Electronic Military & Defense Annual Resource

4th Edition

Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.

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do not typically interfere with Raman measurements, many of the plastic containers and bottles screened at security checkpoints have unique Raman spectra that interfere with identification of the material contained within the bottle. Spatially offset Raman spectroscopy (SORS) was developed to enable measurements through the wall of a container so the potentially hazardous material inside could be identified without the background signal from the container interfering with the Raman spectrum for the sample. SORS is done by measuring Raman at two locations on the surface of the container. The measurements are made within a few millimeters of one another. Subtracting the resulting spectra provides a Raman spectrum for the sample without any background or interference from the container itself. SORS provides the ability to ignore the container while identifying the material contained within. It is a great option for the rapid detection of hazards, like explosives, and other materials hidden in containers during security checks in the field. Where SORS overcomes limitations related to background interference from the container in which the sample is contained, shifted-excitation Raman difference spectroscopy (SERDS) overcomes another limitation associated with Raman spectroscopy — background fluorescence. In SERDS, a sample is measured with laser excitation at two slightly offset wavelengths. Since the fluorescence component of the Raman spectrum is independent of the laser wavelength, it remains consistent upon exposure to the different lasers. The Raman-scattering spectrum, on the other hand, changes with the slight shift in excitation wavelength. The fluorescence background is removed from the Raman spectrum by subtracting the spectra collected from the sample with each of the laser wavelengths. The result is observed in peaks that are resolved from the fluorescence background, leading to more confident sample identification. SERDS provides an alternative to the longer laser wavelengths often used to reduce background fluorescence. With SERDS, fluorescence excited at shorter laser wavelengths is removed so the benefit of higher intensity Raman scattering at shorter laser wavelengths can be achieved. This higher Raman scattering intensity is important for the detection of trace levels of materials. Other Advances In Raman Instrumentation Innovation continues to advance the field of Raman. Even now, as conventional Raman is reaching its pinnacle, advances in Raman instrumentation and innovative sampling techniques continue to propel it forward. Raster orbital scanning (ROS) is a recent advancement to Raman sampling. The ROS sampling technique is accomplished by rastering a tightly focused laser beam over a large sample area — providing higher sensitivity without a loss of the resolution required for confident identification. When the sample is an inhomogeneous sample like a SERS substrate, ROS improves the enhancement achieved with SERS even further by sampling more Raman hotspots with each raster orbital scan. As shown in Figure 3, Raman instruments with ROS offer all the advantages of the tightly focused laser beam used in traditional Raman sampling plus the increased sensitivity achieved by interrogating a larger sample area. ROS also decreases the average laser power to Technology Electronic Military & Defense Annual Resource, 4th Edition 16 Figure 3: ROS sampling acquires Raman signal from a larger sample area while maintaining resolution and minimizing the power buildup of the tightly focused laser. Figure 2: Advanced sampling techniques and SERS substrates make it possible to detect low levels of hazardous materials.

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