Issue Date: March 31, 2008
Building A Better Nose
A ROSE BY ANY OTHER NAME may smell as sweet, but so far only a human nose can make that determination. Analytical scientists, though, would like nothing better than an electronic nose that could consistently identify and characterize an alluring fragrance, such as the scent of a musk rose on a midsummer breeze, rather than leave it to the subjectivity and variability of the human nose.
It's the same with fragrances. Gas chromatography can map out the molecules contained in the fragrance of a strawberry, for instance, notes Gabriella M. Petrick, assistant professor of nutrition and food studies at New York University. Scientists can then try to reproduce the fragrance, "but it never comes out the same as the original. There is always a synthetic character to it," she says. At the end of the day, there is nothing like the real thing, and in the final analysis, no instrument can substitute for the human schnoz.
Undaunted, scientists have been sniffing around the subject of electronic noses since the early 1990s. Many with an entrepreneurial bent are looking for commercial opportunities for odor detectors, and to judge by all the activity, the future smells sweet. In a recent article in the journal Chemical Reviews (2008, 108, 705), Udo Weimar and colleagues at Germany's University of T??bingen list more than 25 makers of electronic noses that sense both odorous and odor-free volatile organic compounds. However, few if any odor-analyzing firms can claim to turn a profit.
So far, scientists' efforts have yielded a number of gas analyzers and specialty sensors that allow users to quickly ferret out an off-note in a wine's bouquet, detect offensive odors in cosmetic raw materials, or identify spoiled meat. They are often not classic analytical instruments that provide detailed breakdowns of chemical constituents. Instead, they distinguish simply between the good, the bad, and the just plain ugly.
The human olfactory system has about 350 different sensor proteins that are difficult to copy, explains Peter Boeker, a lecturer at the University of Bonn, in Germany, and founder of the electronic nose maker AltraSens. And even if scientists succeed in copying human olfactory sensors, they would still face the daunting task of making the artificial proboscis as sensitive as the human kind. About 10,000 of each type of sensor are scattered throughout the human olfactory epithelium and connected to the olfactory bulb.
Thanks to Linda B. Buck and Richard Axel, winners of the 2004 Nobel Prize in Physiology or Medicine, scientists are now beginning to understand how humans and other animals sniff the difference between a rose and a rotten egg. Because of their work, scientists realize that every odorant molecule in a given scent activates several different types of receptor proteins. As a result, every scent gives rise to a unique signature of activated cells, allowing the brain to recognize and remember more than 10,000 smells with only 350 different kinds of odorant receptor proteins.
Boeker says he and like-minded scientists are now looking to apply genetic research to the development of biological sensors. But current electronic noses use relatively simple sensors. For instance, AltraSens' OdorVector system uses an array of chemical sensors based on coated oscillating quartz crystals to measure odors emanating from industrial and waste-processing sites. Known as quartz microbalance sensors, they shift frequency as they absorb gas molecules. Six such sensors with different coatings make it possible to get an individual pattern of an odor sample.
In addition, Boeker points out, the sensor can detect volatile organic compounds that the human nose cannot detect. When offensive-odor control is important, the nose knows, but an instrument must learn. OdorVector and similar instruments have to be calibrated by a panel of human sniffers. Financed by the German government and selling instruments mostly to sewage odor control labs, four-year-old AltraSens will develop a large industrial odor measurement market over the next couple of years, Boeker hopes.
Thierry Zesiger, vice president and founder of 12-year-old SMart Nose, based in Marin-Epagnier, Switzerland, says most of his customers today are university researchers, but he expects various industries to adopt the instruments in the next few years for quality control. The quadrupole fingerprint mass spectrometer his firm supplies can screen about 200 samples per day, he says.
IN SOME CASES, scientists use chemical sensors to ferret out odors that humans cannot sense. For instance, Smiths Detection sells a variety of ion mobility spectrometers to government agencies to detect explosives, narcotics, and chemical warfare agents. Other firms are developing devices for sensing odorless gases. Sweden-based AppliedSensor has developed metal oxide, field effect, and quartz microbalance sensors used, for instance, to monitor air quality in cars and to detect hydrogen gas leaks in fuel-cell-powered vehicles.
But the development of instruments that mimic the human nose's ability to identify complex odors is among the greatest challenges. Four-year-old Scensive Technologies of Leeds, England, has developed conductive-polymer-coated silicon chips that might be used to monitor the quality of odors emitted from cosmetics or household chemicals.
The firm calls its instrument the Bloodhound. Managing Director Viv Hallam says that it can also be put to use for security purposes such as identifying people by their distinct odors. Depending on whether they are designed for a single purpose or greater flexibility, the $15,000 to $40,000 devices need to be "trained" by reference to a human panel or known substance. That way they can pick out the better smelling cosmetic or the real McCoy as opposed to an imposter. The Leeds University spin-off isn't making money yet, Hallam says, but he expects it will as customers now using its product quality control services instead buy their own instruments.
Food and beverage makers are increasingly using electronic noses. Marion Bonnefille, a chemical engineer and marketing executive with Toulouse, France-based Alpha MOS, says her firm's Heracles gas chromatography fingerprint identification scanner can be used to rapidly monitor odor quality of omega-3 food additives. The 14-year-old firm makes a number of other instruments, including metal oxide sensors and fingerprint mass spectrometry systems, that can monitor the quality of orange juice or detect off odors in food packaging, Bonnefille says.
Bala Balasanmugam, a research scientist at ingredients supplier International Specialty Products, uses Alpha MOS instruments to control odor quality in skin care polymers, hair spray fixatives, drug excipients, and beer-clarifying polymers. Odor scanners "can't tell you exactly what causes an odor, but once we've calibrated our instrument and know what odors we are looking for, the electronic nose is a good replacement for a human odor panel."
Still, instrumentation science has some way to go before it comes close to copying the full human olfactory experience. Janice Teal, chief scientific officer of cosmetics and fragrance maker Avon, says electronic noses are not tools most companies would use to develop new fragrances. Fragrance makers will use gas chromatography for quality control, but when it comes to assembling the 50 or so ingredients that make up today's perfumes, nothing can substitute for a well-trained human nose.
For scientists, the big prize in odor detection is an instrument as sensitive as the human olfactory system. And although they have succeeded in designing any number of analyzers to do some very challenging and occasionally unpleasant sniffing jobs, it will be some time, AltraSens' Boeker admits, before scientists master the science of smell.
- Chemical & Engineering News
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