Smell, electronic nose

In the old days. chemists prided themselves on their ability to identify compounds by odor. Smelling unknown chemicals is a bad idea because some vapors are toxic. Chemists are developing electronic noses to recognize odors to assess the freshness of meat, to find out if fruit is internally bruised, and to detect adulteration of food products.13... 

Fig. 13.2 Two kinds of noses at work, (a) The electronic nose can smell tens of samples during the night, while (b) a human nose fatigues more easily.

One may assume that the AQS is a sort of electronic nose . The human nose reacts immediately to bad smells to protect us from rotten food or harmful gases. However, a nose functions rather differently from the AQS. The nose detects primarily ketenes, aldehydes, mercaptans, mecaptans, not mercaptanes and similar substances, often together with carriers such as methane. The nose can detect some gases in incredibly small concentrations of only a few ppb. The evaluation of these gases then is subject to a very complicated process that is determined by a lot of different factors that make it a very subjective, even culturally dependent, process. So a standard or normative evaluation is not possible. 

Breer H., Sense of smell Signal recognition and transduction in olfactory receptor neurons, in Handbook of Biosensors and Electronic Noses Medicine, Food and Environment, ed. E. Kress-Rogers (Boca Raton, EL CRC Press, 1997, 521-532). 

It has to be remarked that in spite of the widely accepted term electronic nose, current devices are still far from the structure and functions of natural olfaction sense. The unique common feature between artificial and natural system is that both are largely based on arrays of nonselective sensors. The concept underlying electronic nose systems has been demonstrated to be independent on the particular sensor mechanism indeed during the last two decades almost all the available sensor technologies have been utilized as electronic noses. Clearly, all these sensors are very different from the natural receptors. These dissimilarities make the perception of electronic nose very different from that of natural olfaction, so that the instrumental perception of the composition of air cannot be called odor measurement because odor is the sensation of smell as perceived by human olfaction. Nonetheless, the term odor analysis with electronic noses is now largely adopted, but it is important to keep in mind, especially in medical applications, that the electronic nose measurement may be very distant from the human perception. 

An array of sensors is called an electronic nose (e-nose) which is an electronic device associated with the ability to smell (Rock et /., 2008) target gases or volatiles in the atmosphere and therefore its function can be compared with that of the human nose. Electronic noses have been commercially available since the mid-1990s and have been used for the purposes of detection, discrimination and recognition of simple and complex gaseous mixtures (Bourgeois et al., 2003). 

Applications While few applications are available in the scientific literature for the AromaScan and Neotronics instruments, numerous applications of the Alpha M.O.S. system occur in the literature. The breadth of the applications is impressive. The Wall Street Journal (27) reported that the electronic nose was being evaluated for possible applications including new car smell (General Motors), deodorants (Unilever), perfume creation (possible patent implications), wines (Wine Magazine), breath (indication of diabetes), infection of woimds (South Manchester University Hospital), sewage treatment plants, fish freshness (FDA) and numerous others. A few of the food related applications follow (Figures 2-6). Most of these applications are self explanatory so little will be said about them. 

Electronic noses (e-noses) and electronic tongues are devices already used in pharmaceutical formulations, in different areas of food industry (oil, tee, wine, dairy discrimination taste or smells), as well as in medicine. Applications in the field of environmental analysis start to emerge. 

For quality control purposes it is the case at present that human smell panels will be used in preference to instrumental analytical techniques which do not yet adequately mimic the human response. There are practical drawbacks to this approach, leading to a lot of research going into electronic noses utilising a number (e.g., 32) of sensors based on organic conducting polymers. These devices enable fingerprints of satisfactory products to be recorded which are used as references for quality checks. 

For various reasons, reliable electronic-nose applications have been slower to develop. Chapter 13 discusses the benefits of MS as a potential e-nose sensor. This book also discusses the value of time-of-flight MS to the study of flavors and odors. Incorporating the human sense of smell with potent analytical systems is invaluable in problem solving. Just as sample preparation procedures and analytical instrumentation have continued to evolve and improve, so have olfactometry techniques. Chapters 11 and 12 cover various olfactometry techniques, including a new, easier-to-implement method called SNIP. 

In the following, only chemical and biochemical sensors are considered They make use of specific "key-lock" interactions which convert chemical to electronic information Three different tasks are usually fulfilled by chemical sensors, i e the quantitative and selective determination of individual particles (such as molecules or ions in gases or liquids), the determination of gross parameters (such as toxicity), or the quantitative characterization of odors (such as smells monitored qualitatively by the human nose) These requirements can only be achieved with sensor systems which in the most general case contain ten components for analyzing gases or liquids [4]... 

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