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Figure 1. Picture of battery operated
portable acoustic wave sensor system for monitoring
trace concentrations of VOCs. |
Overview
A variety of portable environmental monitoring systems
have been designed, built, and field tested using surface
acoustic wave (SAW) devices to provide rapid, reversible,
sensitive, and quantitative detection of individual
volatile organic compounds (VOCs). The latest and smallest
system (shown in Figure 1) is housed in a 5.5"x3.3"x1.5"
plastic case (approximately 27 cubic inches) and includes
batteries for portable or field operation. Although
this system consists of a single SAW sensor, work is
under way to design and build a portable instrument
based on a SAW sensor array using selective coatings
on separate sensors to provide simultaneous chemical
identification and quantitation of one or more VOCs.
Applications
PAWS sensing systems have been shown to provide rapid,
reversible, sensitive, and quantitative detection of
individual VOCs. Their stable calibration and ease of
use make them ideal for selected applications. Demonstrated
and potential applications include:
- on-line monitoring of industrial and environmental
remediation processes
- in-situ environmental monitoring (long term post
closure monitoring)
- industrial hygiene
- analysis of VOC contamination in water
- detection of explosives and chemical weapons (counterterrorism,
nonproliferation)
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Figure 2.
Schematic of a surface acoustic wave (SAW) sensor.
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Technical Approach
As shown in Figure 2, SAW devices consist of a piezoelectric
substrate, typically quartz, and two interdigitated
transducers formed by photolithographic patterning of
a thin metal layer. Application of an alternating voltage
to the input transducer generates an alternating strain
field that launches a surface acoustic wave that travels
along the substrate surface before being converted back
into an electrical signal by the output transducer.
The
velocity and attenuation of the propagating wave are
very sensitive to properties, such as mass and
viscoelasticity,
of thin films formed on the device surface. For example,
increases in surface mass loading cause decreases in
wave velocity that can be used to detect picogram (10-12 g) mass changes. By coating the acoustic
path with a material that sorbs a chemical analyte of
interest, this sensitivity can also be used to develop
chemical sensors. Although the SAW device is the heart
of the portable sensor, a variety of supporting circuitry
and sample handling devices have been incorporated into
the hand-held device.
The system includes the following components: (1) One
polyisobutylene-coated quartz SAW for detecting organic
solvents, (2) a SAW drive circuit (U.S.
Patent #5,763,283) that reduces power needs by more
than 80% over previous designs, (3) a data acquisition
and control (DAC) board optimized for lower power operation
and a 4 line display, (4) a small tubular adsorbent
preconcentrator with a resistive wire heater, (5) a
Nafionä membrane for humidity control, (6) a small rotary
vane pump, (7) three latching relays to activate the
pump at both high and low flow rates and to turn on
the preconcentrator heater and the SAW drive electronics,
and (9) batteries. The total power use for the prototype
is about 11 mA-hr @ 3.6 V for one analysis cycle. |
