Resonance ehhanced Raman spectroscopy for explosives detection. July - December 2009, Clutter measurements

The influence of vapor from clutter substances on the ability to detect explosives vapor by resonance enhanced Raman spectroscopy (RRS) have been investigated. Acetone, ethanol, methanol, diesel fuel, gasoline, kerosene and ethylene glycol were used as clutter substances. Measurements were made on both the clutter samples alone and together with an explosive, where nitromethane was used as a model explosive. Measurements were made in a laboratory setup at a stand-off distance of 1.65 meters and outdoors in a realistic environment at a stand-off distance of 13 meters. The laboratory measurements were performed at room temperature (291-298 K) at a laser wavelength of 232 nm when nitromethane was present, and the outdoor measurements at 220 nm with the samples heated to approximately 328 K. Nitromethane was successfully detected together with every clutter substance in the outdoor measurements though especially kerosene and glycol increased the noise level, probably by "particle scattering". In the laboratory measurements nitromethane was detected together with all the clutter substances except for with gasoline, which obscured the signal from nitromethane, probably by fluorescence. The outdoor measurements were most likely made closer to the resonance wavelength of nitromethane, probably lying around 200 nm, according to the absorption maximum of nitromethane for UV absorption measurements. Measurements closer to the resonance wavelength were probably enabled because of a much lower nitromethane concentration in the outdoor measurements, giving less absorption and self absorption.