Development of a photofragmentation laser-induced-fluorescence laser sensor for detection of 2,4,6-trinitrotoluene in soil and groundwater

Laser photofragmentation (PF) and subsequent nitric oxide (NO) laser-induce d fluorescence (LIF) have been developed to measure the concentration of en ergetic materials CEM's), such as 2,4,6-trinitrotoluene (TNT), in soil and other media. Gas-phase EMs photodissociate, releasing NO2, when exposed to laser radiation near 226 nm. Laser-excited NO2 predissociates to form NO th at gives an intense fluorescence when excited near 226 ma. The EM concentra tion is inferred from the intensity of the NO fluorescence. A PF-LIF laser- based sensor is being developed to be used with the U.S. Army Corps of Engi neers' Waterways Experiment Station's cone penetrometer to measure in situ the concentration of subsurface TNT. Several factors that affect the PF-LIF signal waveforms, such as sample temperature, laser power, and heating tim e, were investigated. Also, effects on the PF-LIF signal of adding water an d fertilizer to the TNT mixtures were studied. Decay times were determined by least-squares fitting of the exponential PF-LIF signal waveforms. The us e of PF-LIF waveforms promises to enable diagnostics of the sample's charac teristics that would otherwise not be possible in situ. (C) 1999 Optical So ciety of America.