Listen hard and listen good if you want to name that smell
IT DOES the job of a nose, works like an ear and displays what it finds as a picture. And in just 10 seconds, the world's fastest electronic sniffer can identify hundreds of scents in a mix, and tell you how much of each is there.
Dubbed the zNose, the new sensor is the first electronic nose accurate enough to be approved by the US Environmental Protection Agency (EPA), the national pollution watchdog. Its manufacturer, Electronic Sensor Technology of Newbury Park, California, says it can be used to sniff out pollutants in everything from plastics and perfumes to foodstuffs.
To sniff out the chemicals in a sample, the zNose uses fast chromatography, which separates them according to their chemical and physical properties. Different chemicals emerge from the chromatography unit at different times.
The key to the zNose's sensitivity is a device called a surface acoustic wave detector, which detects these chemicals, and measures their concentration. It is based on a piezoelectric crystal with a complex electrode at one end that generates 500-megahertz ultrasound waves on the crystal's surface. Another electrode at the far end detects the waves that have passed across the surface. Chemicals that emerge from the chromatography unit are momentarily adsorbed on the surface of the detector crystal. This causes small but significant changes in the tone arriving at the detector electrode, which indicates the amount of chemical present.
This information is then packaged by software into a distinctive, easily recognisable odour profile called a vapour print (see Diagram, showing the vapour prints of various canned soups). The radial amplitudes measure fragrance intensity.
The acoustic detector allows the zNose to pick up chemicals at concentrations as low as a few parts per billion, the Acoustical Society of America was told earlier this month at its meeting in Newport Beach, California. It can be calibrated to EPA standards, and because it uses solid-state technology, it will stay calibrated for months. Rival e-noses have to be recalibrated after only a few hours, the manufacturer says.
Ken Zeiger, who oversees production of the zNose, says it has already proved its worth among Californian winemakers, sniffing out trichloroanisole, a chemical given off by mouldy corks that makes wine taste musty. "TCA doesn't come up often, but when it does it's a big problem," says David Sheppard, assistant winemaker at Inniskillin Wines in Niagara-on-the-Lake, Ontario.
Food producers often employ "sniffing panels" of seven or eight people who try to detect anything amiss in their product. The sensor could to do this job in a far more consistent way, and could spot odours too faint for humans to detect. "A panel can't tell you a quantity," says Zeiger.
Author: Alison Motluk
New Scientist issue: 23/30 December 2000
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