What is the Electronic Nose?
Introduction
Electronic Nose (eNose) is a device used to detect and recognize odours/vapours, i.e. a mechanical olfactory device with a series of chemical sensors.
Alternatively, according to the definition of Gardner and Bartlett, (1994) [3]:
“An electronic nose is an instrument comprising an array of electronic chemical sensors with partial specificity and an appropriate pattern recognition system, capable of recognizing simple or complex odors”
The most common use today for the eNose is within the food and beverage industries. Besides this field, eNose can be used in other areas, such as qualitative and quantitative analysis of petroleum, detection of explosives, studies of classification and degradation of olive oils, development of a field odor detector for environmental applications, control applications quality in the automotive industry. , discrimination between teats of clean and contaminated cows in a milking system, analysis of cosmetic raw materials, in addition to many other important areas, such as in the medical and space field.
The principle of eNose is that it uses an array of sensors, either in the form of different types of polymers or through the use of metal oxide semiconductors, the principle here remains the same.
When molecules of any element are deposited on the sensor surface, the electrical conductivity changes, as the surface expands. This is the basic idea of how eNose works, ie the change in sensor resistance when the sensor is exposed to odours/vapours.
The pattern displayed on the monitor for each particular resistance is unique (ie, the type of odor or vapor from a particular sample). In this way, it is possible to distinguish one sample from another or the state/condition of the sample itself, since the headspace of each sample has a unique signature in the resistance of the eNose sensors.
Brief history
It is difficult to pinpoint the exact date “when and how” the idea of designing a system that could mimic the human nose arose. However, the following dates with devices provide a better understanding of how design for a machine sniffing device (MOD) system progressed. The MOD design ultimately led to the conceptualization of the eNose.
Please note that eNose differs from other types of MOD simply by having multiple sensors, while other devices may have only one sensor or simply the mechanism itself differs substantially from the basic principles of operation of eNose.
The name MOD, therefore, covers devices like eNoses, that is, devices with multiple sensors, as well as devices with single sensors, or those devices that work with different design principles.
The following four dates are important in the history and development of eNose:
1. The manufacture of the first gas sensor, Hartman 1954
2. Construction of a matrix of 6 thermistors, Moncrief 1961
3. First electronic nose, Persaud and Dodd, 1982
4. Ikegami matrix (Hitachi Research Laboratory, J) for odor quality – 1985
Thus, the first recorded scientific attempt to use sensor arrays to emulate and understand mammalian olfaction was made by Persaud and Dodd in 1982. [3]at the University of Manchester Institute of Science and Technology.
A device was built with an array of three metal oxide gas sensors that are used to discriminate between twenty odorous substances. By visually comparing the proportions of the sensor responses, they derived the pattern classification.
The name “Electronic Nose” itself was first used during 1988 and has come into common usage. “as a generic term for a series of chemical gas sensors incorporated into an artificial smelling device” [3][4] after the introduction of this title at a conference covering this field in Iceland in 1991. Since then, the idea and principles of eNose have grown and developed in different fields around the world.
Historically speaking, there are two different types of eNos (Pearce 1997):
- Static odor delivery.
- Mass flow systems.
As the two names suggest, the basic mechanism for the first type is that there is no odor flow, but simply a flask containing the sensor array with a fan on top to distribute the flow within the flask. This type was the design of the first eNose in 1982.
The second type that is very popular now is where the smell flows into the system. Most eNoses designs are made this way.
To round out this brief historical perspective on eNose, it’s a good idea to look at the basic schematic comparison between human and electronic noses. [6]summarized in the next two sections.
the human nose[6]
There are millions of autogenerated receptors (more than 100 million) with selectivity classes that can vary from 10 to 100.
The human nose is very adaptable, but unlike eNose, saturation can occur and that’s one of the reasons why it only works for short periods of time. It can identify a variety of odors, plus it can detect some specific molecules but cannot detect other, simpler types of molecules.
As a biological system, infection can occur, which can affect the ability to smell.
And finally, smell can be associated with various experiences and memories.
the eNose [6]
Approximately 5 – 100 manually replaced chemical sensors. Compared to the human nose, it is not possible to automatically reduce the number of signals to a particular one.
As eNose continues to develop, it may become scalable in the future, it is also unlikely to saturate, and may work for long periods of time.
If pattern recognition hardware is provided within the device, then further signal processing can occur in real time. Unlike the human nose, eNose must be trained for each application. It can detect simple molecules but cannot detect some complex molecules in a low concentration.
The eNose can be poisoned (malfunction of the sensors); at the same time it is possible to associate eNose with multisensors to other functions and recognitions.
How does eNose work?
In general, a number of operating parameters are required in order for the eNose to function “at maximum effect”. These operating parameters can be:
- Setting the temperature for sample incubation
- The size of the sample.
- injection rate.
- The amount of injection.
- The added solvent being used.
- flow rate.
- sensor type
- Sensor operating parameters.
The above are just examples; however, there may be other factors as well.
As briefly mentioned above, the principle of eNose is mainly based on one or more (an array) of detectors (sensors) sensitive to vapor. Typically, the detector is made up of certain types of sensitive materials whose characteristics or behavior change in response to absorbed or adsorbed molecules. As we measure changes in each sensor, unknown odors can be identified by comparing them to library data.
Conclusion
eNose devices have been developed over the last 20 years to perform a variety of identification tasks in various industries. However, just a few years ago, most of the works and publications related to this field were mainly restricted to the research area. These days, you can buy various types of eNose available in the market anywhere in the world.
The reason for the relatively rapid development and commercialization of these devices is because they have attracted new interest in their application in the fields of food, environment, medical diagnosis, industries, security, and other related areas.
