Electronic Nose

Introduction

For ages, the human nose has been an important tool in assessing the quality of many products, food products being good examples. While all others parts of production processes, including these of the food industry, were getting more and more automated, there was still no 'objective' means for using the 'subjective' information confined in the smell of products. This changed in 1982, when Persaud and Dodd introduced the concept of an electronic nose. They proposed a system, comprising an array of essentially non-selective sensors and an appropriate pattern recognition system. Much research was done in order to find new and more diverse sensors, and to date there are several companies offering ready-to-use electronic noses.

An "electronic or artificial nose" is an instrument, which comprises a sampling system, an array of chemical gas sensors with differing selectivity, and a computer with an appropriate pattern-classification algorithm, capable of qualitative and/or quantitative analysis of simple or complex gases, vapors, or odors. An "electronic tongue" uses an array of liquid sensors. The artificial "chemical" senses include taste and olfaction.
This writeup is about the electronic nose and specifically about electronic noses based on electrochemical sensors ("electrochemical nose"), the class with which we are most experienced. The entire genus of electronic noses includes those with conductive polymer, polymer composite, quartz microbalance, surface acoustic wave, calorimetric, and other classes of sensors. In this discussion, we will simply refer to an "E-nose" to indicate artificial olfaction, since many modern E-noses are constructed with more than one class of sensor in them. These latter instruments are said to employ "heterogeneous" sensor arrays. Many sources of multi-parameter chemical data including infrared spectrometers, gas chromatographs, and mass spectrometers have been used to identify odors and therefore called E-noses. So, even though the above definition is broad, it may not be broad enough to describe this entire field of technology.

Biological nose

One cannot discuss the "electrochemical nose" without first discussing the human or biological nose and the factors that led to the development of artificial olfaction technology. Of all the five senses, olfaction uses the largest part of the brain and is an essential part of our daily lives. Indeed, the appeal of most flavors is more related to the odor arising from volatiles than to the reaction of the taste buds to dissolved substances. Our olfactory system has evolved not only to enhance taste but also to warn us of dangerous situations. We can easily detect just a few parts per billion of the toxic gas hydrogen sulfide in sewer gas, an ability that can save our life. Olfaction is closely related to the limbic or primitive brain, and odors can elicit basic emotions like love, sadness, or fear. In fact life-saving nauseous revulsion is at least as ancient and universal in the mammalian world as the animal that takes most exquisite advantage of its existence, "Mephitis mephitis", the striped skunk!

How does it work ?

The operation of the device pumps the sample and records the signals from four electrochemical sensors at four inlet operating conditions over about a minute. This produces 16 responses and these are recorded for four substances: methyl isocyanate, hydrogen cyanide, nitrogen dioxide, and carbon monoxide. These signals were recorded at several different concentrations in the working range of the sensors (about 0 to 1000 parts per million). One can tell the amount of the material by the strength of the signals $150; the stronger the responses, the higher the concentration.

E-nose sensors

• Amperometric
• Chemoresistor
• Chemocapacitor
• Optical
• Gravimetric
• Polymer

Applications

• Medical: To detect lung cancers, vaginal infections, TB detection
• Scientific: To detect ammonia gas leakage on space board station.
• Military: To detect land mines
• Industrial: To detect freshness of fish, food quality
• Others

Future trends

The next generation of e-nose points out accuracy and compactness that's why scientists are researching on new version of e-nose, that is zNose.

Its main feature is that it uses only one physical sensor, instead of sensors array.

Advantages of zNose

• The zNose is fast, able to speciate and measure the chemical composition of collected vapors in 10 seconds.
• Easy to learn and use Win-dows® Software.
• The zNose can perform over 300 measurements in an 8 hour shift
• zNose is sensitive, parts per billion and even parts per trillion for certain aromatic compounds
• zNose provides 2-dimensiional high resolution olfactory images based upon product chemistry.
• Principal component analysis, virtual chemical sensors
• New biofeedback tool and documentation for sensory panels

The electronic nose is a prime example of successful application of neural network technology. It uses many of the rudimentary concepts from bio-logical olfaction including the sniffing, chemical detection and odor recognition process. The range of simple gas sensing devices available is already quite diverse.

Source

Invention Intelligence, September - October 2006