Standoff Imaging of Trace RDX Using Quantum Cascade Lasers

We report on a standoff spectroscopic technique for detecting chemical residues on surfaces. An infrared (IR) camera was used in combination with a wavelength tunable mid-IR quantum cascade laser to acquire multispectral image arrays of a surface with explosive residue. IR spectral signatures of residue of cyclotrimethylenetrinitramine (commonly known as RDX) were extracted in the wavelength range 7.9 to 9.7 \mu \text{m} from the multispectral IR images. Detection of RDX was achieved for a lower surface concentration of 5~\mu \text{g} cm^-2 and a higher surface concentration of 20~\mu \text{g} cm^-2 at distances of 0.15 m and 1.5 m, respectively. We performed no background subtraction and the resulting IR spectra of RDX were compared with reference Fourier Transform IR Spectroscopy data that were acquired by placing 20 \mu \text{g} of RDX on a ZnS window. We found that the detection of RDX can be made with negligible interfering contribution of reflected IR photons from the substrate surface.