Electronic nose
An electronic nose is a technical system for the measurement of odors . For this purpose, microelectronic gas sensors generate electronic signals . The term electronic nose combines the "recognition" of smells with the technical implementation with electronic sensors. It should be noted that there cannot actually be an electronic nose, since smells have to be interpreted by the brain , whereas the technical measuring system only supplies data on gas concentrations , both odorless and odor-active gases.
Biological background
The inspiration for the electronic nose comes from the biological model, the olfactory system . In a very simplified way, the olfactory system consists of olfactory cells in the nose that are activated by odorous substances. About 350 different types of olfactory cells are active in humans, of each type several 10,000 are present in the olfactory mucous membrane. The signals of the related olfactory cell types are grouped together on mitral cells in the olfactory bulb, part of the brain. The signals generated there are primarily processed by the limbic system , which is where the strong emotional connection of the sense of smell originates. The perception of an odor is only the last stage in the effect of odorous substances on the olfactory cells.
The experiences of evolution are represented in the human sense of smell . Like all other senses, the olfactory sense provides vital data about the environment. In order to find food, its specific gas components must be sensitively detected. The sense of smell is therefore particularly sensitive to the smells of ripeness and food aromas. The same applies to odors from hazards ( poisons ) and those associated with social functions. So every person has a very specific smell that is genetically coded. The technical measuring systems lack these evolutionary conditions. Chemically similar things are detected with comparable signal strength. Odorless gases such as methane (CH 4 ), carbon dioxide (CO 2 ) or carbon monoxide (CO) are also measured.
Instead of the olfactory cells, various gas sensors are used in the electronic nose. They cover the largest possible range of gaseous compounds in the air. The intention is not to measure individual gas components, but rather a metrological mapping of the composition of the air sample. In fact, the electronic nose analogy to vision is more accurate. The three color channels of the visual sense create the colored image of the world in the perception process with the three associated intensities. The few gas sensors of an electronic nose (from about 6 to 40) also generate an image of the measured air sample. This image can be combined with additionally measured parameters, in the case of odor measurement with the odorous substance concentration measured by human sensory via olfactometry .
technology
A typical electronic nose consists of a number of gas sensors, the signals of which are processed by mathematical methods in the sense of pattern recognition . The pattern here means the ratio of the signal strengths of the individual sensors to each other, which can be imagined as a geometric pattern plotted around a common center point ( radar plot ) as a star with beams of different lengths. In practice, however, more abstract mathematical methods are used, such as principal component analysis , with which the pattern information is mapped onto a two-dimensional plane in which the pattern similarity is represented in delimited areas. Technical gas sensors do not work like olfactory cells. The olfactory cells contain receptor molecules that interact very specifically with a few gases and thus generate a nerve signal. At the same time, there is a high level of signal amplification, so that a few gas molecules, ie very low concentrations, are sufficient to activate an olfactory cell. A number of different technical sensors are used in electronic noses. The main representatives are:
- Sensors based on semiconducting metal oxides, abbreviated to MOX sensors
- Sensors with electrically conductive polymers, a distinction is made between independently (intrinsically) conductive polymers and those to which a conductive component such as graphite has been added.
- Sensors that utilize a mass effect with the two groups of oscillating quartz sensors (QMB / QCM sensors) and surface acoustic wave sensors (SAW sensors).
Each of these sensor types has its own special features in terms of measurement technology. In particular, the chemical range of the gases measured differs. MOS sensors measure preferentially low molecular weight oxidizable gases, with conductive polymers polar gas components are measured well, mass-sensitive sensors measure higher molecular substances preferentially. In technical applications, however, the measurement stability of the sensors in particular is a crucial parameter over long periods of time, since the calibration information must be obtained. Depending on the operating principle of the sensors and the specific application, measures must therefore be taken to protect and monitor the sensors.
The gas sensors used in electronic noses are often constructed in the form of specially grouped gas sensors, some of which are attached to a single microchip. One speaks here of sensor arrays or chemosensor arrays. Instead of the term electronic nose, the term chemosensor arrays is often used.
Odor measurement with electronic noses
The measuring system of the electronic nose is not designed to be an odor measuring system. The differences to the biological sense of smell are too great for that, the clearest of which is the measurement of completely odorless gases by the broadband gas sensors. However, in many applications, a chemosensory measuring system can be turned into an electronic nose with a coherent methodology and a calibration using olfactometric measurements.
The reference problem of the electronic noses
In order to carry out odor measurements with gas measurement systems, reference data on odor are required. The standardized measuring technique is olfactometry according to the European standard EN 13725. This measuring method measures the odorous substance concentration, which indicates how much an odor sample has to be diluted before it becomes odorless to an average smell. A sample of 3000 UU / m 3 must therefore be diluted in a ratio of 1: 3,000. It turns out that the olfactometric measuring method only delivers a very uncertain measured value. The fluctuations with repeated measurements or when comparing several laboratories with one another are very large. The measurement uncertainty is between four times and a quarter of a measured value. A measured value of 1000 GE / m 3 has a measurement uncertainty interval of 250 to 4000 GE / m 3 . The asymmetry is due to the actually logarithmic nature of the measured value.
This measurement uncertainty of the reference measurement method is of great importance for the calibration of electronic odor measurement systems. This effect must be taken into account in the mathematical modeling.
Applications of the electronic nose
Electronic noses are suitable for continuously observing sources of smell. In contrast to olfactometry with human test smells, which can only make random sample measurements, permanent monitoring is possible with an odor measuring system that is carefully tailored to the application. This is useful for problematic odor sources, such as industrial plants, sewage technology and waste management. Continuous monitoring can help protect local residents.
The first large-scale application of continuous odor monitoring can be found in upper-class automobiles. It is the ventilation flap control which - in the event of a traffic jam or driving through tunnels - automatically shuts off the supply air when exhaust gases are sucked in. In the actual sense, however, odorous substances are not measured, but ratios of the indicator gases such as CO and NO x . The system can therefore only be used for this application.
Another field of application of the electronic nose is quality monitoring. Food and luxury foods can be characterized by outgassing aromas and perfumes and other, also odorless components. Whenever a constant product composition is required, electronic noses can be used here, which are particularly suitable for determining a constant composition of these outgassing.
Electronic noses can now also be used to prove the use of cannabis products .
literature
- JW Gardner, PN Bartlett: (1994) A Brief History of Electronic Noses, Sensors and Actuators B, 18, Nos. 1-3, pp. 211-220
- P. Boeker et al .: (2003) Methodology and technology of online odor measurement, hazardous substances - keeping the air clean , 63, No. 7–8, pp. 283–289 Download (PDF; 780 kB)
- P. Boeker, T. Haas .: (2007) The measurement uncertainty of olfactometry, hazardous substances - keeping the air clean, Volume 67, No. 7–8, pp. 331–340 Download (PDF; 331 kB)
- P. Boeker: (2010) Electronic noses: the methodical concept and its problems, part 1: Introduction and problem situation, hazardous substances - keeping the air clean, Volume 70, No. 7–8, pp. 314–320 Download (PDF; 314 kB )
- P. Boeker: (2010) Electronic noses: the methodical concept and its problems, Part 2: Methodical application, hazardous substances - keeping the air clean, Volume 70, No. 10, pp. 431–436 Download (PDF; 209 kB)
- Use of electronic noses in medicine: Possibilities and limits , article frommedizin & technik, 2/2012
Individual evidence
- ^ A. Voss, K. Witt, T. Kaschowitz u. a .: Detecting cannabis use on the human skin surface via an electronic nose system , Sensors (Basel), 2014, Jul 23; 14 (7), 13256-72, PMID 25057136