Better Security with Ambient Air SensingDecember 1, 2006 By: John S. Kauer, Barbara Talamo Sensors
Explosives, chemical warfare agents, and toxic industrial chemicals present the greatest threats for soft targets, including stadiums, malls, theme parks, airports, subways, train stations, and high-rise buildings. Most screening today uses a "portal" system that generally includes individual walk-through stations. These systems restrict movement, create traffic-flow bottlenecks, are physically large and costly, and are labor-intensive to operate. By definition, portal sensors are also fixed in place and cannot be deployed in a distributed format for continuous sensing over wide areas such as airport terminals. More complete security screens would survey waiting rooms, bathrooms, and shops, using distributed monitors that could detect hazardous materials carried by people moving through these areas.
Although some hazardous items are not volatile (e.g., viruses and some toxins), many explosives, toxic industrial chemicals, and chemical warfare agents fall into this category. If inexpensive, sensitive, and rapid detectors were deployed at multiple locations, networks of sensors and radionuclide detectors could be set up to monitor and report in identified areas. The addition of cameras would help pinpoint the threats.
The Olfactory Technique
One of the most highly developed devices for detecting volatile airborne compounds is the "vertebrate olfactory system," as exemplified by the use of dogs for detecting vapor signatures. The nose can detect and discriminate among many different odors by using broadly responsive sensors that provide a pattern of responses across an array, rather than relying on individual, odor-specific sensors to capture information. The brain then interprets this response pattern to recognize odors. Dogs trained to sniff out and identify specific chemical substances (mostly explosives and narcotics) are often used in military and civilian settings. Yet dogs have a limited duty cycle, can function only with trained handlers, and cannot be used in the presence of chemical threats.
An inexpensive device that can mimic some features of vertebrate olfactory systems (broad sensitivity, pattern recognition), and provide continuous surveillance would be highly desirable. The challenges of creating a biomimetic olfactory device lie in mimicking the broadly responsive sensor array and in finding sensors similar to the olfactory receptor proteins in the vertebrate nose that would allow the device to identify specific odorants of interest and discriminate between targets and interferents.
Arrays vs. Monospecific Sensors
The broadly responsive sensor array has advantages over systems based on "monospecific" sensors. True monospecific sensors are difficult to produce, whereas broadly responsive sensors are readily made. Even if perfect monospecificity could be achieved, detecting and identifying multiple compounds of interest would require a separate sensor for each one. Conversely, a relatively small array of combinatorial, broadly responsive sensors can discriminate among a large number of different compounds.
Solving the Interferent Problem
The challenge of odor discrimination in vapor phase detection can be overcome in a number of different ways. First, a device can be trained to detect a target analyte under a number of background conditions, extremely beneficial when the environmental baseline has been determined. It was used when field-testing the electronic nose's ability to detect landmines by establishing training patterns that represented a number of different environmental contexts (e.g., bare earth vs. long grass).
Second, because sensors can be selected from large banks of potential detecting materials for certain tests, they can be chosen to have intrinsic sensitivities that exclude responses to the interferents defined for those conditions. For example, in developing sensors to detect toxic industrial chemicals and chemical warfare agents for first responders, many of the defined interferents consisted of hydrocarbons (e.g., diesel, gasoline, and toluene). The sensors we have elected as appropriate for the defined target vapors generated negligible responses to these interferents.
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