Toward Sensor to Reproduce Human Sense - The Electronic Nose Artificial Olfaction Technology - page 114

5.2 Electronic Nose
During the last decades artificial nose technology has provided many successful
examples of industrial applications. As this technology becomes cheaper and more
accessible, the possibilities of potential use in medical and clinical studies have
also increased.
When chemicals are detected by particular cells in the nose resulting in nerve
impulses which are sent to the brain for interpretation is called recognition of
smell. Smell is responsible for the perception of 75-80 % of all we taste. Smell
results from molecule of various chemicals floating through air. All substance does
not have smell, only few which include chemicals that are volatile in nature.
E-nose was developed with characteristic alike an inexpensive, quick, and
portable device capable of quantifying complex mixtures of volatile compounds.
The realization of the E-nose was made possible by using the human olfactory
system as a paradigm.
There are several sensors which are specific to particular compounds or classes
of compounds, but using compound-specific sensors to monitor changes in
breathing air is a task which would involve the use of several hundreds of sensors.
In addition, compound-specific sensors are subject to interference from molecules
of structure similar to the target molecule. Analytical instruments such as gas
chromatography—mass spectrometry (GC-MS) have multiple capabilities, but are
not practical for continuous monitoring. Work on the E-nose was designed to fill
the gap between individual, chemically specific devices and analytical instruments
such as GC-MS which have multi-compound capability.
The main section of an E-nose is an array of non-specific chemical sensors.
Sensor array output stimulates by an odor analyte and a pattern of characteristic
response is generated. E-nose sensor made of a compilation of technologies, also
note that in every case a specific physical property is measured and a set of signals
is generated. The last stage is pattern recognition (PARC) process and it's also
similar to biological olfaction, where a sensor type responds to more than one
odorant and one odorant type activates more than one sensor. So this set of process
collectively, activated sensors and their signals characterize the odor (sometimes
referred as an odor fingerprint). Therefore, an important difference between E-
noses and analyte detectors such as gas chromatographs, is that whereas the latter
are aimed at identifying the components that contribute to an odor, E-noses can be
used to identify, as a whole, the mixture of components that together form an odor
[ 2 ].
Multi-compound sensing may be accomplished using an array of partially
specific sensors, or an E-nose. In an E-nose, the distributed response of an array of
several sensor heads is used to identify the changes in the composition of a gaseous
environment. This is accomplished by comparing the response of the array to a
baseline response of known, ''good'' air. Sensors are not particular to any one gas;
it is in the use of an array of sensors with a different sensing medium on each that
gases and gas mixtures can be recognized by the pattern of response of the array.