Investigations of resonance enhanced multiphoton ionization for in situ measurements of volatile organic compounds under ambient conditions

This dissertation describes investigations of resonance-enhanced multiphoton ionization (REMPI) for in situ measurements of volatile organic compounds (VOCs) under ambient conditions. VOCs are common environmental contaminants and the target of extensive cleanup efforts. However, such efforts are hindered by the lack of fast, in situ techniques that can be used for characterization of VOCs at contaminated sites. REMPI is a technique that holds promise for VOC measurement due to its selectivity and sensitivity. REMPI excitation spectra can be used to identify different species while the REMPI signal at an analyte-specific absorption wavelength can be used for quantitative analysis of the species of interest.; In the first part of this study, a three-photon (2+1) REMPI scheme was used to record the spectra of tetrachloroethylene (PCE), dichloroethylene (TCE), cis-1,2-dichloroethylene (cis-1,2-DCE), trans-1,2-dichloroethylene (trans-1,2-DCE), and 1,1-dichloroethylene (1,1-DCE) under ambient conditions. It was shown that REMPI spectra could be used for the discrimination of individual chloroethylene in a mixture. Quantitative detection at part-per-billion levels was achieved despite interference from atmospheric oxygen and nitrogen in certain wavelength regions. Furthermore, the possibility of using near-UV photons to measure vacuum-UV-absorbing chloroethylenes was demonstrated. This is important for application of REMPI for in situ chloroethylene detection using fiber-optic probes.; The potential of REMPI for environmental remediation monitoring was demonstrated by the use of REMPI to monitor the conversion of PCE to TCE using zero-valent zinc, a scheme that has been considered for the removal of PCE from contaminated groundwater. The reaction was carried out in a batch reactor, with the progress of the reaction being monitored by recording REMPI spectra of PCE and TCE vapors in the headspace at selected time intervals. The REMPI results were in general agreement with those from comparative gas chromatography analysis, despite some disparities attributed to slow mass transfer between aqueous and vapor phases.; The possibility of REMPI detection of BTEX compounds in soil following laser desorption was also investigated. REMPI measurements of benzene-spiked clay samples were performed using laser pulses that were also used to desorb benzene from the clay matrix. A linear correlation was obtained between REMPI signal and benzene concentration in preliminary studies.