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LASER FLUORESCENCE EEM PROBE FOR CONE PENETROMETER POLLUTION ANALYSIS
Kenny, J. E. AND B. K. Lien*. LASER FLUORESCENCE EEM PROBE FOR CONE PENETROMETER POLLUTION ANALYSIS. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-99/041 (NTIS PB99-171605), 1999.
A fiber optic LIF (Laser induced fluorescence) EEM (Excitation emission matrix) instrument for CPT deployment has been successfully developed and field tested. The system employs a Nd: YAG laser and Raman shifter as a rugged field portable excitation source. This excitation source simultaneously produce >20 beams of differing wavelength in the UV that can be selectively chosen for launch into fiber optics. The system uses a pair of silica-clad-silica optical fibers for each of its channels (eight of which were used in the work described), one to deliver excitation light from the multi-wavelength laser excitation source to the sample, and the other to conduct contaminant fluorescence to a grating spectrograph which utilizes a charge-coupled device detector to record fluorescence intensity as a function of both excitation and emission wavelength. The system has automatic data acquisition (depth and LIF) and some real time data analysis capability. The instrument had undergone several different calibrations including wavelength, analyte concentration, and standard compound fluorescence as a function of incident energy. Many sample type have been characterized in the laboratory including single components on sands of different particle sizes and clay, and fuel mixtures in solution and on sands. This system has been installed in a CPT vehicle and has undergone four field tests at three different sites. The first field test was at Hill AFB in Utah; hardware failure and low contaminant levels prevented successful demonstration of the technology. The second field test was at Hanscom AFB in Massachusetts, where we achieved our goals of instrument characterization and demonstration. The third test, at the U.S. Coast Guard Station in Elizabeth city, North Carolina, was intended as a demonstration of the system's site characterization capabilities. The fourth and final demonstration was a return visit to Hanscom AFB with the intent to perform a more detailed characterization of the site. The last three field demonstrations were successful in that the instrument was operational and depth - encoded LIF data were collected for a significant number of surface push locations. The matrix-formatted field data are displayed as three-dimensional fingerprints of contamination at given push locations and depths; they are also reduced in dimension by summing over one or both wavelength axes and plotting summed fluorescence vs. depth to facilitate visualization of the approximate extent of the contaminant plume. The summed fluorescence data have been photon-normalized to the incident excitation energy during the push in which they were measured. Selected EEMs from each push location are examined for characteristic patterns and are qualitatively compared to standard EEMs of pure compounds and fuels. Several hardware and software challenges remain unmet and the subject of ongoing research.