Electronic Design

Saw-Based Electronic Nose Accurately Characterizes Vapors In Seconds

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.

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