citation

Smith, James N. (2000) A. Computational chemistry applied to the analysis of air pollution reaction mechanisms ; B. Fundamental studies of droplet evaporation and discharge dynamics in electrospray ionization. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-09202007-080810

Abstract

The first part of the thesis investigates the use of theoretical quantum chemical calculations for the study of the reaction pathways of the atmospheric aromatic-OH reaction. The computational model is comprised of a semi-empirical PM3 geometry optimization followed by a single point calculation performed using the Becke3LYP hybrid density functional and a 6-31G(d,p) basis. Zero-point energies were calculated using PM3, and transition states barrier heights estimated by a constrained optimization procedure developed for this study. Full mechanisms for the OH-initiated photooxidation of toluene, m-xylene, p-xylene, 1,2,4-trimethylbenzene and m-ethyltoluene are developed. The lowest energy intermediates have been determined and predicted products from these structures are compared to available experimental product data. These studies serve to refine proposed mechanisms currently available for toluene, mxylene and p-xylene, while providing new information on the 1,2,4-trimethylbenzene and m-ethyltoluene reaction pathways. In the second part of the thesis, an experimental technique is described for probing the mechanism and dynamics of charged droplet breakup in electrospray ionization. At its core is an instrument that can perform simultaneous, in situ measurements of size and charge on individual droplets. Charged droplets are sampled directly from the spray source into a drift cell with a uniform electric field. A simultaneous diameter and charge measurement is made on individual droplets at the center of the cell using phase Doppler anemometry. By reversing the field in the drift chamber once the initial size and charge measurement is made, the droplet can be made to pass again through the measurement region of the phase Doppler anemometer. In this way, repeated measurements of the size and charge can be made on a single droplet. This technique has been applied to a variety of solutions used commonly in electrospray ionization and lead to the following generalizations. (1) The discharge dynamics of droplets with the same initial diameter and charge are highly reproducible for all solvents and analyte/solvent combinations studied to date. (2) Published over a hundred years ago, Lord Rayleigh's analysis of charged droplet instabilities resulting from solvent evaporation is remarkably accurate in predicting observed discharge events for all three solvents. (3) Droplet discharge events are characterized by loss of 15-20% of the charge from methanol and acetonitrile droplets, and 20-40% from water droplets, with little accompanying loss of solvent. (4) Discharge events occur in a reproducible temporal pattern, with decreasing time intervals between successive events, dictated by solvent evaporation and the approach to the Rayleigh limit. The droplet size decrease agrees well with a model of solvent evaporation. (5) The addition of biomolecules or salt (up to 10[superscript -3] M to the solution does not significantly alter discharge dynamics. The size-charge correlation and discharge dynamics of negatively charged droplets formed in electrospray ionization of 10[superscript -4] M NaCl in methanol are similar to those observed for positively charged droplets. (6) The life history of a single electrosprayed droplet can be followed through numerous discharge events (sequences with up to 50 measurements and 6 discharges have been observed) in the absence of radial confinement. This implies that no significant displacement of the droplet arises from discharge events and is inconsistent with any notion of high energy chaotic fission processes. (7) Droplet size-charge correlations show that, in certain instances, droplets from an electrospray fall into discrete groupings that can be attributed to fission events.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:chemistry
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Environmental Science and Engineering
Awards:The Herbert Newby McCoy Award, 2000
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Flagan, Richard C. (advisor)
  • Beauchamp, Jesse L. (advisor)
Thesis Committee:
  • Unknown, Unknown
Defense Date:11 May 2000
Record Number:CaltechETD:etd-09202007-080810
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-09202007-080810
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3665
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:20 Sep 2007
Last Modified:26 Dec 2012 03:01

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