Sensitivity, MDL, and Response Dynamic Range

The response dynamic range of the typical SAW sensor is very broad and could be up to six orders of magnitude. The sensor reaches its saturation point only when the target chemical in the sample is sufficiently concentrated to occupy most or all active sorption sites available on the polymer-coated surface. 

Once the chemical vapor comes into contact with the SAW sensors, a response is developed within seconds. A complete analysis cycle for a SAW-based detector includes sampling time, desorption time from the preconcentrator, analysis time by the sensors, and recovery time to clear the sorbed chemicals from the polymer. Additional elution time is added when a GC column is incorporated. Analysis time is only a very small portion of the entire cycle. Detection speed depends on sampling time, especially when a preconcentrator or GC column is used for low-concentration samples. While it is possible to bypass the preconcentrator to reduce the analysis cycle time, the probability of false responses by the sensors significantly increases. Recovery time depends on the quantity of chemicals sorbed, strength of the bond between the sorbed molecules and sorption polymer, and the temperature necessary for releasing sorbed chemicals. Recovery time ranges from a few seconds to several minutes. 

The HAZMATCAD has two modes of detection. The Fast mode requires 20 sec per sample cycle. The High Sens mode requires 120 sec per cycle of sample and purging. This mode can be used to monitor low-concentration situations. During 

Testing conducted under the Domestic Preparedness Program has confirmed that the SAW sensor can detect and differentiate the HD and the G nerve agents. Results indicated that sensitivity toward the more volatile GB is lower than that of the less volatile GA. Humidity effects on detection were avoided with the use of a preconcentrator. Temperature affected some of the detection algorithms due to the insufficient size of the heater used. 

The HAZMATCAD technology has further been incorporated in the development of the CW Sentry system that is used for monitoring buildings or subway systems where the unit can operate continuously around the clock 7 days a week. The SAW sensors fit well in this application because it requires very minimal maintenance and practically no consumables except for an occasional dust filter change. 

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