Standoff detection of explosive residues on unknown surfaces

Standoff identification of explosive residues may offer early warnings to many hazards plaguing present and future military operations. The greatest challenge is posed by the need for molecular recognition of trace explosive compounds on real-world surfaces. Most techniques that offer eye-safe, long-range detection fail when unknown surfaces with no prior knowledge of the surface spectral properties are interrogated. Inhomogeneity in the surface concentration and optical absorption from background molecules can introduce significant reproducibility challenges for reliable detection when surface residue concentrations are below tens of micrograms per square centimeter. Here we present a coupled standoff technique that allows identification of explosive residues concentrations in the sub microgram per square centimeter range on real-world surfaces. Our technique is a variation of standoff photoacoustic spectroscopy merged with ultraviolet chemical photodecomposition for selective identification of explosives. We demonstrate the detection of standard military grade explosives including RDX, PETN, and TNT along with a couple of common compounds such as diesel and sugar. We obtain identification at several hundred nanograms per centimeter square at a distance of four meters.