Laser-based detection of TNT and RDX residues in real time under ambient conditions.

We detect thin films of 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-hexanitro-1,3,5-triazine (RDX) by one- and two-laser photofragmentation-fragment detection spectroscopy in real time at ambient temperature and pressure. In the one-laser technique, a laser tuned to 226 nm excites the energetic material and both generates the characteristic NO photofragments and facilitates their detection by resonance-enhanced multiphoton ionization (REMPI) using their A-X (0,0) transitions near 226 nm. In contrast, in the two-laser technique, a 454 nm laser generates the analyte molecule in the gas phase by matrix-assisted desorption, and a second laser tuned to 226 nm both photofragments it and ionizes the resulting NO. We report the effects of laser energy, analyte concentration, and matrix concentration on the ion signal and determine the rotational temperatures of the NO photofragments from Boltzmann, rotational distribution analysis of the REMPI spectra. We achieve limits of detection (S/N = 3) of hundreds of ng/cm(2) for both techniques under ambient conditions with a positive signal identification as low as 70 pg using a single 226 nm laser pulse of approximately 50 microJ.