Saw-Based Electronic Nose Accurately Characterizes Vapors In Seconds

July 24, 2000
Typically, engineers who want to electronically sense and characterize odors expose an array of SAW crystals with different polymer coatings to the target vapor. Yet this method suffers from limited sensitivities in the nanogram range, as well as...

Typically, engineers who want to electronically sense and characterize odors expose an array of SAW crystals with different polymer coatings to the target vapor. Yet this method suffers from limited sensitivities in the nanogram range, as well as from lengthy analysis times. Electronic Sensor Technology of Newbury Park, Calif., though, has resolved these flaws.

The company has developed a SAW detector that samples the analyte's concentration in direct proportion to the detector's frequency. This system, the 7100 zNose, can sense and analyze odors from volatile compounds in the parts-per-billion (ppb) range, along with semivolatile compounds in the parts-per-trillion (ppt) range, within 10 seconds.

The instrument system employs a very fast separation of chemicals in sampled vapors. Through fast-gas chromatography (FGC), it separates odors into individual chemical-vapor pressure spectrum responses and identifies them. Direct-column heating creates these spectra in seconds, while conventional sensing instruments require minutes.

The detector consists of a 500-MHz acoustic interferometer or resonator bonded to a Peltier thermoelectric heat pump that both heats and cools the quartz substrate (Fig. 1). The substrate's temperature is maintained during chromatography. Users can vary detector sensitivity by controlling the substrate's temperature. A two-step process of sample collection and sample analysis assists in sensing and identifying the observed analyte. Each step in the process corresponds to the position of a six-port, two-position rotary valve.

Conventional SAW detectors utilize several polymer coatings. Each coating adsorbs the vapors differently. By comparing response patterns from the array of sensing crystals, the detector can identify the vapors. But polymer coatings reduce the sensitivity of the SAW crystals and limit their detection levels. The collected vapor sample also must be split between many sensing crystals. That reduces their sensitivity as well.

The lack of specificity of polymer coatings means that in general, each coated crystal response overlaps the response of other crystals to some extent. In this case, pattern recognition with overlapping responses is very difficult. Coated crystals also suffer from lengthy analysis times because the analyte needs to diffuse into and out of the coating. The new SAW detector technique, used in the 7100 zNose, eliminates these problems. The portable benchtop electronic instrument simulates a sensor array containing hundreds of orthogonal (nonoverlapping) sensors (Fig. 2).

During sample collection, inlet air containing target-material vapors is pumped through a small section of a capillary tube, which traps and preconcentrates the vapors. At this time, pure helium carrier gas flows through the gas-chromatograph capillary to the SAW detector. The internal supply of helium is enough to perform 300 chromatograms. Sample pumping time is carefully con-trolled to produce a re-peatable and accurate collection of am-bient vapors for analysis.

Next, the ro-tary valve is switched to the analysis position. This causes the helium gas to flow backwards through the trap before passing through the capillary column to the SAW detector. The gas chromatograph column's initial temperature is kept at a nominal 40°C. Immediately afterward, the device passes a 10-ms pulse of high current through the trap, heating and releasing the trapped vapors. These vapors are then swept by the helium gas into the gas-chromatograph column, where they're again trapped and focused by the column's relatively low temperature.

At this point, the column temperature is programmed to follow a linear rise to its maximum temperature. This releases the different chemical species, which then travel through the column. The right sensitivity and stability levels are achieved because no coatings are used on the SAW detector. Coatings tend to reduce the resonator's Q, introduce instability, and require excessive time for equilibrium. The 7100 zNose can typically repeat the same measurements with a variation of no more than 1% to 2%.

An example of the instrument's sensitivity can be seen in the table, which lists the minimum detection levels for 10 common volatile organic compounds found in air and water. In fact, the instrument is sensitive enough to determine safe drinking-water levels by simply smelling the headspace vapors above a water sample.

The 7100 zNose has been validated by the U.S. Environmental Protection Agency (EPA) and the White House Office of National Drug Control Policy (ONDCP). For more details, contact Edward J. Staples or Ken Zeiger at (805) 480-1994. Or, point your browser to www.estcal.com.

About the Author

Roger Allan

Roger Allan is an electronics journalism veteran, and served as Electronic Design's Executive Editor for 15 of those years. He has covered just about every technology beat from semiconductors, components, packaging and power devices, to communications, test and measurement, automotive electronics, robotics, medical electronics, military electronics, robotics, and industrial electronics. His specialties include MEMS and nanoelectronics technologies. He is a contributor to the McGraw Hill Annual Encyclopedia of Science and Technology. He is also a Life Senior Member of the IEEE and holds a BSEE from New York University's School of Engineering and Science. Roger has worked for major electronics magazines besides Electronic Design, including the IEEE Spectrum, Electronics, EDN, Electronic Products, and the British New Scientist. He also has working experience in the electronics industry as a design engineer in filters, power supplies and control systems.

After his retirement from Electronic Design Magazine, He has been extensively contributing articles for Penton’s Electronic Design, Power Electronics Technology, Energy Efficiency and Technology (EE&T) and Microwaves RF Magazine, covering all of the aforementioned electronics segments as well as energy efficiency, harvesting and related technologies. He has also contributed articles to other electronics technology magazines worldwide.

He is a “jack of all trades and a master in leading-edge technologies” like MEMS, nanolectronics, autonomous vehicles, artificial intelligence, military electronics, biometrics, implantable medical devices, and energy harvesting and related technologies.

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