Sensors – Detection from afar

A new instrument able to detect chemical residues from a distance overcomes a number of problems that have plagued laser-based detectors of the past, according to Marissa Morales of Oak Ridge National Laboratory’s Measurement Science and Systems Engineering Division. Using a tunable mid-infrared quantum cascade laser and an infrared camera, Morales and colleagues were able to identify as little as 5 micrograms per square centimeter of an explosive residue on a stainless steel surface. The ORNL system avoids safety problems associated with high-power lasers and approaches that require the laser to methodically scan a large area, a slow process. ORNL’s technique also avoids problems of interference in the infrared region from background material.

We have investigated an infrared (IR) microcalorimetric spectroscopy technique that can be used to detect the presence of trace amounts of target molecules. The chemical detection is accomplished by obtaining the IR photothermal spectra of molecules absorbed on the surface of uncooled thermal micromechanical detectors. IR microcalorimetric spectroscopy requires no chemical specific coatings and the chemical specificity of the presented method is a consequence of the wavelength-specific absorption of IR photons from tunable quantum cascade lasers due to vibrational spectral bands of the analyte. We have obtained IR photothermal spectra for trace concentrations of RDX and a monolayer of 2-mercaptoethanol, over the wavelength region from 6 to 10 μm. We found that in this wavelength region both chemicals exhibit a number of photothermal absorption features that are in good agreement with their respective IR spectra.

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