Laser photofragmentation and heterogeneous chemiluminescence for nitro-based explosive detection

The purpose of this research was to develop a simple, fast, reliable, sensitive and potentially portable explosive detection device employing laser photofragmentation (PF) followed by heterogeneous chemiluminescence (CL) detection. The PF process involves the release of NOx(x=1,2) moieties from explosive compounds such as TNT, RDX, and PETN through a stepwise excitation-dissociation process using a 193 nm ArF laser. The NOx(x=1,2) produced upon PF is subsequently detected by its CL reaction with basic luminol solution. The intensity of the CL signal was detected by thermoelectrically cooled photomultiplier tube with high quantum efficiency and negligible dark current counts.;The research work was divided into four stages: (1) discerning the PF pathways of nitro-based explosives (2) luminol-CL method development and optimization for improving NO2 sensitivity (3) construction of CrO3 oxidizer for NO to NO2 conversion (4) evaluation of the device in detecting nitro-based explosives in various media including air and soil matrix.;In the first stage of the research, the following PF pathways were verified using classical colorimetric analysis and a fast time resolved absorbance method. R--NO2 + hv → R + NO 2; hv = 193 nm (1) NO2 + hv → NO + O*; hv = 193 nm (2);The second stage involved the development and optimization of NO 2 detection using luminol CL. In this system, a stream of NO2 gas passed through a concentric nebulizer and was used to aspirate a spray of luminol solution. The aspiration process maximized NO2/luminol contact area enhancing the CL signal. The current system was able to improve the detection limit through optimization of all the feasible physical and chemical parameters involved in the CL reaction between luminol and NO 2. Detection limit (LOD) of 19 ppt NO2 at (S/N) = 3 was reported. The optimal reagent solution from the viewpoint of sensitivity of the response to NO2 (maximum signal/signal noise ratio) is 5 x 10-3 M luminol + 0.01 M p-iodophenol + 0.2 M KOH. A Luminol/H2O 2 CL set-up was also explored where a bundle of porous polypropylene fibers was used to bring the NO2 into contact with luminol solution. A LOD of 178 ppt NO2 at (S/N) = 3 was obtained.;In the third stage, a CrO3 oxidizer was constructed for NO to NO2 conversion. Photolysis of nitro compounds produced NO (reaction 2) which is not detectable by luminol-CL system. To further increase the sensitivity of the device for NO2 fragments, NO must be converted back to NO 2.;Finally, the fourth stage was integration of PF and CL units into an explosive detection device via a CrO3 oxidizer. The system was able to detect energetic materials in real time at ambient conditions. Detection limits of 3.4 ppbv for PETN, 1.7 ppbv for RDX, and 34.5 ppbv for TNT were obtained. It was also demonstrated that the presence of PETN residue within the range of 61 to 186 ng/cm2 can be detected at a given signal to background ratio of 10 using a few micro joules of laser energy. The technique also demonstrated its potential for direct analysis of trace explosive in soil. LOD range of 0.5 to 4.3 ppm for PETN was established, an analytical capability comparable to currently available techniques.