Sensors - May 1999 - A New Electronic Nose

An innovative electronic nose based on gas
chromatography and a surface acoustic wave
sensor functions as the equivalent of more
than 500 chemical detectors.

A n array of sensors simulating the human olfactory response has become known as an electronic nose [1]. Conventional chemical sensor systems are built around substance-specific detectors that filter out interfering stimuli and indicate simply whether the chemical of interest is present. Electronic noses are designed to quantify and characterize all substances present in the sample. They can provide an output in the form of a recognizable visual image

Figure 1. An electronic nose with only a few sensors produces overlapping responses as more than one sensor responds to the same vapor. It is therefore nearly impossible to calibrate this type of nose with test vapors containing several compounds. The ideal nose produces peaks that do not overlap, permitting compound identification and quantification

of specific vapor mixtures that can consist of hundreds of different chemical species. The sensors constituting the array are selected for their chemical affinities and are typically based on chemisorbing polymer films. Many such sensors can be used, and a serial polling of each sensor reading creates a histogram of outputs (see Figure 1). In the ideal array response there is no overlap of sensor outputs; each output corresponds to only one analyte or chemical compound.

The GC/SAW Electronic Nose
Operating Principle. The gas chromatography/surface acoustic wave (GC/SAW) electronic nose system [2] is based on fast gas chromatography. Gas chromatography encompasses various techniques, all of which entail separating the components of a mixture by preferential adsorption in an ascending molecular-weight sequence onto a solid adsorbent material applied as a coating to the interior of the chromatography column [3]. Each gas is identified by its unique retention time, or the time at which the center of a symmetrical peak appears on the chromatogram.

Conventional electronic noses incorporate sensor arrays with a different adsorbent coating on each sensor. In the technique presented here, the gases are first separated in a small capillary loop trap filled with Tenax, an adsorbing compound that captures condensable vapors. The gases next pass through the chromatography column and then to a single, uncoated SAW sensor for analysis.

System Components. The system consists of a six-port, two-position valve; the loop trap; a sampling pump for pulling vapors into the loop trap; a source of clean helium for use as a carrier gas; the GC column, which is a short section of glass or metal capillary tubing ~0.25 mm in diameter; and the temperature-controlled SAW vapor sensor. A sample cycle is followed by an injection and analysis cycle, corresponding to the two positions of the six-port valve.

Figure 2. When the two-position valve is placed in the sample position, a pump draws inlet air and vapors through a cooled trap where the vapors are absorbed and collected. By controlling the sampling time, the system can evaluate both low and high concentrations. During sampling, pure helium carrier gas, the only expendable, is allowed to flow through a GC column and onto a cooled SAW detector crystal. This allows the sensor to stabilize and establishes a zero signal baseline.

Sample Cycle. The test sample can be drawn from ambient air, desorbed vapor samples, or headspace vapors from liquid or water samples. During the sample cycle (see Figure 2), a sample is pulled through the loop trap, where the Tenax adsorbs and concentrates the vapors of interest. By controlling the sample time, vapor concentrations over a wide range can be collected. Sampling times of 510 s typically produce parts per billion (ppb) sensitivity for most volatile organic compounds; concentrations in the high parts per million (ppm) range, e.g., automobile exhaust, can be sampled in 12 s.

Injection and Analysis Cycle. The injection and analysis cycle is initiated by moving the valve controlling the direction of helium flow (see Figure 3). Rapid heating of the loop

Figure 3. After a suitable sample time the GC valve is rotated to the inject position, allowing the helium to flow back through the trap and into the column. A short heat pulse applied to the trap causes the collected vapors to evaporate into the helium carrier gas in a 10 ms burst. As the vapor impulse travels through the GC column, individual compounds are slowed by differing amounts by the coating inside the column. Compounds exiting the column are detected by the SAW sensor and their characteristic retention time is recorded. Identification is accomplished by comparing the retention time with known standards.

trap releases the adsorbed vapors, which are carried by the helium in a single 10 ms burst into the GC column. The internal surfaces of the column are coated with a compound whose specific chemical properties enable the individual analytes to spatially separate as they pass through the column and exit it at different times. The column temperature is closely controlled to optimize the separation process. The solubility of specific analytes in this bonded phase determines their retention time. Retention times are typically 10 s, and the retention time of each analyte is measured to within 20 ms. In effect, the GC/SAW nose produces responses equivalent to those of a 500-element sensor array.

SAW Sensor. The surface acoustic wave sensor performs quantitative and qualitative analyses of each vapor constituent as it exits the GC column. This sensor is an uncoated, high-Q piezoelectric quartz crystal with a natural resonating frequency of 500 MHz. Its surface temperature is controlled by a small thermoelectric element that cools the surface to promote vapor condensation and then heats it for cleaning between analyses.

The added mass of an analyte condensing on the crystal's surface lowers the vibrational frequency in

The GC/SAW is the first electronic nose to receive validation from the U.S. EPA for monitoring volatile organic compounds in water and PCBs in soil. The White House Office of National Drug Control Policy has validated it for detecting vapors associated with illegal drugs such as cocaine, heroin, or methamphetamines. For U.S. government and law enforcement efforts involving drug interdiction, the nose can now be purchased under Section 1122 from the Government Services Administration (GSA).

direct proportion to the amount of condensate. This frequency is mixed with a reference frequency, and the intermediate frequency (typically 100 kHz) is counted by a microprocessor.

The specificity of the uncoated SAW sensor is based on its ability to detect the vapor pressure of the condensate. Vapor pressure is a function of both the substance itself and its temperature. Hence, the surface temperature of the crystal must be tightly controlled. Analytes with vapor pressures close to the crystal temperature will remain on the crystal surface longer and be more easily detected than those with vapor pressures well above that temperature. This property provides a general method for separating volatile from nonvolatile vapors. Volatile organic vapor residence times are <1 s with crystal temperatures in the 0ºC-40ºC range. Parts per billion sensitivity is possible for volatile organics; parts per trillion for semivolatile compounds. (These values are 1000 × those reported from investigations into polymer-coated SAW crystals.)

Because the SAW sensor measures the integral of the chromatogram peaks, it is called an integrating detector. The lower trace in Figure 4 displays the frequency histogram; the upper trace plots the derivative of frequency (column flux) and produces the familiar peaks of chromatography.

Figure 4. The SAW sensor is the first integrating GC detector. Frequency changes (lower trace) are used to quantify the amount of vapor detected; the derivative of frequency (upper trace) measures the vapor flux and is used to determine retention time. A VaporPrint, a unique fingerprint image of the vapor, is created by mapping the frequency and flux chromatograms to a radial coordinate system.

Also shown are VaporPrints images [4] of both the detector frequency and derivative of frequency. These images are formed by transforming the time variable to a radial angle with the beginning and end of the analysis occurring at 0º.

VaporPrint Images and
Pattern Recognition
A polar plot of chromatogram time in which the radial direction represents the sensor signal or its derivative is well suited to electronic nose pattern-recognition algorithms [5,6]. The SAW crystal is the only integrating GC detector; all others detect the flux of column flow. The derivative of the detector output is used only to determine retention time. The amount of analyte detected is determined by sensor frequency.

Figure 5. Vapor analysis can be illustrated using mushrooms as an example. A 5 s sample of mushroom vapors is decomposed into a series of compound peaks, mostly sulfurous, and are displayed as either VaporPrint images or full chromatograms. The compounds' retention times are used to define a reduced set of sensor signals in software. In this way, software can define simple sensor arrays and set alarm levels relevant to the object being analyzed.

The process of vapor identification and recognition for the GC/SAW nose is shown in Figure 5 using mushroom vapors as an example. After 5 s of sampling the vapor is analyzed to form VaporPrint images and chemical chromatograms, and to post the status of selected sensor alarms within 10 s. The system's ability to form a sensor array with alarms enables it to monitor only those analytes of interest, e.g., illegal drugs or explosives. For the mushroom example we select five sulfur compounds as the five sensor vectors we wish to monitor. A simple array of sensor meters is then displayed, representing a

Figure 6. VaporPrint images provide a means of fingerprinting unique odors associated with common substances. For example, automobile exhaust always forms a well-defined image representing a mixture of benzene, toluene, ethylbenzene, xylene, and trimethylbenzene. In a similar manner, food, beverages, bacteria, drugs, and even money form easily recognized Vapor-Print images that can be read by a trained operator.

"mushroom nose." Only a software click is needed to load another set of sensor alarm settings and form a different nose for, say, garlic.

Several electronic pattern-recognition algorithms based on sliding sets of correlations that use known compound patterns associated with complex fragrances were evaluated in an effort to identify the best one. Thus far, nothing approaching the performance of a human interpreter has been found. Although human operators must be trained to recognize VaporPrint patterns, they excel when properly taught. The

Figure 7. Perfume VaporPrint images can be used to distinguish among various perfumes. Many, such as Chanel No. 5, can be seen to consist of a single chemical compound; others, such as Guerlain's Shalimar, are much more complex and contain a well-defined mixture of compounds.

images became richer with volatile compounds such as those in gasoline and auto exhaust (see Figure 6) and perfumes (see Figure 7). The sharp peak at ~1.5 s (1 o'clock) in some of the perfume images indicates the presence of alcohol. (Inexpensive perfumes have a significant alcohol content.) The VaporPrint images illustrate the way that mixing different base odors can create perfumes. Givenchy, for example, appears to be a mixture of Joy and Chanel No. 5.

Commercial Availability

Photo 1. The GC/SAW nose is commercially available in two different configurations. The handheld system is designed for use in the field. The portable benchtop system is intended for applications such as water quality measurements as well as serving as an adjunct to complex laboratory instrumentation.

The nose is currently available in two configurations (see Photo 1). Intended for field as well as lab operation, the system includes a Pentium computer with preinstalled Office98 and PCAnywhere software for remote operation. Options also include a low-cost GPS receiver for accurately recording the location of each measurement. A benchtop GC/SAW system designed for laboratory use or detecting water contamination is also available.

Summary
A commercially available electronic nose provides a recognizable visual image of specific vapor mixtures containing hundreds of different chemical species. The nose is fast, operates over a wide range of vapor concentrations, has picogram sensitivity, and is simple to use and calibrate. The analyses are displayed as VaporPrints that are derived from an integrating, solid-state SAW sensor. After a period of training, the sensor operator can readily identify and interpret the displayed data.

References
1. H.T. Nagle et al. Sept. 1998. "The How and Why of Electronic Noses," IEEE Spectrum :22-33.

2. E.J. Staples. Oct. 1998. "Dioxin/Furan Detection and Analysis Using a SAW-Based Electronic Nose," Proc 1998 IEEE International Ultrasonics Symposium, Sendai, Japan.

4. E.J. Staples. "Method and Apparatus for Analyzing Vapor Elements," U.S. Patent Pending.

5. P. Keller et al. 1012 May 1994. "Three Neural Network Based Sensor Systems for Environmental Monitoring," Proc IEEE Electro94 Conference, Boston, MA.

6. J. Gardner and P. Bartlett. Nov. 1998. Electronic Noses: Principles and Applications , Oxford University Press.