Explosive Residue Detection by Laser Surface Photo-fragmentation-Fragment Detection Spectroscopy: II. In Situ and Real-time Monitoring of RDX, HMX, CL20, and TNT, by an Improved Ion Probe

Report No. ARL-TR-3478
Authors: Jerry B. Cabalo and Rosario C. Sausa
Date/Pages: April 2005; 30 pages
Abstract: We measured trace concentrations of hexahydro-1,3,5-hexanitro-1,3,5-triazine [RDX], octohydro-1,3,5,7-octonitro-1,3,5,7- octazocine [HMX], hexanitrohexazaisowurtzitane [CL20], and 2,4,6-trinitrotoluene [TNT] by surface laser photo-fragmentation- fragment detection (SPF-FD) spectroscopy at ambient temperature and pressure. In this technique, a low power, 248-nm laser photo-fragments the target molecule on the surface of a substrate, and a low power, 226-nm laser ionizes the resulting nitricoxide (NO) fragment by resonance-enhanced multi-photon ionization by means of its A-X (0,0) transitions near 226 nm. Two different probes collect the ions: one with miniature, square electrodes mounted vertical to the substrate and the other with similar electrodes mounted horizontal to the substrate. The horizontal electrodes contain 2-mm holes for laser beam passage. We tested both probes on the selected explosives and modeled their electric field in the presence of a substrate with an ion optics simulation program. Our results show that the horizontal probe is about ten times more sensitive than the vertical probe because its electric field is perturbed by the substrate to a lesser extent than is the electric field of the vertical probe. The limits of detection of the various explosives range from 1 to 15 ng/cm2 (S/N=3) at 1 atm and room temperature and depend mostly on the mechanism for NO formation. NO is formed with a room-temperature, Boltzmann, rotational-vibrational, population distribution. We also present an ultraviolet absorption spectrum of CL20 and report its extinction coefficient at 248 nm.
Distribution: Approved for public release
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Last Update / Reviewed: April 1, 2005