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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/