Odour sensors

Equation (1) shows a basic reaction in which the target analyte A binds to the substrate material S. The progress of this reaction can be monitored in a wide variety of manners. The most common t q)e of odour sensor is a resistive one, i.e. the bulk resistance (or conductance) of the substrate changes when the analyte is introduced. Examples of this are the tin oxide... 

One simple approach has been that of Cole et al. who propose the use of two sensors run in a ratiometric mode [12]. Figure 9 shows a CMOS chip that drives a pair of resistive odour sensors one polymer sensor is active while the other is passivated. The output of the ratio of the resistances is amplified rather than the absolute resistance of just one sensor. This removes at the input stage of the amplifier any common mode signals, such as temperature drift. 

Figure 9. Principle of ratiometric CMOS odour sensor to enhance signal-to-noise ratio and photograph of realized CMOS chip [12].

Sanchez-Montanes, M.A., Pearce, T.C. Fisher Information and Optimal Odour Sensors. Neurocomputing 38, 335-341 (2001)... 

Stussi E, Stella R, De Rossi D (1997) Chemoresistive conducting polymer-based odour sensors influence of thickness changes on their sensing properties. Sens Actuators B 43 180-185... 

Bartlett, P.N. Gardner, J.W. Odour sensors for an electronic nose. NATO ASI Series, Sensors and Sensory Systems for an Electronic Nose, Reykjavik, Island, 1991, p. 31. 

K. C. Persaud, J. Bartlett, and P. Payne, Design strategies for gas and odour sensors, in Robots and Biological Systems (P. Dario, ed.), NATO ASI Series, Springer-Verlag, Berlin, 1992, p. 579. 

Fig. 36.1 An odour station being visited by a U. caudimaculatus, showing the odour source in cage at front of picture and IR sensors on each side... 

A working group on odours of the Inspectorate for the Environment in The Netherlands proposed, in 1983, an air quality standard for odour concentration in dwellings around odour sources (3). Because odour concentrations always have to be assessed sensorically, a guideline for olfactometric measurement was also included in the same report. Detailed recommendations were not included. In 1985 a draft chapter on odours will be pubished in the Air Pollution Control Manual with more details and comments on the measuring methods in use in The Netherlands (4). 

Although some research institutes started already in the middle of the seventies to measure sensorically odour emissions, it was not until 1980 before 4 different olfactometers were compared with each other for the first time. Great differences were found ranging from a factor of 3 to 40. A relationship between the differing compounds tested and the factor number was also noticed. The differences between the two TNO olfactometers almost disappeared after setting both on the same flow rate of 16 1/min. (7). 

The use of the human subject as a sensor in olfactometry leads to a number of physical, methodological and panel composition requirements. Olactometers used in odour pollution assessment should be able to deliver ranges of concentrations varying in dilution from 1 to 10° at volumes between 1.8 and 3.6 m3/h without changing the pressure at the nose entrance more than 5 mbar. 

N. A. Rakow and K. S. Suslick. 2000. A Colorimetric Sensor Array for Odour Visualization. Nature 406 710. 

Nitz, S., Kollmannsberger, H., Lachermeier, C., Horner, G. (1999) Odour assessment with piezoelectric quartz crystal sensor array, a suitable tool for quality control in food technology Adv. Food. Sci. 21 136-150. 

J.W. Gardner and P.N. Bartlett, Pattern recognition in odour sensing. In Sensors and Sensory Systems for an Electronic Nose, Vol. 212, Kluwer Academic Publishers, Dordrecht, 1992, pp. 161-179. 

Atema, J., Eddy chemotaxis and odour landscapes exploration of nature with animal sensors, Biol. 

Rakow NA, Sushck KS. A colorimetric sensor array for odour visualization. Nature 2000 406 710-12. 

Given that an array sensor is being developed, how may they be validated I know of no system that has undergone validation for accreditation purposes. Even compared with environmental analysis, these sensors present difficulties that need to be resolved, before a consensus on suitable validation protocols is reached. Gardner and Bartlett have considered the problem of how to define the performance of electronic noses using standard odour mixtures [21]. They propose two indicators of performance, the range of different odours that may be detected by the array, and the ability to discriminate between similar odours (the resolving power). 

Buhlmann, K. Schlatt, B. Cammann, K. Shulga, A., Plasticised polymeric electrolytes New extremely versatile receptor materials for gas sensors (VOC monitoring) and electronic noses (odour identification discrimination), Sens. Actuators B 1998, 49, 156-165. 

The artificial intelligence systems to which sensor arrays are coupled supply the closest likeness to the human olfactory system. Some of the recent theories on olfaction require that the human nose has only relatively few types of receptor, each with low specificity. The activation of differing patterns of these receptors supplies the brain with sufficient information for an odour to be described, if not recognized. As a consequence of this belief, the volatile chemical-sensing systems commercially available only contain from 6 to 32 sensors, each having relatively low specificity. Statistical methods such as principal component analysis, canonical discriminant analysis and Euclidian distances are used for mapping or linked to artificial neural nets as an aid to classification of the odour fingerprints . 

Friedman J H 1991 Multivariate adaptive regression splines Ann. Stat. 19 1-141 Ema K, Yokoyama M, Nakamoto T and Moriizumi T 1989 Odour-sensing system using a quartz-resonator sensor array and neural-network pattern recognition Sensors Actuators 18 291-6... 

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