Toward Sensor to Reproduce Human Sense - The Electronic Nose Artificial Olfaction Technology - page 117

principles based on electrical measurements, including changes in current, voltage,
resistance or impedance, electrical fields and oscillation frequency are most
common methods which are utilized. Other methods are measurements of mass
changes, temperature changes or heat generation. Optical sensors measure the
modulation of light properties or characteristics such as changes in light absor-
bance, polarization, fluorescence, optical layer thickness, color or wavelength
(colorimetric) and other optical properties [ 5 ].
Note that the effect of slight changes in the specific amounts of chemical type
within an odor mixture often can be detected by the human nose as a change in
odor by trained panel experts, but changes in odorless materials are not detectable.
The added advantage of E-nose is detecting definite odorless compounds that are
not detectable by the human nose. Each vapor presented to the sensing system
generates a signature or ''fingerprint''. Presenting many different chemicals to the
sensor yields a database of fingerprints, which the pattern-recognition system uses
to recognize and automatically identify each chemical.
Many E-noses are commercially offered today and have an extensive range of
applications in different markets and industries ranging from food processing,
industrial manufacturing, quality control, environmental protection, security,
safety and military applications to various pharmaceutical, medical, microbio-
logical and diagnostic applications.
Electronic noses with different types of sensor arrays are differentially
responsive to a wide variety of possible analytes and have a number of advantages
over classical analytical instruments. E-nose sensors do not require chemical
reagents, have high-quality sensitivity and definite, give rapid results, and allow
non-destructive sampling of odorants or analytes e-noses generally are far less
expensive than analytical systems, easier and cheaper to operate, and have greater
potential for portability and field use compared with complex analytical laboratory
instruments. E-noses have good potential to be used in the long term by inexpe-
rienced users for numerous practical applications in residential and public settings.
Several disadvantages of e-nose sensing contain problems with reproducibility,
improvement, effects of humidity and temperature on the sensor responses, and
incapability to recognize individual chemical class with sample gases. Thus, E-
noses may never completely replace complex analytical equipment or odor panels
for all applications, but offer quick real-time detection and discrimination solutions
for applications requiring accurate, rapid and repeated determinations.
The E-nose consists of five key components: sampling chamber, sensor
chamber, data acquisition system and controller unit, power supply and graphic
user interface on a computer. Figure 5.1 shows the block diagram of the E-nose
system.
Human nose is elegant, sensitive, and self-repairing, but the E-nose sensors do
not fatigue or get the ''flu''. Further, the E-nose can be sent to detect toxic and or
else hazardous situations that humans may desire to keep away from i.e., sensors
can detect toxic carbon monoxide, which is odorless to humans.