product review
Seeing SAW Potential SAW sensors occupy a small market but give other chemical vapor detectors a run for their money. Cheryl M. Harris
S
oon after sarin gas killed a dozen people and injured hundreds in a Tokyo subway in 1995, Hank Wohltjen and colleagues from a small Kentucky company were among a group of researchers called to protect U.S. citizens from similar terrorist attacks. With collaborators from Argonne, Sandia, and Lawrence Livermore National Laboratories, as well as the University of Illinois at Urbana–Champaign, they began a discreet, federally sponsored project to protect subway commuters in Washington, D.C. But then came September 11, 2001 (9-11), and the program quickly turned into a highly publicized effort to secure the railways of the Washington Metropolitan Area Transit Authority (WMATA). Today, Wohltjen’s company, Microsensor Systems, Inc. (MSI), in Bowling Green, Ky., has installed surface acoustic wave (SAW) sensor devices for chemical vapor detection throughout a railway labyrinth that serves ~650,000 people daily. “We’ve been actually involved in Washington since 1997,” says Wohltjen, adding that MSI continues to supply chemical detection technology for WMATA. “It was confidential for many years, but then after 9-11 . . . everything is out in the open.” After having invented SAW sensor technology in the mid-1970s, Wohltjen isn’t alone in the business. BAE Systems, headquartered in Bristol, England, is busily meeting a quota of 270,000 SAW sensor devices for the U.S. Department of Defense. In 1999, the company acquired Femtometrics, which had also invested in SAW sensor
technology. For Craig Curtis of BAE Systems’ Integrated Defense Solutions branch in Austin, Texas, SAW sensor technology— which allows compact size, ruggedness, and high selectivity—offers a lower-cost alternative to portable GC and GC/MS systems that have proliferated in the chemical vapor detection market (1). In the aftermath of this year’s war in Iraq and the 9-11 terrorists hijackings and anthrax attacks in 2001, activity surrounding SAW sensors has increased, say company representatives. “Pre-9-11, this stuff wasn’t really that important because there wasn’t a civilian market for chemical warfare agent detection systems,” says Wohltjen. “But SAW is really poised to be the next generation of sensor technology that’s going into those kinds of markets.” In its first Product Review on SAW sensor systems, Analytical Chemistry examines the allure and future behind this mass-sensitive analytical tool. Table 1 lists examples of devices currently on the market.
The SAW sensor market During the 1980s at the Naval Research Laboratory (NRL), Wohltjen and his col-
leagues, including Arthur Snow and Lynn Jarvis, who were responsible for developing the polymer coatings used in MSI’s detectors, began experimenting on SAW sensor devices as chemical vapor detectors. The devices were—and still are—largely used in radio frequency filtering applications in the wireless communications industry. A range of SAW devices use bounded acoustic waves that propagate on solid–liquid or solid–air interfaces, explain experts. Examples include Rayleigh surface wave devices, thickness shear mode devices, shear horizontal acoustic plate mode devices, and flexural plate wave devices. The majority of SAW sensor devices use Rayleigh surface wave, which is the
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product review
Table 1. Selected saw sensor systems.1 Product
HAZMATCAD
JCAD ChemSentry
Company
Microsensor Systems 62 Corporate Ct. Bowling Green, KY 42103 270-745-0099 www.microsensorsystems.com
BAE Systems Integrated Defense Solution 6500 Tracor Ln. Austin, TX 78725 512-929-4371 www.ids.na.baesystems.com
Price (U.S.D.)
$4580–$7590
$9850 (complete system with detector unit and accessories, volume discounts available)
Size h w d (cm)
20 7 6.5
10.2 17.8 3.8; 11.7 34.3 8.6 with preconcentrator and interface cradle
Weight (kg)
~0.63
~1.0 with rechargeable battery; ~0.8 with non-rechargeable battery
CW agents
Nerve and blister agents; (optionally) AC or CK blood agents (TICs with HAZMATCAD Plus only)
Nerve and blister agents (VX, GA, GB, GD, GF, HD, L); blood agents (HN3, AC, CK)
3 Alarm levels (mg/m )
Nerve and blister agents: 0.2–1.0 (fast mode) and 0.03–0.15 (high-sensitivity mode); blood and choking agents:
