Introduction
Electronic Nose (eNose) is a device used to detect and recognize odours/vapours, i.e. a mechanical sniffing 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 odours”
The most common use today for the eNose is within the food and beverage industries. In addition to this field, eNose can be used in other areas, such as qualitative and quantitative analysis of petroleum, detection of explosives, classification and degradation studies of olive oils, development of a field odor detector for environmental applications, control applications quality in the automotive industry. , discrimination between clean and contaminated cow teats in a milking system, analysis of cosmetic raw materials, in addition to many other important areas, such as in the medical and space fields.
The principle of eNose is that it uses an array of sensors, whether 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 surface of the sensor, the electrical conductivity changes as the surface expands. This is the basic idea of how eNose works, i.e. the sensor resistance changes when the sensor is exposed to odours/vapours.
The pattern displayed on the monitor for each particular resistor 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 on the resistance of the eNose sensors.
Brief history
It is difficult to pinpoint the exact date of “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 the design for a machine olfaction 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 just the mechanism itself differs substantially from the basic working principles of eNose.
The name MOD therefore covers devices like eNoses, i.e. multi-sensor devices, as well as single-sensor devices, or those devices that work on 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 an array 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 itself “Electronic Nose” was first used during 1988 and has come into common use. “as a generic term for an array of chemical gas sensors incorporated into an artificial olfactory device” [3][4] after the presentation of this title at a conference covering this field in Iceland in 1991. Since that time, 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 at the 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 odor flows into the system. Most of the eNoses designs are made this way.
To complete this brief historical perspective on eNose, it is 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 self-generated receptors (over 100 million) with selectivity classes that can range from 10 to 100.
The human nose is very adaptable, but unlike eNose, saturation can occur and that’s one of the reasons it works only for short periods of time. It can identify a variety of odors, it can also detect some specific molecules but cannot detect other types of simpler 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 nose [6]
Approximately 5 – 100 chemical sensors replaced manually. 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 is possible that it will become adaptable in the future, it is also unlikely to become crowded and can work for long periods of time.
If pattern recognition hardware is provided within the device, then new 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 (sensors malfunction); at the same time it is possible to associate eNose with multisensors to other functions and recognitions.
How does eNose work?
Typically, a number of operating parameters are required for the eNose to function “to full 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 that is being used.
- Flow rate.
- sensor type.
- Sensor operating parameters.
The above are just examples; however, there may also be other factors.
As briefly mentioned above, the eNose principle is mainly based on one or more (an array) of vapor sensitive detectors (sensors). 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 the data in the library.
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 mostly restricted to the area of research. These days, various types of eNose available in the market can be purchased anywhere in the world.
The reason for the relatively rapid development and commercialization of these devices is because they attracted new interest in their application in the fields of food, environment, medical diagnostics, industries, security, and other related areas.
