27-12-2012, 03:30 PM
Electronic Nose
[attachment=45449]
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. The development of artificial neural
network (ANN), in which the nervous system is
electronically implemented is one among them.
The scientists realized the importance of the
detection and identification of odor 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 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 odors 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
nose’s olfactory epithelium, where ends if the nerve cells
that sense odorants.
The volatile organic compounds (VOCs) basic to
odors 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.
ELECTRONIC NOSE PRINCIPLES
Enter the gas sensors of the electronic nose. This
speedy, reliable new technology undertakes what till now
has been impossible – continuous real monitoring of odor
at specific sites in the field over hours, days, weeks or even
months.
An electronic device can also circumvent many other
problems associated with the use of human panels.
Individual variability, adaptation (becoming less sensitive
during prolonged exposure), fatigue, infections, mental
state, subjectivity, and exposure to hazardous compounds
all come to mind. In effect, the electronic nose can create
odor exposure profiles beyond the capabilities of the
human panel or GC/MS measurement techniques.
The electronic nose is a system consisting of three
functional components that operate serially on an odorant
sample- a sample handler, an array of gas sensors, and a
signal processing system. The output of the electronic nose
can be the identity of the odorant, an estimate of the
concentration of the odorant, or the characteristic
properties of the odor as might be perceived by a human.
Sensing an odorant
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 odorant,
producing a transient response as the VOCs interact with
the surface and bulk of the sensor’s 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.
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 polymer’s thickness. To speed response
times, micrometer – size conducting polymer bridges are
formed between the contract electrodes.
[attachment=45449]
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. The development of artificial neural
network (ANN), in which the nervous system is
electronically implemented is one among them.
The scientists realized the importance of the
detection and identification of odor 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 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 odors 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
nose’s olfactory epithelium, where ends if the nerve cells
that sense odorants.
The volatile organic compounds (VOCs) basic to
odors 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.
ELECTRONIC NOSE PRINCIPLES
Enter the gas sensors of the electronic nose. This
speedy, reliable new technology undertakes what till now
has been impossible – continuous real monitoring of odor
at specific sites in the field over hours, days, weeks or even
months.
An electronic device can also circumvent many other
problems associated with the use of human panels.
Individual variability, adaptation (becoming less sensitive
during prolonged exposure), fatigue, infections, mental
state, subjectivity, and exposure to hazardous compounds
all come to mind. In effect, the electronic nose can create
odor exposure profiles beyond the capabilities of the
human panel or GC/MS measurement techniques.
The electronic nose is a system consisting of three
functional components that operate serially on an odorant
sample- a sample handler, an array of gas sensors, and a
signal processing system. The output of the electronic nose
can be the identity of the odorant, an estimate of the
concentration of the odorant, or the characteristic
properties of the odor as might be perceived by a human.
Sensing an odorant
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 odorant,
producing a transient response as the VOCs interact with
the surface and bulk of the sensor’s 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.
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 polymer’s thickness. To speed response
times, micrometer – size conducting polymer bridges are
formed between the contract electrodes.