John Hepburn


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Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Development and Characterization of Two Versions of a New Single Particle Mass Spectrometer for Organic Aerosol Analysis that Incorporate a 3D Ion Trap (2010)

Aerosol particles are ubiquitous throughout the atmosphere and play an important role in human health, climate, and the chemistry of the atmosphere. A significant mass fraction of these particles is composed of organic species, which remain poorly characterized due to the number and diversity of species present. This thesis describes the development and characterization of two versions of a new single particle mass spectrometer with a 3D ion trap for organic aerosol studies.Version I combines CO₂ laser desorption and electron impact ionization in an ion trap. Mass spectra obtained for four species are comparable to NIST EI spectra. Tandem mass spectrometry studies are also demonstrated. The effects of vaporization energy, ionization delay time, and electron pulse width on the mass spectra and fragmentation patterns are examined. The detection limit of the instrument is found to be ~1x10⁸ molecules (350 nm diameter particle) for 2,4-dihydroxybenzoic acid. Version II integrates CO₂ laser desorption and tunable VUV ionization in an ion trap and was used for a detailed study of oleyl alcohol, oleic acid and mixtures thereof. Both the degree of fragmentation in the mass spectra and the translational energy of the vaporized molecules are found to vary as a function of desorption energy in the pure particles and as a function of composition in the mixed particles. These changes can be described by the energy absorbed per particle during desorption. We show that these effects hinder the quantitative response of the instrument and have important implications for other two step laser desorption/ionization systems.The final part of this thesis presents preliminary results from atmospherically relevant particles. Mass spectra of cigarette sidestream smoke, fulvic acid, meat cooking, and ammonium bisulfate aerosols are collected using both versions of the instrument. The two step desorption/ionization process only worked for two types of aerosols, while CO₂ only mass spectra were obtained for all four aerosol types. The suitability of CO₂ desorption strongly depended on particle composition, which will affect the applicability of the technique to atmospherically realistic aerosols. The results also suggest that CO₂ only laser desorption/ionization may be useful for field studies.

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Development and Characterization of a Single Particle Mass Spectrometer with Soft Photoization for Organic Aerosol Studies (2009)

No abstract available.

New methods of coherent anti-Stokes Raman spectroscopy based on broadband pulses (2009)

The research work of this Ph.D. thesis is centered on coherent anti-Stokes Raman spectroscopy (CARS) with broad band coherent pulses. After a mathematical derivation of the formula that is responsible for CARS, four new approaches were proposed. The first method of Noise-autocorrelation spectroscopy with coherent Raman scattering utilizes spectral noise to reveal vibrational level spacings through autocorrelation. Its variation of Narrowband spectroscopy by all-optical correlation of broadband pulses, uses the technique of optical processing based on noisy probe pulse of special shape to obtain high resolution CARS spectra. The method of complete characterization of molecular vibration, can measure the phase of laser induced vibration through amplitude and phase retrieval on a time-frequency spectrogram. It is also a high resolution method. The final method Background free coherent Raman spectroscopy by detecting spectral phase of molecular vibrations is the spectroscopic application of detected spectral phase of laser induced vibration.

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