A procedure to evaluate and optimize the performance of a chemical identification algorithm is presented. The Joint
Contaminated Surface Detector (JCSD) employs Raman spectroscopy to detect and identify surface chemical
contamination. JCSD measurements of chemical warfare agents, simulants, toxic industrial chemicals, interferents and
bare surface backgrounds were made in the laboratory and under realistic field conditions. A test data suite, developed
from these measurements, is used to benchmark algorithm performance throughout the improvement process. In any one
measurement, one of many possible targets can be present along with interferents and surfaces. The detection results are
expressed as a 2-category classification problem so that Receiver Operating Characteristic (ROC) techniques can be
applied. The limitations of applying this framework to chemical detection problems are discussed along with means to
mitigate them. Algorithmic performance is optimized globally using robust Design of Experiments and Taguchi
techniques. These methods require figures of merit to trade off between false alarms and detection probability. Several
figures of merit, including the Matthews Correlation Coefficient and the Taguchi Signal-to-Noise Ratio are compared.
Following the optimization of global parameters which govern the algorithm behavior across all target chemicals, ROC
techniques are employed to optimize chemical-specific parameters to further improve performance.
UV Raman spectroscopy is being applied to the detection of natural and man-made surfaces contaminated with chemical agents. In support of these efforts, we have measured the UV Raman signatures of chemical agents and their simulants. In addition, we have measured both the UV Raman and UV absorption cross sections of these compounds for determining their relative limits of detection. The UV Raman measurements were made using a doubled Argon ion laser operating at 248 nm. Spectra were collected on an echelle spectrograph equipped with a CCD array detector. Based on the data collected, we also discuss the suitability of currently accepted agent simulants for testing UV Raman detection instruments.
Efforts to develop a single solution for detecting hazardous chemicals and biological organisms for both military and civilian communities often produce conflicting requirements. The detection of biological threats, specifically spores, presents us with the most challenging problem. Raman spectroscopy is an excellent method for unique chemical and biological identification. The applicability of Raman spectroscopy to bacterial identification and analysis has been previously demonstrated. Surface-enhanced Raman scattering (SERS) is a well-known method for improving the signal level in Raman scattering. In order to form a uniform noble metal surface architecture, and therefore reproducible surface enhanced spectra, novel fabrication techniques have been developed. Here we report on our recent efforts using silver-shells around latex spheres as a SERS substrate for bacterial endospores.
Surface enhanced Raman spectroscopy (SERS) has been used as a tool to investigate spectral differences of bacterial endospores. Ultimately, this method could be used as a smart and rapid on-site detector for biological warfare agents. However, due to the spectral complexity and the relative size of spores to the substrate features, a rigidly defined substrate is necessary for reproducible characterization. We are investigating many of the reported substrate classes such as: Nano-sphere lithography (NSL), Film over nano-sphere (FONS), nano-shells, electrochemically roughened metals, and dispersed and immobilized colloids. The key aspects of this work include discerning what architectural pattern provides the largest enhancement and reproducibility when sampling the spore coat and whether some method of immobilization, or attraction, of the spores to the surface is necessary. We will present preliminary results of bacterial spore identification as well as a comparison of the substrates studied.
Raman spectroscopy has proven to be a plausible solution to the difficult challenge of on-site detection of biological threats. Adding to the challenge is the fact that many biological species, spores specifically, have relatively low scattering cross sections. The intrinsic need to detect these threats at low concentrations and in the presence of strong background signals necessitates the need for surface enhancement schemes. With an available technique to quickly identify bacterial spores, we hope to find spectral differences between target species in order to incorporate library technologies with the on-site sensor. We are investigating many of the reported substrate classes such as: Nano-sphere lithography (NSL), Film over nano-sphere (FONS), nano-shells, electrochemically roughened metals, and dispersed and immobilized colloids. The key aspects of this work include discerning what architectural features provide the largest enhancement and reproducibility. We will present preliminary results of bacterial spore identification as well as a comparison of the substrates studied.
The threat of biological agents to soldiers and the civilian community was amply demonstrated in the fall of 2001. We are examining the feasibility of using surface-enhanced Raman spectroscopy (SERS) to detect and identify bacteria. In order to use SERS for bacterial detection and identification, it is necessary to determine the most appropriate type of SERS substrate to use. We are examining gold colloids in suspension, immobilized gold colloids, electrochemically roughened gold, periodic particle arrays (PPA), and film over nanosphere substrates (FONS). Briefly, PPA’s are prepared by depositing gold or silver in the interstitial spaces in a close-packed array of polystyrene nanospheres, while FONS are prepared by depositing approximately half a nanosphere diameter of gold or silver on top of a close-packed array of polymer nanospheres. We are evaluating each of these substrate types to determine which will have a high affinity for bacteria, whether we need to modify the surface of the substrate to attract bacteria, and the degree to which each type of substrate enhances the Raman scattering from the bacterial targets. We will present the results of our initial evaluations of substrates and the spectra obtained for several species of bacteria.