loaded on inert particles will still emit a vapor signature. Solid agents, such as BZ (3-quinuclidinyl benzilate), will often be aerosolized as an inhalable powder. Many of the techniques for delivering solids are designed to defeat the standard vapor-detection devices used by most military forces. Liquid agents can also leave ground contamination. Agents such as mustard and VX are designed for use in terrain denial; they persist in the environment because of their low volatility. Precursors and by-products can be found in any of these physical forms.

Unfortunately, because the response to direct exposure to CW agents is typically very fast and violent, humans are excellent detectors for the presence of such substances in the immediate environment. It is certainly preferable to be able to detect the presence of CW agents through some other form of interaction. There are numerous other physical mechanisms that can be exploited to produce robust detection, classification, and identification signatures. Although CW agents can appear in many different physical forms—vapors, solids, or liquids, with or without inert co-components—such chemical substances typically have distinctive mass, chemical, and electromagnetic (EM) properties that can be measured.

The electromagnetic interactions, which probe the energy-level structure of the chemical molecules, are particularly useful, leading to various forms of classical spectrometry, which are known to be capable of both excellent sensitivity and specificity. In addition, because of the propagation properties of EM waves, spectroscopic approaches can be successfully applied both locally, for point detection, and remotely, for standoff detection.

The mass and chemical and physical interaction properties of CW agents are also quite distinctive and can permit sensitive detection, classification, and identification. Sensors based on such properties require that physical samples of the suspected agent be placed into intimate contact with the measurement equipment, however, and so they are suitable only as point detectors.

The two popular forms of mass-based CW point detection—ion mobility spectrometry (IMS) and mass spectrometry (MS)—utilize similar principles. The unknown CW agent is collected and ionized in some way, and the ions are allowed to propagate under the influence of an electric field to a collector. Ions with different mobilities arrive at the collector at different times, and the timedependent collection current provides a signature which depends both on the specific ions that are present (i.e., position time of arrival peaks) and the numbers of each (i.e., signal level). The fundamental difference between the two techniques lies in the properties of the propagation medium. IMS uses local ambient air at atmospheric pressure, while MS uses a vacuum. As compared with the MS technique, the atmospheric pressure ion chemistry associated with the IMS technique greatly modifies the distribution of ion fragments or clusters that are generated by the ionization process, as well as the mobilities of the resulting fragments. As a result, the signatures and sensitivities of the two approaches are typically quite different—although signatures unique to individual CW agents can be obtained from each. Typically, the IMS approach is less sensitive, but it can be



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