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INVESTIGATION OF OPEN-PATH FTIR FOR FAST DEPLOYMENT EMERGENCY RESPONSE TO CHEMICAL THREATS AND ACCIDENTS.
Citation:
KAGANN, R. H., R. HASHMONAY, AND B. HARRIS. INVESTIGATION OF OPEN-PATH FTIR FOR FAST DEPLOYMENT EMERGENCY RESPONSE TO CHEMICAL THREATS AND ACCIDENTS. Presented at A&WMA'S 98th Annual Conference & Exhibition, Minneapolis, MN, June 21 - 24, 2005.
Impact/Purpose:
To inform the public.
Description:
We have performed a series of experiments to determine the tradeoff in detection sensitivity for implementing design features for an Open-Path Fourier Transform Infrared (OP-FTIR) chemical analyzer that would be quick to deploy under emergency response conditions. The fast-deployment concept is to use a small, single ended OP-FTIR sensor that receives its IR signals from ambient or quickly-deployed IR sources. These sources are warm or hot objects that may be found in a terrorist-attack environment, such as the hot surfaces of a motor vehicle or remotely deployed heat sources from a mobile laboratory. The main goal of this study is to determine whether the use of existing ambient sources at relatively low temperatures is feasible, or if the deployment of hotter IR sources would be necessary for the application. Another goal is to determine how the design parameters necessary for smaller and faster responding instruments impact the detection sensitivity. The experiment consisted of measuring the detection limits for sulfur hexaflouride, a chemical warfare agent (CWA) surrogate, and for ethylene, a toxic industrial chemical (TIC) surrogate, over a range of IR source temperatures. Two OP-FTIR systems, one bistatic and the other monostatic, were set up side-by-side and aimed to the bistatic source and monostatic retroreflector array, which were placed at a 76-meter distance. A mixture of the two surrogates was introduced into a partially closed, 6-meter tent that was set up between the sensors and the IR source and retroreflector. The surrogate concentration levels in the tenet were allowed to decay sequentially and slowly so that the detection limits could easily be determined. The monostatic system provides the path integrated concentration determinations at the point when level falls below the bistatic system's detection limit. These tests were performed over a range of source temperatures from 4 to 512 degrees C above the ambient temperature, using two different signal averaging times; 10 seconds and 30 seconds. A 10-second measurement by a bistatic system with an IR source that is only 4 degrees C above ambient can detect sulfur hexafluoride at path-integrated levels as low as 0.61 ppm meter, corresponding to a path-averaged level of 8 ppb. Detection limits can be lowered by a factor of 5 by raising the source temperature by 28 degrees C, from 4 degrees C to 32 degrees C. If a source that is 64 degrees C above ambient is available, the improvement will be by a factor of 13 for a path averaged detection of 630 ppt. As the source temperature increases to levels higher than 64 degrees C, the improvement in the detection sensitivity asymptotically diminishes.