What is Electronic Nose?

What is Electronic Nose?




Introduction

Electronic Nose (eNose) is a device used to detect and recognize odours/vapours, i.e. a machine olfaction device with an range of chemical sensors.

Alternatively, according to Gardner and Bartlett, (1994) definition [3]:

“An electronic nose is an instrument which comprises an range of electronic chemical sensors with uncompletely specificity and an appropriate pattern recognition system, capable of recognizing simple or complicate odours”

The most shared use at the present time for the eNose is within the food and drink industries. In addition to this field, eNose can be used in other areas such as petroleum qualitative and quantitative examination, 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 polluted cows’ teats in a milking system, cosmetic raw materials examination, plus many other important areas such as in the medical and space fields.

The rule of eNose is that it uses an range of sensors, whether in the form of different types of polymers or via the use of metal oxide semi conductors, the rule here is nevertheless the same.

When molecules from any component deposited on the surface of the sensor, the electrical conductivity changes, as and when the surface expands. This is the basic 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 rare (i.e. the kind of odour or vapour of a particular sample). In this way it is possible to discriminate a sample from another or the state/condition of the sample itself, as the headspace from each sample has a rare identifying characteristics on the eNose sensors resistance.

fleeting History

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 ultimately for the conceptualisation of the eNose.

Please observe 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 significantly from the eNose basic working principles.

The name MOD, consequently, cover devices such as eNoses i.e. devices with multiple sensors, in addition as devices with single sensors – or those devices which function 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 range of 6 termistors, Moncrief 1961

3. First Electronic Nose, Persaud and Dodd, 1982

4. Ikegami (Hitachi Research Laboratory, J) range for odour quality – 1985

consequently, the first recorded scientific attempt to use sensor arrays to emulate and understand mammalian olfaction was carried out by Persaud and Dodd in 1982 [3], at the University of Manchester Institute of Science and Technology.

A device was built with an range 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 shared usage “as a generic term for an range of chemical gas sensors incorporated into an artificial olfaction device” [3][4] after the introduction of this title at a conference covering this field in Iceland 1991. From 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 kind is that there is no odour flow but simply a flask contains the sensors range with a fan at the top to spread the flow within the flask. This kind was the design of the first eNose in 1982.

The second kind which is very popular now is where the odour flows within the system. Most eNoses designs are made in this way.

To complete this fleeting historical outlook concerning the eNose, it is a good idea to look at the basic schematic comparison between human and electronic noses [6], summarized in the following two sections.

The Human Nose[6]

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 function only for a short periods of time. Variety of odours can be identified, plus it can detect some specific molecules but it cannot detect some other types of simpler molecules.

As a biological system, infection can take place, which may affect the ability to smell.

And finally, smelling can be associated with various experience and memory.

The eNose [6]

Approximately 5 – 100 chemical sensors manually replaced. In comparison with the human nose, it is not possible to reduce automatically the number of signals to a particular one.

As the eNose continue to develop, it is possible in the future to become adaptive, it is also doubtful to become saturated and can work for a long periods of time.

If pattern recognition hardware provided within the device, then new real-time signal treatment can occur. Unlike the human nose, eNose needs to be trained for each application. It can detect simple molecules but it cannot detect some complicate molecules at a low concentration.

The eNose can get poisoned (sensors’ malfunction); at the same time it is possible for eNose with multi-sensors to be associated with other roles and recognitions.

How eNose work?

A number of operation parameters are usually required in order for the eNose being able to function “to a maximum effect”. These operation parameters can be:

  1. Setting up the temperature for the sample incubation
  2. The size of the sample.
  3. The rate of injection.
  4. The quantity of injection.
  5. The additional solvent being used.
  6. Flow rate.
  7. Sensor kind.
  8. Sensor operational parameters.

The above are just examples; however, there can be other factors in addition.

As mentioned briefly earlier on, the rule of eNose is mainly rests with the one or more (an range) of vapour-sensitive detectors (sensors). Usually the detector is made up from certain kind 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 can be made for the unknown odour(s) by comparing it with the library data.

Conclusion

eNose devices have been developed over the past 20 years to perform a variety of identification responsibilities in various industries. However, merely a few years ago, the majority of work and publication related to this field were mostly restricted to the area of research. These days, various types of commercially obtainable eNoses can be purchased anywhere in the world.

The reason for the comparatively fast development and commercialization of these devices is because they attracted new interest in their application in the fields of food, ecosystem, medical diagnosis, industries, security and other related areas.




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