Development and characterization of a single particle mass spectrometer with soft photoionization for organic aerosol studies

• Main Author: Hanna, Sarah Jane
• Language: English
• Published: University of British Columbia 2009
• Online Access: http://hdl.handle.net/2429/11882

Aerosol particles, which are ubiquitous in the Earth’s atmosphere, can be 20-90% organic carbon by mass. These organic aerosols are thought to play an important role in climate, human health, and the chemistry of the atmosphere. Their composition, however, can be extremely complex, presenting a significant challenge to standard analytical techniques. Over the past several decades aerosol mass spectrometry has become an important tool for determining organic aerosol chemical composition. This thesis describes the development and characterization of a new aerosol mass spectrometer designed for analysis of individual organic aerosol particles. A unique vacuum UV source and custom monochromator, fully tunable from 7.4 to 10.2 eV (168 to 122 nm), was developed and characterized using gas phase analytes. The VUV source was coupled to a single particle mass spectrometer which uses a tunable CO₂ laser for particle vaporization and an ion trap for mass analysis. Initial aerosol experiments were carried out using caffeine particles. The appearance energy of caffeine molecular ions from the vaporized particle was measured by scanning the VUV photon energy. The impact of increasing vaporization energy was also studied. Following the caffeine experiments, a detailed study of oleic acid and 2,4-dihydroxybenzoic acid (DHB) aerosols was undertaken. The appearance energies of both the molecular and fragment ions were measured and the impact of ionization wavelength was determined. In addition, the results were compared to those from similar studies done with time-of-flight mass analyzers, allowing observation of the impact of long ion storage times on the mass spectra. The final part of this thesis compares the IR laser vaporization of small, solid caffeine and DHB aerosols with larger, liquid oleic acid particles. The translational energy of the vaporized aerosol plume was followed by changing the delay between the vaporization and ionization events. The extent of fragmentation was monitored and was found to be dependent on both vaporization energy and ionization delay time. Both translational energy and the degree of fragmentation were seen to change with particle type, an effect which has important implications for pulsed laser desorption in aerosol mass spectrometry.