Seminar on Electronic Nose

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    Chapter 1

    Introduction

    The electronics field is developing at a fast rate. Each day the industry is coming with new

    technology and products. The electronic components play a major role in all fields of life. The

    scientists had started to mimic the biological world. Until now online communication involved

    only two of our senses, sense of hearing and sense of sight. Soon it will involve the third, the sense

    of smell. A new technology is being developed to appeal to our sense of smell. Bringing alive our

    experience, technology now targets on the sense of smell. The development of artificial neural

    network (ANN), in which the nervous system is electronically implemented in one among them.The scientists realized the importance of the detection and identification of odour in many fields. In

    human body it is achieved with the help of one of the sense organ, the nose. So scientists realized

    the need of imitating the human nose. The concept of the electronic nose appeared for the first time

    in a nature paper by Persuade and Dodd (1982). The authors suggested and demonstrated with a

    few examples that gas sensor array responses could be analyzed with artificial neural networks

    thereby increasing sensitivity and precision in analysis significantly. This first publication was

    followed by several methodological papers evaluating different sensor types and combinations. The

    scientists saw the last advances in the electronic means of seeing and hearing. Witnessing this fast

    advances they sent a marker for systems mimicking the human nose. The harnessing of electronics

    to measure odour is greatly desired.

    Using Electronic-nose we can sense a smell and with a technology called Digital scent technology

    it is possible to sense, transmit and receive smell through internet, like smelling a perfume online

    before buying them, sent scented E-cards through scent enabled websites, and to experience the

    burning smell of rubber in your favorite TV games etc.

    If this technology gains mass appeal no one can stop it from entering into virtual world. Just

    imagine you are able to smell things using a device connected to your computer.

    With Digital scent technology this can be made a reality. There is complete software and hardware

    solution for scenting digital media and user.

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    The E nose offers objectivity and reproducibility. The electronic nose technology goes several

    steps ahead of the conventional gas sensors. The electronics nose system detects and sensing

    devices with pattern recognition sub system. The electronic nose won quickly considerable interest

    in food analysis for rapid and reliable quality classification in manufacturing testing. Later, the

    electronic noses have also been applied to classification of micro organisms and bio-reactor

    monitoring. Even though the electronic nose resembles its biological counter part nose too closely

    the label electronic nose or E-nose has been widely accepted around the world.

    An electronic nose is an instrument which comprises an array of electronic chemical sensors with

    partial specificity and an appropriate pattern recognition system capable of recognizing simple or

    complex odour. It can be regarded as a modular system comprising a set of active materials which

    detect the odour, associated sensors which transduce the chemical quantity into electrical signals,

    followed by appropriate signal conditioning and processing to classify known odours or identify

    unknown odours.

    The "electronic nose" is a relatively new tool that may be used for safety, quality, or process

    monitoring, accomplishing in a few minutes procedures that may presently require days to

    complete. Therefore the main advantage of this instrument is that in a matter of seconds, it delivers

    objective, reproducible aroma discrimination with sensitivity comparable to the human nose for

    most applications. The term "electronic nose" was first used in a jocular sense with sensor arrays in

    the 1980's. As the technology developed, it became apparent that the animal and human olfactory

    systems operate on the same principle: A relatively small number of non- selective receptors allow

    the discrimination of thousands of different odours.

    1.1) HISTORY:

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    It is difficult to pin point the exact date of "when and how" the idea of designing a system, which

    can mimic the human nose, came about. However, the following dates with devices give a better

    understanding of how the design progressed for a machine olfaction devices (MOD) system. The

    MOD design led eventually for the conceptualization of the eNose.

    Please note that an eNose differ from other types of MOD by simply having multiples sensors,

    while other devices may have one sensor only or simply the mechanism itself differ substantially

    from the eNose basic working principles.

    The name MOD, therefore, cover devices such as eNoses i.e. devices with multiple sensors, as well

    as devices with single sensors - or those devices which operate on a different design principles.

    The four following dates are important in the history and development of the eNose:

    1. The making of the first gas sensor, Hartman 1954

    2. Constructing array of 6 termistors, Moncrief 1961

    3. First Electronic Nose, Persaud and Dodd, 1982

    4. Ikegami (Hitachi Research Laboratory, J) array for odour quality - 1985

    Therefore, the first recorded scientific attempt to use sensor arrays to emulate and understand

    mammalian olfaction was carried out by Persaud and Dodd in 1982, at the University of

    Manchester Institute of Science and Technology.

    A device was built with an array of three metal-oxide gas sensors used to discriminate among

    twenty odorous substances. Using visual comparison for the ratios of the sensor responses, they

    obtained the pattern classification.

    The name itself "Electronic Nose" used for the first time during 1988 and has come into common

    usage"as a generic term for an array of chemical gas sensors incorporated into an artificial olfaction

    device" after the introduction of this title at a conference covering this field in Iceland 1991. From

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    that point, the idea and the principles of the eNose has grown and developed into different fields

    across the globe.

    Historically speaking, there are two different types of eNoses (Pearce 1997):

    1. Static odour delivery.

    2. Mass-flow systems.

    As the two names suggest, the basic mechanism for the first type is that there is no odour flow but

    simply a flask contains the sensors array with a fan at the top to distribute the flow within the flask.

    This type was the design of the first eNose in 1982.

    The second type which is very popular now is where the odour flows within the system. MosteNoses designs are made in this way.

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    Chapter 2

    BIOLOGICAL NOSE

    To attempt to mimic the human apparatus, researchers have identified distinct steps that

    characterize the way humans smell. It all begins with sniffing, which moves air samples that

    contain molecules of odours past curved bony structures called turbinate. The turbinate create

    turbulent airflow patterns that carry the mixture of volatile compounds to that thin mucus coating

    of the noses olfactory epithelium, where ends if the nerve cells that sense odourants.

    The volatile organic compounds (VOCs) basic to odours reach the olfactory epithelium in gaseous

    form or else as a coating on the particles that fill the air we breathe. Particles reach the olfactory

    epithelium not only from the nostrils but also from the mouth when food is chewed. As VOCs and

    particles carrying VOCs pass over the mucus membrane lining the nose, they are trapped by the

    mucus and diffuse through to the next layer, namely, the epithelium, where the sensory cells lie in

    wait. The cells are covered in multiple cilia- hair like structures with receptors located on the cells

    outer membranes. Olfactory cells are specialized neurons that are replicated approximately every

    30 days.

    The transformation of a molecule into an odour begins when this odourant molecule, as it is called,

    binds to a receptor protein. The event initiates a cascade of enzymatic reactions that result in

    depolarization of the cells membrane. (Ion pumps within the cells membrane keep the cell

    polarized in its rest, or steady state, with a typical rest potential of about 90 mV across the

    membrane). There are more than 100 million protein receptors in all and perhaps 1000 types. For

    example, one receptor type is sensitive to a small subset of odourants, one of which is the organic

    compound octanal. The sensory cells in the epithelium respond by transmitting signals along neural

    wires called axons. Such an axon first traverses a small hole in a bony structure in the base of the

    skull, known as the cribriform plate. Then the rest of the neuron wends its way to the brains

    olfactory bulb, where it terminates in a cluster of neural networks called glomeruli.

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    The 2000 or so glomeruli of the olfactory bulb represent the first tier of central odour information

    processing. All sensory neurons containing a specific odour an receptor are thought to converge on

    two or three glomeruli in the olfactory bulb.

    Note that olfactory sensory neurons in the epithelium can each respond to nose than one odourant.

    It is therefore the pattern of response across multiple glomeruli that codes olfactory quality.

    Olfactory information ultimately arrives higher up in the brain, first at the hypothalamus, which

    also processes neural signals related to food intake, and then at still higher processing centres. The

    use of noninvasive techniques to study the brain suggests that different chemical stimuli activate

    different brain regions to different degrees.

    As the new electronic technology emerges, conventional approaches to measuring odour are

    challenged. As noted earlier, current methods generally involve either the use of human odour

    panel to quantify and characterize the odour or gas chromatography and mass spectrometry to

    precisely identify the odourants producing it. The concentration of an odour may be expressed as a

    multiple of either its detection on its recognition threshold. The recognition threshold is defined by

    the American Society for Testing and Materials (ASTM) as the lowest concentration at which an

    odour is first detected recognition is no necessary by 50% of human sniffing it. The detection

    threshold is considered the absolute threshold of sensation for an odour. The odour concentration at

    this threshold is defined to be 1.0 odour unit / m3. The value is established by averaging the

    responses over a population of individuals. Panels of trained human sniffers are the gold standard

    of odour measurement. The recognition threshold is defined by ASTM as the lowest concentration

    at which an odour is first identified by 50% of the population sniffing

    an odourant. In this case, the recognition threshold is often 5-10 odour units or 5-10 times as high

    as the detection threshold.

    Gas chromatography and mass spectrometry have also been used to identify the chemical

    constituents of an odourous mixture. Air samples are collected in special canisters or bags and

    taken to the laboratory for analysis afterward. The odourant may be concentrated in the field or

    laboratory by using a vapor trap consisting of an absorbent maternal or cryogenic device.

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    In either case, a measured volume of the sample is forced through the trap where odourant

    molecule are removed from the gas sample and collected on the absorbent material or cryogenic

    surface. Heating the trap releases the concentrated molecules rapidly into the gas chromatography.

    Fig.1: Major sensing components in humans

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    Chapter 3

    ELECTRONIC

    NOSE

    Electronic Nose is a smart instrument that is designed to detect and discriminate among complex

    odours using an array of sensors. The array of sensors consists of a number of broadly tuned (non-

    specific) sensors that are treated with a variety of odour-sensitive biological or chemical materials.

    An odour stimulus generates a characteristic fingerprint from this array of sensors. Patterns or

    fingerprints from known odours are used to construct a database and train a pattern recognition

    system so that unknown odours can Neural Network based Soft Computing Techniques are used to

    tune near accurate co-relation smell print of multi-sensor array with that of Tea Tasters scores.

    The software framework has been designed with adequate flexibility and openness so that tea

    planters themselves may train the system with their own system of scoring so that the instrument

    will, then on, reliably predict such smell print scores. subsequently be classified and/or identified.

    Electronic nose is a device that identifies the specific Components of an odour and

    analyzes its chemical makeup to identify it.

    An electronic nose consists of mechanism for identification of chemical detection such as

    an array of electronic sensors and a mechanism for pattern recognition.

    An electronic nose is such an array of non-specific chemical sensors, controlled and analyzed

    electronically, which mimics the action of the mammalian nose by recognizing patterns of response

    to vapors. The sensors used in the device discussed here are conductometric chemical sensors

    which change resistance when the composition of its environment changes. The sensors are not

    specific to any one vapor; it is in the use of an array of sensors, each of which responds differently,

    that gases and gas mixtures can be identified by the pattern of response of the array.

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    Electronic Noses have been discussed by several authors, and may be applied to environmental

    monitoring as well as to quality control in such wide fields as food processing and industrial

    environmental monitoring.

    In the device designed and built for crew habitat air monitoring, a baseline of clean air is

    established, and deviations from that baseline are recorded as changes in resistance of the sensors.

    The pattern of distributed response of the sensors is deconvoluted, and contaminants identified and

    quantified by using a set of software analysis routines developed for this purpose. The overall goal

    of the program at JPL/Caltech has been the development of a miniature sensor which may be used

    to monitor the breathing air in the International Space Station, and which may be coordinated with

    the environmental control system to solve air quality problems without crew intervention.

    An electronic nose can be a modular system comprising of active materials which operate serially

    on an odourant sample. These active materials can be classified into two: an array of gas sensors

    and a signal processing system. The output of the electronic nose can be the identification of the

    odourant, an estimation of the concentration of the odourant or the characteristic of the odour as

    might be perceived by the human. Fundamental of artificial nose is that each sensor in the array has

    different sensitivity. The pattern of response across the sensors is distinct for different odours. This

    distinguishably allows the system to identify the unknown odour from the pattern of sensor

    responses. The pattern of response across all the sensors in the array is used to identify the odour.

    Different e-noses use different types of gas sensors which form heart of e-nose.

    Electronic Nose developed in the early 1980s, the operating principle consists of an array of

    chemical sensors that are coupled to an appropriate pattern recognition program that emulates the

    human olfactory system. The individual sensors consist of conductive polymers which have

    defined adsorptive surfaces that, when interacting with volatile chemicals, display a change of

    electrical resistance that can be recorded. Even though each individual sensor responds more

    selectively to a certain group of chemicals, they all show an overlapping response (this is called

    cross-selectivity ).

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    Fig. 2: Electronic Nose

    Fig. 3: Electronic Nose

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    How the electronic nose actually works is that, for each complex aroma, the array of sensors

    produces a unique response pattern -called a fingerprint- which reflects the aroma complexity of

    that sample. An electronic nose, therefore, acts more like a human nose in that it does not measure

    the amount of an individual aroma compound, but rather, gives a global and qualitative idea of the

    whole aroma profile. The electronic nose consists of two components, (1) an array of chemical

    sensors (usually gas sensors) and (2) a pattern recognition algorithm. The sensor array "sniffs" the

    vapors from a sample and provides a set of measurements; the pattern-recognizer compares the

    pattern of the measurements to stored patterns for known materials. Gas sensors tend to have very

    broad selectivity, responding to many different substances. This is a disadvantage in most

    applications, but in the electronic nose, it is a definite advantage. Although every sensor in an array

    may respond to a given chemical, these responses will usually be different. Figure shows sets of

    responses of a typical sensor array to different pure chemicals.

    ACETONE BENZENE CHLOROFORM

    Fig. 4: Responses of a typical sensor array to different pure chemicals

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    Chapter 4

    WORKING OF ELECTRONIC NOSE

    4.1) ODOUR:

    An odour is composed of molecules, each of which has a specific size and shape. Each of these

    molecules has a correspondingly sized and shaped receptor in the human nose. When a specific

    receptor receives a molecule, it sends a signal to the brain and the brain identifies the smell

    associated with that particular molecule. An odour or odour (see spelling differences) is a

    volatilized chemical compound, generally at a very low concentration, that humans or other

    animals perceive by the sense of olfaction. Odours are also called smells, which can refer to bothpleasant and unpleasant odours. The terms fragrance, scent, and aroma are used primarily by the

    food and cosmetic industry to describe a pleasant odour, and are sometimes used to refer to

    perfumes.

    4.2) COMPONENTS:

    Main components of electronic nose

    o Sensing System

    o Pattern Recognition System

    Sub components of electronic nose

    o Sample Delivery System

    o Detection System

    o Computing System

    4.3) WORKING PRINCIPLE:The signals generated by an array of odour sensors need to be processed in a sophisticated manner.

    The electronic nose research group has obtained considerable experience in the use of various

    parametric and non-parametric pattern analysis techniques. These include the use of linear and non-

    linear techniques, such as discriminant function analysis, cluster analysis, genetic algorithms, fuzzy

    logic, and adaptive models. An odour is composed of molecules, each of which has a specific size

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    and shape. Each of these molecules has a correspondingly sized and shaped receptor in the human

    nose. When a specific receptor receives a molecule, it sends a signal to the brain and the brain

    identifies the smell associated with that particular molecule.

    Electronic noses based on the biological model work in a similar manner, albeit substituting

    sensors for the receptors, and transmitting the signal to a program for processing, rather than to the

    brain. Electronic noses are one example of a growing research area called biomimetics, or

    biomimicry, which involves human-made applications patterned on natural phenomena.

    Fig 5: Block Diagram of Electronic Nose

    In a typical e-nose, an air sample is pulled by a vacuum pump through a tube into a small

    chamber housing the electronic sensor array. The tube may be of plastic or stainless steel.

    A sample-handling unit exposes the sensors to the odourant, producing a transient response

    as the volatile organic compounds interact with the active material.

    The sensor response is recorded and delivered to the Signal-processing unit.

    Then a washing gas such as alcohol is applied to the array for a few seconds or a minute, so

    as to remove the odourant mixture from the active material.

    The more commonly used sensors include metal oxide semiconductors, conducting

    polymers, quartz crystal microbalance, surface acoustic wave, and field effect transistors.

    In recent years, other types of electronic noses have been developed that utilize mass

    spectrometry or ultra fast gas chromatography as a detection system.

    The computing system works to combine the responses of all of the sensors, which

    represents the input for the data treatment. This part of the instrument performs global

    fingerprint analysis and provides results and representations that can be easily interpreted.

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    Moreover, the electronic nose results can be correlated to those obtained from other

    techniques.

    An electronic nose system primarily consists of four functional blocks, viz., Odour Handling and

    Delivery System, Sensors and Interface Electronics, Signal Processing and Intelligent Pattern

    Analysis and Recognition. The array of sensors is exposed to volatile odour vapour through

    suitable odour handling and delivery system that ensures constant exposure rate to each of the

    sensors. The response signals of sensor array are conditioned and processed through suitable

    circuitry and fed to an intelligent pattern recognition engine for classification, analysis and

    declaration.

    The most complicated parts of electronic olfaction process are odour capture and associated sensor

    technology. Any sensor that responds reversibly to chemicals in gas or vapour phase, has the

    potential to be participate in an array of sensor in an electronic nose. For black manufactured tea,

    an array of Metal Oxide Semiconductor sensors have been used for assessment of volatiles.

    Fig 6: Specified block diagram of electronic nose

    An electronic nose can be regarded as a modular system comprising a set of active materials which

    detect the odour, associated sensors which transduce the chemical quantity into electrical signals,

    followed by appropriate signal conditioning and processing to classify known odours or identify

    unknown odours, see Using variants of molecules found in biology it is possible to create 'senses'

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    from electrical charges caused by the binding of the molecules to mimic the human nose. With this

    approach, the sensitivity of the device can be a thousand times better than the currently available

    electronic nose.

    The receptors, which will be housed within an artificial membrane, remain in a closed steady state

    until approached by smell molecules, when they will open and transmit an electrical signal which

    will indicate the nature of the odour.

    Electronic Nose uses a collection of 16 different polymer films. These films are specially designed

    to conduct electricity. When a substance -- such as the stray molecules from a glass of soda -- is

    absorbed into these films, the films expand slightly, and that changes how much electricity they

    conduct. Because each film is made of a different polymer, each one reacts to each substance, or

    analyte, in a slightly different way. And, while the changes in conductivity in a single polymer film

    wouldn't be enough to identify an analyte, the varied changes in 16 films produce a distinctive,

    identifiable pattern.

    4.4) SENSING AN ODOURANT:

    In a typical electronic nose, an air sample is pulled by a vacuum pump through a tube into a small

    chamber housing the electronic sensor array. The tube may be made of plastic or a stainless steel.

    Next, the sample handling unit exposes the sensors to the odourant, producing a transientresponse as the VOCs interact with the surface and bulk of the sensors active material. (Earlier,

    each sensors has been driven to a known state by having clean, dry air or some other reference gas

    passed over its active elements.) A steady state condition is reached in a few seconds to a few

    minutes, depending on the sensor type.

    During this interval, the sensors response is recorded and delivered to the

    signal processing unit. Then, a washing gas such as an alcohol vapor is applied

    to the array for a few seconds to a minute, so as to remove theodourant

    mixture from the surface and bulk of the sensors active material. (Some

    designers choose to skip this washing step) Finally, the reference gas is

    applied to the array, to prepare it for a new measurement cycle. The period

    during which the odourant is applied is called the response time of the sensor

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    array. The period during which the washing and reference gases are applied is

    termed the recovery time.

    Chapter 5

    SIGNAL PROCESSING AND PATTERN RECOGNITION

    The task of an electronic nose is to identify an odorant sample and perhaps to estimate its

    concentration. The means are signal processing and pattern recognition. For an electronic nose

    system this two steps may be subdivided into four sequential stages. They are preprocessing,

    feature extraction, classification and decision making. But first a data base of the expected odorant

    must be compiled, and sample must be presented to the noses sensor array.

    Preprocessing compensates for sensor drift compress the transient response of the sensor array, and

    reduces sample to sample variations. Typical techniques are manipulation of sensor base lines,

    normalization of sensor response ranges of all the sensors in an array (the normalization constant

    may some times be used to estimate the odorant concentration), and compression

    sensor transients.

    Feature extraction has two purposes; they are to reduce the dimensionality of the measurement

    space, and to extract information relevant for pattern recognition. To illustrate, in an electronic

    nose with 32 sensors, the measurement space has 32 dimensions. This space can cause statistical

    problem if odor database contains only a few examples, typical in pattern recognition applications

    because of the cost of data collection. Further more, since the sensors have overlapping

    sensitivities there is high degree of redundancy in these 32 dimensions. Accordingly is it

    convenient to project the 32 on to a few informative and independent axes. This low dimensional

    projection (typically 2 or 3 axes) has the added advantage that it can be more readily inspectedvisually.

    Feature extraction is generally performed with linear transformations such as the classical principal

    component analyses (PCA) and linear discriminate analysis (LDA). PCA finds projections of

    maximum variance and is the most widely used linear feature extraction techniques. But it is not

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    optimal for classification since it ignores the identity (class label) of the odor examples in the

    database. LDA, on the other hand, looks at the class label of each example. Its goal is to find

    projections that maximize the distance between examples from different odorants yet minimize the

    distance between examples of the same odorant. As in example, PCA may do better with a

    projection that contain high variance random noise whereas LDA may do better with a projection

    that contain subtle, but maybe crucial, odor discriminatory information. LDA is therefore more

    appropriate for classification purposes.

    5.1) ELECTRONIC NOSE INSTRUMENTATION:

    Fig 7: General measurement system

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    The basic element of a generalized electronic instrument system to measure odours are shown

    schematically in the figure. First there is an odour from the source material to the sensor chamber.

    There are tow main ways in which the odour can be delivered to the sensor chamber, namely head

    space sampling and flow injection. In head space sampling, the head space of an odorant material is

    physically removed from a sample vessel and inserted into the sensor chamber using either a

    manual or automated procedure. Alternatively, a carrier gas can be used to carry the odorant from

    the sample vessel into the sensor by a method called flow injection. The sensor chamber houses the

    array of chosen odour sensors, e.g. Semi conducting polymer chemo resisters, etc. The sensor

    electronic not only convert the chemical signal into an electrical signal into an electrical signal into

    an electrical signal into an electrical signal but also, usually, amplify and condition it. This can be

    done using conventional analogue electronic circuitry (e.g. operational amplifiers) and the output is

    then a set of an analogue outputs, such as 0 to 5v d.c. although a 4 to 20mA d.c. current output of

    preferable if using a long cable. The signal must be converted into a digital converter (e.g. a 12

    bit converter) followed by a multiplexer to produce a digital signal which either interfaces to a

    serial port on the microprocessor or digital bus. The microprocessor is programmed to carry out a

    number of tasks.

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    Chapter 6

    COMPARISION OF ELECTRONIC NOSE WITH BIOLOGICAL NOSE

    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 odour 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 odours can elicit basic emotions like love,

    sadness, or fear The term,"electronic nose" has come into common usage as a generic term for an

    array of chemical gas sensors incorporated into an artificial olfaction device, after its introduction

    in the title of a landmark conference on this subject in Iceland in 1991.There are striking analogies

    between the artificial noses of man and the "Biological-nose" constructed by illustrates a biological

    nose and points out the important features of this "instrument". electronic nose. Comparing the two

    is instructive.

    Fig: 8: Human Nose

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    The human nose uses the lungs to bring the odour to the epithelium layer; the electronic nose has a

    pump. The human nose has mucous, hairs, and membranes to act as filters and concentrators, while

    the E-nose has an inlet sampling system that provides sample filtration and conditioning to protect

    the sensors and enhance selectivity. The human epithelium contains the olfactory epithelium,

    which contains millions of sensing cells, selected from 100-200 different genotypes that interact

    with the odourous molecules in unique ways. The E-nose has a variety of sensors that interact

    differently with the sample. The human receptors convert the chemical responses to electronic

    nerve impulses. The unique patterns of nerve impulses are propagated by neurons through a

    complex network before reaching the higher brain for interpretation. Similarly, the chemical

    sensors in the E-nose react with the sample and produce electrical signals. A computer reads the

    unique pattern of signals, and interprets them with some form of intelligent pattern classification

    algorithm. From these similarities we can easily understand the nomenclature. However, there are

    still fundamental differences in both the instrumentation and software. The Bionose can perform

    tasks still out of reach for the electronic-nose, but the reverse is also true.

    Electronic Nose is a device that can learn to recognize almost any compound or combination of

    compounds. It can even be trained to distinguish between Pepsi and Coke. Like a human nose, the

    E-Nose is amazingly versatile, yet it's much more sensitive .

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    Fig 9: Comparison of human nose and electronic nose

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    Chapter 7

    SENSORS

    A sensor is a device which can respond to some properties of the environment and

    transform the response into an electric signal.

    Fig 10: sensor array

    7.1) WORKING MECHANISM:

    The general working mechanism of a sensor is illustrated by the following scheme:

    In the field of sensors, the correct definition of parameters is of paramount importance because of

    these parameters:

    allow the diffusion of more reliable information among researchers or sensor operators.

    allow a better comprehension of the intrinsic behavior of the sensors help to propose new

    standards, give fundamental criteria for a sound evaluation of different sensor

    performances. The output signal is the response of the sensor when the sensitive material

    undergoes modification.

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    Fig 11: working mechanism of a sensor

    The sensors in the electronic nose are polymer films which have been loaded with a conductive

    medium, in this case carbon black. A baseline resistance of each film is established; as the

    constituents in the air change, the films swell or contract in response to the new composition of the

    air, and the resistance changes. In the electronic nose, sensing films were deposited on co-fired

    ceramic substrates which were provided with eight Au-Pd electrode sets.

    7.2) ELECTRONIC NOSE SENSORS:

    Electronic nose sensors fall in four categories:-

    Conductivity Sensors

    Piezo Electric Sensors

    MOSFET Sensors and,

    Optical Sensors.

    7.2.1) CONDUCTIVITY SENSORS:There are two types of conductivity sensors.

    a. Metal Oxide Sensor

    b. Polymer Sensor

    Both of them exhibit a property of change in assistance when exposed to volatile organic

    compounds.

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    a. Metal Oxide Sensor:

    Fig 12: Metal oxide sensor

    Metal Oxide Semi conductor sensors have been used more extensively in electronic nose

    instruments and are widely available commercially. Typical metal Oxide sensors include oxides of

    tin, zinc, titanium, tungsten and Iridium with a noble metal catalyst such as platinum or palladium.

    The doped semi conducting material with which the VOCs interact is deposited between two metal

    contacts over a resistive heating element, which operates at 200oc to 4000c. At these elevated

    temperature, heat dispersion becomes a factor in the mechanical design of the sensing chamber.

    Micro machining is often used to thin the sensor substrate under the active material, so that power

    consumption and heat dissipation requirements are reduced. As a VOC passes over the doped oxide

    material, the resistance between the two metal contacts changes in proportion to the concentration

    of the VOC.

    The recipe for the active sensor material is designed to enhance the response to specific odourants,

    such as carbon monoxide or ammonia. Selectivity can be further improved by altering the

    operating temperature. Sensor sensitivity ranges from 5 to 500 parts per million. The sensor also

    respond to water, vapor, more specifically to humidity differences between the gas sample being

    analyzed and a known reference gas used to initialize the sensor. The baseline response of metal

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    oxide sensors is prone to drift over periods of hours to days, so signal processing algorithms should

    be employed to counteract this property.

    The sensors are also susceptible to poisoning (irreversible binding) by sulphur compounds present

    in the odourant mixture. But their wide availability and relatively low cost make them the most

    widely used gas sensors today.

    b. Polymer Sensor

    Conducting polymer sensors, a second type of conductivity sensor, are also commonly used in

    electronic nose systems. Here, the active material in the above figure is a conducting polymer from

    such families as the polypyroles, thiophenes, indoles or furans. Changes in the conductivity of

    these materials occur as they are exposed to various types of chemicals, which bond with the

    polymer backbone. The bonding may be ionic or in some cases, covalent. The interaction affects

    the transfer of electrons along the polymer chain, that is to say its conductivity is strongly

    influenced by the counter ions and functional groups attached to the polymer backbone.

    In order to use these polymers in a sensor device, micro fabrication techniques are employed to

    form two electrodes separated by a gap of 10 to 20 micrometre. Then the conducting polymer is

    electro polymerized between the electrodes by cycling the voltage between them. For example,

    layers of polypyrroles can be formed by cycling between -0.7 and +1.4 V. Varying the voltage

    sweep rate and applying a series of polymer precursors yields a wide variety of active materials.

    Response time is inversely proportional to the polymers thickness. To speed response times,

    micrometer size conducting polymer bridges are formed between the contract electrodes.

    Because conducting polymer sensors operated at ambient temperature, they do not need heaters and

    thus are easier to make. The electronic interface is straight forward, and they are suitable for

    portable instruments. The sensors can detect odours at sensitivities of 0.1 parts per million (ppm),

    but 10 to 100 ppm is more usual.

    The main drawback of existing conducting polymer sensor is that it is difficult and time consuming

    to electro polymerize the active material, so they exhibit undesirable variations from one batch to

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    another. Their responses also drift over time and they are usually greater sensitivity than metal

    oxides to water vapor renders them susceptible humidity. This susceptibility can mask the

    responses to odourous volatile organic compounds. In addition, some odourants can penetrate the

    polymer bulk, dragging out the sensor recovery time by slowing the removal of the VOC from the

    polymer. This extends the cycle time for sequentially processing odourant samples.

    7.2.2) PIEZO ELECTRICAL SENSORS:

    There are two types of piezo electrical sensors.

    a. QCM Sensor

    b. SAW Sensor

    A piezoelectric sensor is a device that uses the piezoelectric effect to measure pressure,

    acceleration, strain or force by converting them to an electrical signal, they are configured as

    mass-change sensing device.

    Fig 13: A piezoelectric disk generates a voltage when deformed

    Piezoelectric sensors have proven to be versatile tools for the measurement of various processes.

    They are used for quality assurance, process control and for research and development in many

    different industries. From the Curies initial discovery in 1880, it took until the 1950s before the

    piezoelectric effect was used for industrial sensing applications. Since then, the utilization of this

    measuring principle has experienced a constant growth and can be regarded as a mature technology

    with an outstanding inherent reliability. It has been successfully used in various applications as for

    example in medical, aerospace, nuclear instrumentation and in mobile's touch key pad as pressure

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    sensor. In the automotive industry piezoelectric elements are used as the standard devices for

    engine indicating in developing internal combustion engines. The combustion processes are

    measured with piezoelectric sensors. The sensors are either directly mounted into additional holes

    into the cylinder head or the spark/glow plug is equipped with a built in miniature piezoelectric

    sensor. The rise of piezoelectric technology is directly related to a set of inherent advantages. The

    high modulus of elasticity of many piezoelectric materials is comparable to that of many metals

    and goes up to 105 N/m. Even though piezoelectric sensors are electromechanical systems that

    react on compression, the sensing elements show almost zero deflection. This is the reason why

    piezoelectric sensors are so rugged, have an extremely high natural frequency and an excellent

    linearity over a wide amplitude range.

    Additionally, piezoelectric technology is insensitive to electromagnetic fields and radiation,

    enabling measurements under harsh conditions. Some materials used (especially gallium phosphate

    or tourmaline) have an extreme stability over temperature enabling sensors to have a working range

    of upto 1000C.Tourmaline shows pyroelectricity in addition to the piezoelectric effect; this is the

    ability to generate an electrical signal when the temperature of the crystal changes. This effect is

    also common to piezo ceramic materials.

    Principle of operation:

    Depending on how a piezoelectric material is cut, three main modes of operation

    can be distinguished: transverse, longitudinal, and shear.

    Transverse effect: A force is applied along a neutral axis (y) and the charges are generated

    along the (x) direction, perpendicular to the line of force. The amount of charge depends on

    the geometrical dimensions of the respective piezoelectric element. When dimensions a, b,

    c apply,

    Cx = dxyFyb / a,

    where a is the dimension in line with the neutral axis, b is in line with the charge

    generating axis and dis the corresponding piezoelectric coefficient.

    Longitudinal effect: The amount of charge produced is strictly proportional to the applied

    force and is independent of size and shape of the piezoelectric element. Using several

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    elements that are mechanically in series and electrically in parallel is the only way to

    increase the charge output. The resulting charge is

    Cx = dxxFxn,

    where dxx is the piezoelectric coefficient for a charge in x-direction released by forces

    applied along x-direction. FX is the applied Force in x-direction [N] and n corresponds to

    the number of stacked elements.

    Shear effect: Again, the charges produced are strictly proportional to the applied forces

    and are independent of the elements size and shape. Forn elements mechanically in series

    and electrically in parallel the charge is

    Cx = 2dxxFxn.

    In contrast to the longitudinal and shear effects, the transverse effect opens the possibility

    to fine-tune sensitivity on the force applied and the element dimension.

    a) QCM Sensor:

    Fig 14: QCM Sensor

    The QCM types consist of a resonating disk a few millimeters in diameter, with metal elect odes on

    each side connected to dead wise. The device resonate at a characteristic frequency (10MHz to

    30MHz) when excited with an oscillating signal.

    During manufacture, a polymer coating is applied to the disk-polymer, device and thereby

    reducing the resonance frequency. The reduction is inversely proportional to odourant mass

    absorbed by the polymer for example; a 166um-thick quartz crystal cut along a certain axis will

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    resonate at 10 MHz positive 0.01 percent change in mass, a negative shift of 1 KHz will occur in

    its resonance frequency. Then when the sensor is exposed to a reference gas, the resonance

    frequency returns to its baseline value. A good deal is known about QCM device. The military for

    one has experimented with them for years, using them for one detection of trance amounts of

    explosive and other hazardous compounds and measuring mass changes to a resolution of 1

    picogram.

    For example, 1pg of methane in a 1 liter sample volume at standard temperature and pressure

    produces a methane concentration of 1.4ppb. In addition, QCM sensors are remarkably linear once

    wide dynamic range. Their response to water it dependent upon the absorbent material employed.

    And their sensitivity to changes in temperature can be made negligible. Tailoring the QCM for

    specific application is done by adjusting its polymer coating. Fortunately, a large number of

    coating its available from those developed of GC Column. The response and recovery times of the

    resonant structure are minimized by reducing size and mass of the quartz crystal along with the

    thickness of the polymer coating. Batch-to-batch variability is not a problem because these device

    measure normalized frequency change, a differential measurement that removes commonmode-

    noise. Care must be taken when making three dimensional devices by micro electromechanical

    system (MEMS) processing techniques. When the dimensions are scaled down to micrometer

    levels, the surface to volume ratios increase, and the larger the surface-to-volume ratio; the noiser

    the device gets because of surface processes that cause instabilities. Hence, signal-to-noise ratio

    degrades with increasing surface-to volume ratio, thereby hampering measurement accuracy. It

    should be noted that this phenomenon applies to most micro fabricated devices.

    b) SAW Sensor

    The Saw Sensor differs from QCMs in several important ways. First, A Rayliegh (Surface)

    wave travels over the surface of the device; not throughout its volume. SAW sensors operate at

    much higher frequencies, and so can generate a larger change in frequency. A typical SAW

    device operates in the hundreds of megahertz, while 10MHZ is more typical for a QCM, but

    SAW device can measure changes in mass to the same order of magnitude as QCMs. Even

    though the frequency range is larger, increased surface-to-volume ratios mean the Signal-to

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    noise ratio is usually poorer. Hence, SAW device be less sensitive the QCMs in some

    instances.

    Fig 15: SAW Sensor

    Being planar, SAW device can be fabricated with photolithographic methods developed by the

    micro electronics industry. The fact that there dimensional MEMS processing is unnecessary

    makes them relatively cheaper then their QCM counterparts when large quantities are produced.

    Being planar, SAW device can be fabricated with photolithographic methods developed by the

    micro electronics industry. The fact that there dimensional MEMS processing is unnecessary

    makes them relatively cheaper than their QCM counterparts when large quantities are produced. Aswith QCMs, many polymer coatings are available, and as with the other sensor types, differential

    measurements can eliminate commonmode effects. For example, two adjacent SAW devices on the

    same substrate (one with an active membrane and another without) can be operated as a differential

    pair of remove temperature variations and power line noise.

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    A disadvantage of both QCM and SAW device is more complete electronics than are needed by the

    conductivity sensors. Another is their need for frequency detectors, whose resonant frequencies can

    drift as the active membrane ages.

    7.2.3) MOSFET SENSORS:

    MOSFET odour sensing device are based on the principle that VOCs in contact with a catalytic

    metal can produce a reaction in the metal and the reactions products can diffuse through the gate

    of the MOSFET to change the electrical properties of the device. A typical MOSFET structure has

    p-type substrate with two n doped regions with metal contacts labeled source and drain as shown in

    fig.

    Fig 16: MOSFET Sensors

    The sensitivity and selectivity of the device can be optimized by varying the type and thickness of

    the metal catalyst and operating them at different temperatures. MOSFET sensors have been

    investigated for numerous applications; but to date few have been used in commercial electronic

    nose systems because of a dearth of sensors variants.

    The advantages of MOSFETs is that they can be made with 1C fabrication processes, so that batch

    to batch variation can be minimized.

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    The disadvantages is that the catalyzed reaction products such as hydrogen must penetrate the

    catalytic metal layer in order to influence that charge at the channel, so that the package must

    therefore have a window to permit gas to interact with the gate structure on the IC chip. Thus it is

    important to maintain a hermetic seal for the chips electrical connections in harsh environments.

    The requirements may be satisfied by using photo-definable polymers such as polyamide, to seal

    all areas of the chip that are not to be intentionally exposed to the environment. MOSFET sensors

    also undergo baseline drift similar to that of conductivity sensors.

    7.2.4) OPTICAL SENSORS:

    Fig 17: Optical sensor

    Optical fiber sensors, yet another type, utilize glass fibers with a thin chemically active material

    coating on their sides or ends as shown in Fig. A light source at a single frequency (or at a narrow

    band of frequencies) is used to interrogate the active material, which in turn responds with a

    change in color to the presence of the VOCs to be detected and measured. The active materials

    contain chemically active fluorescent dyes immobilized in an organic polymer matrix. As VOCs

    interact with it, the polarity of the fluorescent dyes is altered and they respond by shifting their

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    fluorescent emission spectrum. When a pulse of light from and external source interrogates the

    sensor, the fluorescent dye responds by emitting light a different frequency. As the source intensity

    is much greater than sensor response great care must be taken to ensure that the response photo-

    detectors are protected from the source emissions. Favouring the optical sensors is the availability

    of many different dyes of biologic research, so that the sensors themselves the cheap and easy to

    fabricate. It is also possible to couple fluorescent dyes to antibody antigen binding (the recognition

    of a specific molecule, and only that molecule, as in the human immune system). Thus an array of

    fiber sensors can have wide range sensitivities, a feature not easily obtained with other sensor

    types. As with other types, differential measurements can also be used to remove common mode

    noise effects. In their disfavor are the complexity of the instrumentation control system, which

    adds to fabrication cost, and their lifetime due to the photo bleaching. The sensing process slowly

    consumes the fluorescent dyes, the way sunlight bleaches fabric dyes.

    Chapter 8

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    APPLICATIONS OF ELECTRONIC NOSE

    The electronic nose finds lot of application in many fields. They have been used in a variety of

    applications and could help solve problems in many fields including food product quality

    assurance, health care, environmental monitoring, pharmaceuticals etc. The major applications are:

    Electronic Nose For Environmental Monitoring

    Electronic Nose Used In Detection Of Bombs

    Electronic Nose For Multimedia Application

    Electronic Nose For Medical Application

    Electronic Nose For The Food Industry

    Electronic Nose Created To Detect Skin Vapours

    In Resources And Development Laboratories

    In Quality Control Laboratories

    Pharmaceutical Industry Application

    Safety And Security Application

    I. Electronic Nose For Environmental Monitoring:

    Enormous amounts of hazardous waste (nuclear, chemical, and mixed wastes) were generated by

    more than 40 years of weapons production in the U.S. Department of Energies weapons complex.

    The Pacific Northwest National Laboratory is exploring the technologies required to perform

    environmental restoration and waste management in a cost effective manner. This effort includes

    the development of portable, inexpensive systems capable of real-time identification of

    contaminants in the field. Electronic noses fit this category.

    The environmental applications of the electronic nose will include-

    Identification of toxic wastes.

    Analysis of fuel mixtures.

    Detection of oil leaks.

    Identification of household odours.

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    Monitoring air quality.

    Monitoring factor emission.

    II. Electronic Nose Used In Detection Of Bombs:

    A possible alternative is using an electronic nose to sniff out possible explosives so that only

    selected bags need to be searched by staff. The concept has been around for a long time, and was

    initially ridiculed. The basic idea is a device that identifies the specific components of an odour and

    analyzes its chemical makeup to identify it. One mechanism would be an array of electronic

    sensors would sniff out the odours while a second mechanism would see if it could recognize the

    pattern.

    III. Electronic Nose For Multimedia Application:

    Multimedia systems are widely used in consumer electronics environments today, where humans

    can work and communicate through multi-sensory interfaces. Unfortunately smell detection and

    generation systems are not part of today's multimedia systems. Hence we can use electronic nose in

    multimedia environment.

    IV. Electronic Nose For Medicine:

    Since the sense of smell is an important sense it the physician, an electronic nose has applicability

    as a diagnostic tool. An electronic nose can be used to analyze the odours from the body and

    identify the possible problems. Odour in the breath can be indicative of gastrointestinal problems,

    sinus problem, infection, diabetes, liver problems etc, infected wounds and tissues will emit

    distinctive smell, which can be detected by the electronic nose. Odours coming from the body

    fluids such as blood and urine can indicate liver and bladder problems. The electronic nose will

    give the doctor a sixth sense. By sensing the smell of the breath doctor will be able to identify thedisease. As an example, it is found that the fruity, nail-varnish remover smell found of the breathe

    of a diabetic about to enter a sever coma. The tin traces of illness-related chemicals on your breath

    could indicate diseases such as schizophrenia when detected by a new generation of electronic

    noses. A more futuristic application of electronic noses has been recently proposed for telesurgery

    V. Electronic Nose For The Food Industry:

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    Currently, the biggest market for electronic nose is in the food industry. In some instances

    electronic noses can be used to augment or replace panels of human experts. In food production

    especially when qualitative results will do.

    The applications of electronic noses in food industry are numerous. They include:-

    Inspection of food by odour

    Grading quality of food by odour

    Fish inspection

    Fermentation control

    Checking mayonnaise for rancidity

    Automated flavor control

    Monitoring cheese ripening

    Beverage container inspection

    Grading whiskey

    Microwave over cooking control

    VI. Electronic Nose Created To Detect Skin Vapours:

    A team of researchers from the Yale University (United States) and a Spanish company have

    developed a system to detect the vapours emitted by human skin in real time. The scientists think

    that these substances, essentially made up of fatty acids, are what attract mosquitoes and enable

    dogs to identify their owners.

    "The spectrum of the vapours emitted by human skin is dominated by fatty acids. These substances

    are not very volatile, but we have developed an electronic nose' able to detect them",

    This electronic nose can be used to identify many of the vapour compounds emitted by a hand, for

    example. "The great novelty of this study is that, despite the almost non-existent volatility of fatty

    acids, which have chains of up to 18 carbon atoms, the electronic nose is so sensitive that it can

    detect them instantaneously".

    The results show that the volatile compounds given off by the skin are primarily fatty acids,

    although there are also others such as lactic acid and pyretic acid. The researcher stresses that the

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    great chemical wealth of fatty acids, made up of hundreds of different molecules, "is well known,

    and seems to prove the hypothesis that these are the key substances that enable dogs to identify

    people". The enormous range of vapours emitted by human skin and breath may not only enable

    dogs to recognize their owners, but also help mosquitoes to locate their hosts, according to several

    studies.

    VII. In Resources And Development Laboratories:

    Formulation or reformulation of products Benchmarking with competitive products

    Shelf life and stability studies

    Selection of raw materials

    Packaging interaction effects

    Simplification of consumer preference test

    VIII. In Quality Control Laboratories:

    Batch to batch consistency

    Conformity of raw materials, intermediate and final products

    Detection of contamination, spoilage, adulteration

    Origin or vendor selection

    Monitoring of storage conditions.

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    Fig 18: Conformity of raw materials through electronic nose

    IX. Pharmaceutical Industry Application:

    In the pharmaceutical industry the electronic nose could be used to screen the incoming raw

    materials, monitor production process, maintain security in storage and distribution areas, quality

    assurance, testing the employees in critical occupations for drug use or abuse, use to detect

    unpleasant smell in the industrial area.

    X. Safety And Security Application:

    The electronic nose can help in the safety and security applications. They include

    Hazardous alarms for toxic and biological agents

    Screening airline passengers for explosive

    Examining vehicles for drugs.

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    Monitoring indoor air quality.

    Smart fire alarms

    Fire alarms in nuclear plants.

    Biological and chemical detection in battlefield

    Chapter 9

    ADVANTAGES & DISADVANTAGES OF ELECTRONIC NOSE

    :

    9.1) ADVANTAGES:

    The electronic nose is best suited for matching complex samples with subjective endpoints

    such as odour or flavor.

    The electronic nose can match a set of sensor responses to a calibration set produced by the

    human taste panel or olfactory panel routinely used in food science.

    The electronic nose is especially useful where consistent product quality has to be

    maintained over long periods of time, or where repeated exposure to a sample poses a

    health risk to the human olfactory panel.

    Although the electronic nose is also effective for pure chemicals, conventional methods are

    often more practical.

    Helpful in identification of spilled chemicals in commerce.

    Helpful in identification of Quality classification of stored grain.

    Helpful in identification of Water and wastewater analysis.

    Helpful in identification of source and quality of coffee.

    Helpful in monitoring of roasting process. Helpful in Rancidity measurements of olive oil.

    Helpful in Detection and diagnosis of pulmonary infections.

    Helpful in Diagnosis of ulcers by breath tests.

    Helpful in identification of Freshness of fish.

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    Helpful in identification of Process control of cheese, sausage, beer, and bread

    manufacture.

    Helpful in identification of Bacterial growth on foods such as meat and fresh vegetables.

    9.2) DISADVANTAGES:

    The cost of an e-nose ranges from $5000 to $100,000.

    Another disadvantage has been the delay, the time delay ranging between 2 to 10

    minutes.

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    Chapter 10

    FUTURE SCOPE OF ELECTRONIC NOSE

    There are numerous potential applications of electronic noses from the product and process control

    to the environmental monitoring of pollutants and diagnosis of medical complaints. However, this

    requires the developments of application-specific electronic nose technology, that is electronic

    noses that have been designed for a particular application. This usually involves the selection of the

    appropriate active material, sensor type and pattern recognition scheme. The work has led to

    several commercial instruments, one employing commercial tin oxide sensors and another

    employing conducting polymer sensors Future developments in the use of hybrid micro-sensor

    arrays and the development of adaptive artificial neural networking techniques will lead to superior

    electronic noses.

    The major areas of research being carried out in this field are:

    i. Improved sensitivity for use with water quality and sensitive microorganism

    detection applications.

    ii. Identification of microorganisms to the strain level in a number of matrices,

    including food.

    iii. Improvement in sensitivity of the E-Nose for lower levels of organisms or smaller

    samples.

    iv. Identification of infections such as tuberculosis in noninvasive specimens (sputum,

    breath).

    v. Development of sensors suitable for electronic nose use, and evaluation of

    unexploited sensors.

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    Fig 19: Next Generation Products

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    Chapter 11

    CONCLUSION

    Advantages of the electronic nose can be attributed to its rapidity, objectivity, versatility, non

    requirement for the sample to be pretreatment, easy to use etc. And now scientists at the University

    of Rome have developed a sensor, which, they claim, can detect those chemicals flowing out of a

    cancerous lung. Their tests, on a group of 60 people - half with lung cancer - pinpointed every

    single cancer patient. They suggested that an 'e-nose' could one day form the basis of a screening

    test for smokers and others at risk of lung disease. The only way of doing this reliably at the

    moment is to use a bronchoscope to look directly at the insides of the lungs for signs of cancer.

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    Chapter 12

    REFERENCES

    1. www.smartnose.com

    2. www.askjeeves.com

    3. www.enose.info

    4. Hand book on applications and uses of arrays by Jeffery.H

    5. www.scienceweek.com

    6. Electronic nose and their applications By P.E.Keller, L.J. Kangas, L.H. Liden

    http://www.askjeeves.com/http://www.enose.info/http://www.askjeeves.com/http://www.enose.info/