Detection of organic explosive residues from outdoor detonations using confocal Raman microscopy

The detection of post-blast residues in the aftermath of an explosion involving organic explosives with spectroscopic techniques is challenging as, typically, no microscopically visible unreacted particles remain after the explosion. However, some low-order explosions may leave visible particles behind, as well as the presence of significant amounts of unreacted material. In this study, four authentic open-air detonations using two simulated improvised explosive devices (IEDs) containing a mixture of military explosives (TNT and RDX), and two IEDs containing smokeless powder were conducted. The various materials they contained, including plastic, wood, and metal, were swabbed and extracted with acetone to create post-blast liquid extracts. The extracts were then dried and examined using confocal Raman microscopy, alongside a 50 ppm reference mixture of smokeless powder constituents, which was created to evaluate the effects of Raman scattering within the full smokeless powder mixture. Smokeless powder constituents, such as ethyl centralite, diphenylamine, nitroglycerin, and dibutyl phthalate, were successfully identified by comparison to the reference mixture on most substrates, with the exception of the paint stick (wood) substrate. TNT/RDX was also able to be identified in the extracts, with RDX crystals being observed in some dried extracts after solvent evaporation. However, the detection of TNT/RDX in the second detonation was unsuccessful, possibly due to an explosive chain reaction that was highly efficient. No trends were seen in substrate affinity for TNT/RDX. The challenges and benefits with the developed methodology for the detection of organic explosive residues from a variety of substrates are discussed in detail.