Augustus W. Fountain

Raman Chemical Imaging of Explosive-Contaminated Fingerprints

Raman chemical imaging (RCI) has been used to detect and identify explosives in contaminated fingerprints. Bright-field imaging is used to identify regions of interest within a fingerprint, which can then be examined to determine their chemical composition using RCI and fluorescence imaging. Results are presented where explosives in contaminated fingerprints are identified and their spatial distributions are obtained. Identification of explosives is obtained using Pearsons cosine cross-correlation technique using the characteristic region (500–1850 cm−1) of the spectrum. This study shows the ability to identify explosives nondestructively so that the fingerprint remains intact for further biometric analysis. Prospects for forensic examination of contaminated fingerprints are discussed.

Raman Spectroscopy for Homeland Security Applications

Raman spectroscopy is an analytical technique with vast applications in the homeland security and defense arenas. The Raman effect is defined by the inelastic interaction of the incident laser with the analyte molecule’s vibrational modes, which can be exploited to detect and identify chemicals in various environments and for the detection of hazards in the field, at checkpoints, or in a forensic laboratory with no contact with the substance. A major source of error that overwhelms the Raman signal is fluorescence caused by the background and the sample matrix. Novel methods are being developed to enhance the Raman signal’s sensitivity and to reduce the effects of fluorescence by altering how the hazard material interacts with its environment and the incident laser. Basic Raman techniques applicable to homeland security applications include conventional (off-resonance) Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), resonance Raman spectroscopy, and spatially or temporally offset Raman spectroscopy (SORS and TORS). Additional emerging Raman techniques, including remote Raman detection, Raman imaging, and Heterodyne imaging, are being developed to further enhance the Raman signal, mitigate fluorescence effects, and monitor hazards at a distance for use in homeland security and defense applications.

Bioaerosol Analysis with Raman Chemical Imaging Microspectroscopy

Raman chemical imaging microspectroscopy is evaluated as a technology for waterborne pathogen and bioaerosol detection. Raman imaging produces a three-dimensional data cube consisting of a Raman spectrum at every pixel in a microscope field of view. Binary and ternary mixtures including combinations of polystyrene beads, gram-positive Bacillus anthracis, B. thuringiensis, and B. atrophaeus spores, and B. cereus vegetative cells were investigated by Raman imaging for differentiation and characterization purposes. Bacillus spore aerosol sizes were varied to provide visual proof for corroboration of spectral assignments. Conventional applications of Raman imaging consist of differentiating relatively broad areas of a sample in a microscope field of view. The spectral angle mapping data analysis algorithm was used to compare a library spectrum with experimental spectra from pixels in the microscope field of view. This direct one-to-one matching is straightforward, does not require a training set, is independent of absolute spectral intensity, and only requires univariate statistics. Raman imaging is expanded in its capabilities to differentiate and distinguish between discrete 1-6 microm size bacterial species in single particles, clusters of mixed species, and bioaerosols with interference background particles.

Recent Advances and Remaining Challenges for the Spectroscopic Detection of Explosive Threats

In 2010, the U.S. Army initiated a program through the Edgewood Chemical Biological Center to identify viable spectroscopic signatures of explosives and initiate environmental persistence, fate, and transport studies for trace residues. These studies were ultimately designed to integrate these signatures into algorithms and experimentally evaluate sensor performance for explosives and precursor materials in existing chemical point and standoff detection systems. Accurate and validated optical cross sections and signatures are critical in benchmarking spectroscopic-based sensors. This program has provided important information for the scientists and engineers currently developing trace-detection solutions to the homemade explosive problem. With this information, the sensitivity of spectroscopic methods for explosives detection can now be quantitatively evaluated before the sensor is deployed and tested.

Fate dynamics of environmentally exposed explosive traces.

The chemical and physical fates of trace amounts (<50 μg) of explosives containing 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and pentaerythritol tetranitrate (PETN) were determined for the purpose of informing the capabilities of tactical trace explosive detection systems. From these measurements, it was found that the mass decreases and the chemical composition changes on a time scale of hours, with the loss mechanism due to a combination of sublimation and photodegradation. The rates for these processes were dependent on the explosive composition, as well as on both the ambient temperature and the size distribution of the explosive particulates. From these results, a persistence model was developed and applied to model the time dependence of both the mass and areal coverage of the fingerprints, resulting in a predictive capability for determining fingerprint fate. Chemical analysis confirmed that sublimation rates for TNT were depressed by UV (330-400 nm) exposure due to photochemically driven increases in the molecular weight, whereas the opposite was observed for RDX. No changes were observed for PETN upon exposure to UV radiation, and this was attributed to its low UV absorbance.

Assessing Metal Nanofabricated Substrates for Surface-Enhanced Raman Scattering (SERS) Activity and Reproducibility

Surface-enhanced Raman spectroscopy (SERS) has been shown to be an effective technique for increasing the detection sensitivity in chemical and biological applications. SERS has a distinct advantage over normal Raman spectroscopy, with enhancements typically greater than 104 over the normal Raman signal; however, this advantage in sensitivity comes with a caveat: controlling the spectroscopic reproducibility and enhancement activity of metal nanostructured substrates can be difficult. We present a survey and subsequent data analysis performed on several nanostructured substrates designed for SERS, including silver and gold colloids, silver nanorods, gold nanoshells, and commercially manufactured gold nanostructures.

Critical role of adsorption equilibria on the determination of surface-enhanced Raman enhancement.

Surface-enhanced Raman spectroscopy (SERS) is a useful technique for probing analyte-noble metal interactions and determining thermodynamic properties such as their surface reaction equilibrium constants and binding energies. In this study, we measure the binding equilibrium constants and Gibbs free energy of binding for a series of nitrogen-containing aromatic molecules adsorbed on Klarite substrates. A dual Langmuir dependence of the SERS intensity on concentration was observed for the six species studied, indicating the presence of at least two different binding energies. We relate the measured binding energies to the previously described SERS enhancement value (SEV) and show that the SEV is proportional to the traditional SERS enhancement factor G, with a constant of proportionality that is critically dependent on the adsorption equilibrium constant determined from the dual Langmuir isotherm. We believe the approach described is generally applicable to many SERS substrates, both as a prescriptive approach to determining their relative performance and as a probe of the substrates affinity for a target adsorbate.

Surface-Enhanced Raman Scattering (SERS) Evaluation Protocol for Nanometallic Surfaces

We present the results of a three-year collaboration between the U.S. Army Edgewood Chemical Biological Center and the U.S. Army Research Laboratory-Aldelphi Laboratory Center on the evaluation of selected nanometallic surfaces developed for the Defense Advanced Research Projects Agency Surface-Enhanced Raman Scattering (SERS) Science and Technology Fundamentals program. The primary role of the two Army labs was to develop the analytical and spectroscopic figures of merit to unambiguously compare the sensitivity and reproducibility of various SERS substrates submitted by the program participants. We present the design and implementation of an evaluation protocol for SERS active surfaces enabling an enhancement value calculation from which different substrates can be directly compared. This method was established to: (1) collect physical and spectral characterization data from the small number of substrates (performer supplied) typically encountered, and (2) account for the complex fabrication technique and varying nature of the substrate platforms encountered within this program.

Trace explosive detection in fingerprints with Raman chemical imaging

Wide-field Raman chemical imaging (RCI) has been used to detect and identify the presence of trace explosives in contaminated fingerprints. A background subtraction routine was developed to minimize the Raman spectral features produced by surfaces on which the fingerprint was examined. The Raman image was analyzed with a spectral angle mapping routine to detect and identify the explosives. This study shows the potential capability to identify explosives non-destructively so that the fingerprint remains intact for further biometric analysis.

Review of chemical signature databases

This review assesses the current state of chemical signature databases, the primary characteristics that determine their applicability, characterization of their capability to support spectral identifications, and the target audience to which they are directed. Database file formats, spectrometer operating conditions, and spectral matching tools are found to be primary characteristics that determine the applicability of databases and their ability to support spectral identifications. Chemical signature databases have evolved in two very different directions. One movement offers a single portal for chemical signature determinations by multiple analytical techniques. The other movement is toward highly specialized databases that address narrow scientific disciplines. Both movements are necessary, and serve distinctly different needs in the analytical community.