Electronic Nose (eNose) is a device used to detect and recognize odours/vapours, i.e. a machine olfaction device with an array of chemical sensors.
Alternatively, in line with Gardner and Bartlett, (1994) definition :
“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 odours”
The most common use at the current time for the eNose is within the food and drink industries. Along with this field, eNose might be utilized in other areas resembling petroleum qualitative and quantitative analysis, detection of explosives, classification and degradation studies of olive oils, development of a field odour detector for environmental applications, quality control applications in the automotive industry, discrimination between clean and contaminated cows’ teats in a milking system, cosmetic raw materials analysis, plus many other important areas akin to in the medical and space fields.
The principle of eNose is that it uses an array of sensors, whether in the form of various kinds of polymers or via the usage of metal oxide semi conductors, the principle here is still the identical.
When molecules from any element deposited on the surface of the sensor, the electrical conductivity changes, as and when the surface expands. This is the essential idea of how eNose works i.e. change of the sensor resistance when the sensor exposed to odours/vapours.
The pattern displayed on the monitor for each particular resistance is exclusive (i.e. the type of odour or vapour of a specific sample). In this manner it is feasible to differentiate a sample from another or the state/condition of the sample itself, because the headspace from each sample has a novel signature on the eNose sensors resistance.
It’s difficult to pin point the exact date of “when and the way” the idea of designing a system, which can mimic the human nose, happened. 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 conceptualisation of the eNose.
Please note that an eNose differ from other kinds of MOD by simply having multiples sensors, while other devices may have one sensor only or just the mechanism itself differ substantially from the eNose basic working principles.
The name MOD, therefore, cover devices corresponding to eNoses i.e. devices with multiple sensors, in addition to devices with single sensors – or those devices which operate on a distinct 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 make use of 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 that time, the idea and the principles of the eNose has grown and developed into different fields across the globe.
Historically speaking, there are two several types of eNoses (Pearce 1997):
1. Static odour delivery.
2. Mass-flow systems.
As the 2 names suggest, the basic mechanism for the primary type is that there is no such thing as a odour flow but simply a flask contains the sensors array with a fan at the highest to distribute the flow throughout the flask. This type was the design of the primary eNose in 1982.
The second type which is very fashionable now is where the odour flows within the system. Most eNoses designs are made in this way.
To complete this brief historical outlook concerning the eNose, it is a good idea to look at the fundamental schematic comparison between human and electronic noses , summarized in the following two sections.
The Human Nose
There are millions of self generated receptors (over 100 million) with selectivity classes can range from 10 to 100.
The human nose is very adaptive but unlike the eNose, saturation can happen and that is one of the reasons why it operate only for a brief periods of time. Number of odours will be identified, plus it might detect some specific molecules but it surely cannot detect some other varieties of simpler molecules.
As a biological system, infection can take place, which can affect the ability to smell.
And at last, smelling will be related to various experience and memory.
The eNose 
Approximately 5 – 100 chemical sensors manually replaced. As compared with the human nose, it’s not possible to reduce automatically the number of signals to a specific one.
As the eNose continue to develop, it is feasible sooner or later to become adaptive, it’s also unlikely to become saturated and might work for an extended periods of time.
If pattern recognition hardware provided inside the device, then new real-time signal treatment can occur. Unlike the human nose, eNose must be trained for each application. It may well detect simple molecules but it cannot detect some complex molecules at a low concentration.
The eNose can get poisoned (sensors’ malfunction); at the identical time it is feasible for eNose with multi-sensors to be associated with other functions and recognitions.
How eNose work?
Plenty of operation parameters are usually required in order for the eNose with the ability to function “to a maximum effect”. These operation parameters can be:
1. Organising the temperature for the sample incubation
2. The dimensions of the sample.
3. The rate of injection.
4. The quantity of injection.
5. The added solvent getting used.
6. Flow rate.
7. Sensor type.
8. Sensor operational parameters.
The above are just examples; however, there may be other factors as well.
As mentioned briefly earlier on, the principle of eNose is mainly rests with the a number of (an array) of vapour-sensitive detectors (sensors). Usually the detector is made up from certain type of sensitive materials which its characteristic or behaviour change in response to absorbed or adsorbed molecules. As we measure the changes in each sensor, identification will be made for the unknown odour(s) by comparing it with the library data.
eNose devices have been developed over the past 20 years to perform a wide range of identification tasks in various industries. However, merely a number of years ago, the majority of labor and publication related to this field were mostly restricted to the area of research. Today, various types of commercially available eNoses can be purchased anywhere in the world.