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Surface Acoustic Wave Mercury Vapor Sensor

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TTP#: FT0-6-IP01; Tech ID#: 2170
Project Overview
The first objective of this project was to design, fabricate, and demonstrate a prototype instrument for the detection and measurement of gaseous mercury. The instrument was designed for room temperature gas samples; for response times of less than 5 seconds; and for limits of detection of less than 10 µg/dry standard m3 . The sensing element was a chemiresistive film applied to a Surface Acoustic Wave (SAW) resonator that provided a low-power, portable, inexpensive, and accurate way of monitoring vapor-phase elemental mercury. The second objective was to configure the instrument for use as a Continuous Emission Monitor (CEM) accepting sampled and preconditioned gas from a previously developed apparatus at the University of North Dakota’s Energy and Environmental Research Center (UNDEERC).
Technology Description
 The underlying sensing mechanism for gaseous mercury is based upon surface acoustic wave (SAW) technology with a thin gold film used as an ultrasensitive microbalance. By coating the delay line with a thin gold film and configuring it as an RF oscillator, mass changes in the film can be measured by monitoring the oscillation frequency. Because of gold’s strong interaction with mercury (amalgamation), this configuration can be used to sensitively detect gaseous mercury. Mercury can be driven from the film by heating it to a high temperature. Furthermore, by operating the device at an elevated temperature, the reaction kinetics of mercury amalgamation and desorption can be balanced so that an equilibrium surface mass can be quickly realized. The oscillation frequency then becomes a direct measure of the instantaneous mercury concentration. A second SAW delay line without a sensing film is also incorporated into the device to act as a reference so that extraneous environmental effects which perturb both delay lines equally (i.e. temperature fluctuations) can be subtracted out.

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