UVM Theses and Dissertations
Format:
Print
Author:
LaFranchi, Brian W.
Dept./Program:
Chemistry
Year:
2006
Degree:
PhD
Abstract:
Atmospheric aerosols have been found to play an important role in global climate, atmospheric chemistry, and human health. Significant improvements in the understanding of atmospheric aerosols have come about as a result of the development of aerosol mass spectrometers that are capable of providing correlated size and chemical information on individual particles on-line and in real time. One of the commonly cited shortcomings with these methods, however, is their inability to adequately characterize the organic fraction of atmospheric particulate matter, mainly as a result of the extensive fragmentation that occurs during ionization. Photoelectron resonance capture ionization (PERCI) is a soft ionization method that has been developed primarily for use in an online aerosol mass spectrometer (AMS) for the analysis of organic particulate matter. In contrast to conventional ionization methods used in AMS, PERCI results in minimal fragmentation of the organic analyte molecules, greatly facilitating molecular identification in complex mixtures. PERCI relies on the generation of photoelectrons by pulsing a tunable UV laser onto a conducting surface, where the photoelectron energy is dependent upon the photon energy and the work function of the surface. Attachment of these low energy photoelectrons (< 1 eV) to molecular species above the photoelectrode surface leads to the formation of negative ions either through an associative or dissociative mechanism, producing the molecular anion or fragment anions, respectively. In many cases, dissociation leads to the [M-H]⁻ ion or some other ion fragment that closely resembles the parent molecule.
This dissertation focuses on the development, characterization, and demonstration of PERCI-AMS for the analysis of organic particles. First, the utility of PERCI as a soft ionization method is demonstrated using gas phase test compounds such as 2-nitrophenol, SF₆, and a series of n-aldehydes and ketones. The adaptation of PERCI for aerosol analysis, using thermal vaporization, is demonstrated using a well studied heterogeneous reaction between ozone gas and oleic acid particles. Direct detection of the four major ozonolysis products as well as some key high molecular weight intermediates further demonstrates the soft nature of PERCI for compound identification. In addition to these studies highlighting the qualitative benefits, a critical evaluation of the performance and quantitative abilities of the PERCI-AMS is given. This is done through a series of photoelectron, gas phase, and particle phase measurements. These experiments have helped to understand the fundamentals of the PERCI process as well as the limitations of the PERCI-AMS, from which strategies for future design and optimization of the PERCIAMS can be assessed.
This dissertation focuses on the development, characterization, and demonstration of PERCI-AMS for the analysis of organic particles. First, the utility of PERCI as a soft ionization method is demonstrated using gas phase test compounds such as 2-nitrophenol, SF₆, and a series of n-aldehydes and ketones. The adaptation of PERCI for aerosol analysis, using thermal vaporization, is demonstrated using a well studied heterogeneous reaction between ozone gas and oleic acid particles. Direct detection of the four major ozonolysis products as well as some key high molecular weight intermediates further demonstrates the soft nature of PERCI for compound identification. In addition to these studies highlighting the qualitative benefits, a critical evaluation of the performance and quantitative abilities of the PERCI-AMS is given. This is done through a series of photoelectron, gas phase, and particle phase measurements. These experiments have helped to understand the fundamentals of the PERCI process as well as the limitations of the PERCI-AMS, from which strategies for future design and optimization of the PERCIAMS can be assessed.