Surface Acoustic Wave (SAW) Sensor Technology for Threat Detection
Surface Acoustic Wave (SAW) sensors demonstrate superior selectivity for the detection of nerve and blister agents. Due to their solid state design and fabrication, SAW chemical sensors are extremely reliable. This paper describes the technology behind SAW and how this technology can be used for the detection of chemical warfare agents.
SAW Technology
SAW sensors are piezoelectric crystals that detect the mass of chemical vapors absorbed into chemically selective coatings on the sensor surface. This absorption causes a change in the resonant frequency of the sensor. The internal microcomputer measures these changes and uses them to determine the presence and concentration of chemical agents. The SAW sensor coatings have unique physical properties which allow reversible adsorption of chemical vapors.
The polymer films are normally chosen so that each will have a different chemical affinity for a variety of organic chemical classes such as hydrocarbon, alcohol, ketone, oxygenated, chlorinated, and nitrogenated. The selectivity of a polymer coating to a specific chemical vapor is determined by the type of molecular interaction between them. If the polymer films are properly chosen, then each chemical vapor of interest will have a unique overall effect on the set of devices. The SAW sensor coatings have unique physical properties which allow a reversible adsorption of chemical vapors. Therefore, the sensor will recover from exposure to the gas of interest.
To make a SAW instrument which is sensitive to nerve or blister agents, an array of SAW devices is used with each device having a different polymer film. Each film is chosen to have chemical absorption characteristics different from the others. Therefore, a SAW instrument designed for detection of nerve agents will have reduced cross- sensitivity to other compounds in the background stream. The array is housed within a package that has a pathway for vapors to enter the array, pass over all the SAW devices and then exit the device. The sensitivity of the system can be enhanced for low vapor concentrations by using a chemical concentrator before the array.
SAW Performance
SAW detector performance varies from manufacturer to manufacturer; therefore, the performance information below is only a guideline for expected performance and is not a specification.
Detection Limit
The sensitivity to nerve agents or blister agents varies from agent to agent; however, nerve agents can usually be detected below 1 mg/m3 where as the detection limits for blister agents are approximately 1 to 2 mg/m3.
Speed of Response
The time for the sensor to respond to mass changes in the selective polymer coating is typically less than a millisecond. However, in typical vapor-sensing applications it is more likely that the response time will be determined by the time required for the vapors to be transported to the polymer coating surface and for equilibrium to be established. Depending upon the way the gas is sampled, a true response time can range from 20 to 120 seconds.