Measurement of Raman and native fluorescence emission using ultraviolet (UV) sources (<400 nm) on targeted materials is suitable for both sensitive detection and accurate identification of explosive materials. When the UV emission data are analyzed using a combination of Principal Component Analysis (PCA) and cluster analysis, chemicals and biological samples can be differentiated based on the geometric arrangement of molecules, the number of repeating aromatic rings, associated functional groups (nitrogen, sulfur, hydroxyl, and methyl), microbial life cycles (spores vs. vegetative cells), and the number of conjugated bonds. Explosive materials can be separated from one another as well as from a range of possible background materials, which includes microbes, car doors, motor oil, and fingerprints on car doors, etc. Many explosives are comprised of similar atomic constituents found in potential background samples such as fingerprint oils/skin, motor oil, and soil. This technique is sensitive to chemical bonds between the elements that lead to the discriminating separability between backgrounds and explosive materials.
The unique combination of the wavelength, optics, mechanical configurations, and chemometrics enables standoff (1 to 5 m) identification of trace amounts of explosive materials with rapid spatial scanning capability. Each data point, which can include both the native fluorescence and Raman signals, is automatically identified in <100 μs by the real-time analysis engine. The rapid acquisition and real-time analysis allows a user to scan the instrument over a large region such that the probability of false negatives resulting from a heterogeneous distribution of explosive material on a surface is dramatically reduced.
The hand-held or robot-mounted instrument has been tested using a number of experimental conditions. In one example, a car panel doped with RDX (an explosive nitroamine) was placed 1 m away from the instrument. The car panel segment was rotated as the instrument collected data to mimic scanning from a fixed distance. The composite traces from the detectors are used by the analysis to show a relatedness index (high values indicate a high match) of each data point as a function of time and spatial position. This sample was part of a blind test to determine whether it was possible to identify RDX on the car panel in the presence of Arizona dust (a standardized interferrant sample). As the sample is scanned, the RDX is found only in specific areas. In the other related experiments where the RDX samples were not so heterogeneous, the RDX relatedness index was consistently high. In other tests, a limit of detection less than 100 ng/cm2 was demonstrated at a standoff distance of 1 meter using a 38 mm diameter collection aperture. Using a modestly larger aperture or at higher concentration amounts, the instrument can detect and identify explosives at longer standoff distances.
The developed sensor has an excitation wavelength of 248 nm from a transversely excited hollow cathode (TEHC) laser. An alternate excitation wavelength of interest is 224 nm, also from the TEHC laser. Although the optimum excitation wavelength is less than 250 nm at present, there is also an expectation that longer wavelengths up to about 400 nm may also be relevant for some applications.
This work was done by William Hug and Ray Reid of Photon Systems, Inc., and Rohit Bhartia and Arthur Lane of Caltech for NASA’s Jet Propulsion Laboratory under an Army Phase I STTR contract (No. W81XWH-06-C-0395 CM: Dr. Gary Gilbert) with Photon Systems.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-45166, volume and number of this NASA Tech Briefs issue, and the page number.