Chemical sensors electronic nose

L.B. Kish, R. Vajtai, C.-G. Granqvist, "Extracting information from noise spectra of chemical sensors single sensor electronic noses and tongues", Sensors and Actuators B 71 (2000) 55. 

Analysis of the headspace above a sample is an alternative to direct measurement and chemical extraction techniques. Instrumentation for the detection of headspace volatiles includes an array of gas sensors (electronic noses), online real-time chemical ionization apparatus, etc. Some of these instrumental analysis methods are described below. 

Using the principles of biological olfaction, electronic nose systems contain arrays of different types of cross-reactive vapor-sensitive sensors. While it is difficult to discriminate analytes entirely by their responses to a single type of sensor, using an array of sensors yields response patterns that can readily distinguish many different vapors. Ideally, the response mechanisms of the sensors are highly varied and encompass both physical and chemical phenomena1. 

A sensor array named the electronic nose is a rapid and relatively simple technique that can be used for monitoring wastewater odors (Stuetz et al., 2000). The electronic nose uses sensors of varying affinities to characterize an odor without reference to its chemical composition. 

A chemical sensor array (consisting of eight conducting polymer sensors) derived from an electronic nose [62], for the characterization of headspace gas from a sparged liquid sample... 

Chemical sensors are becoming more and more important in any area where the measurement of concentrations of volatile compounds is relevant for both control and analytical purposes. They have also found many applications in sensor systems called electronic noses and tongues. 

Varieties of polymers are also employed as sensitive material for electronic nose applications, and the operating temperature may reach about 100 ° C. In the case of quartz microbalance-based sensors a large role is played by the chemically interactive material (CIM) on which it is deposited. A rather efficient room temperature operating CIM is the metal-porphirin, by which it is possible to construct varieties of nostrils, just changing the type of coordinated metal. Interesting metals success-... 

Arrays were introduced in the mid-eighties as a method to counteract the cross-selectivity of gas sensors. Their use has since become a common practice in sensor applications [1], The great advantage of this technique is that once arrays are matched with proper multivariate data analysis, the use of non-selective sensors for practical applications becomes possible. Again in the eighties, Persaud and Dodds argued that such arrays has a very close connection with mammalian olfaction systems. This conjecture opened the way to the advent of electronic noses [2], a popular name for chemical sensor arrays used for qualitative analysis of complex samples. 

Keywords electronic nose principal component analysis pattern recognition chemical sensors sensor arrays olfaction system multivariate data analysis. 

Zuppa et al.60 have used SOMs in the assessment of data from an electronic nose. Six chemicals—water, propanol, acetone, acetonitrile, butanol, and methanol—were presented at varying concentrations to a 32-element conducting polymer gas sensor array. The output was used to train a group of SOMs, rather than a single SOM, to avoid the problems of parameter drift. One SOM was associated with each vapor, and with suitable use of smoothing filters, the SOM array was found to perform effectively. 

In the past few decades, a precise methodology has been developed for sensory evaluation and has proved to give reliable results [826]. Increasingly, in recent years chemical sensor systems ( electronic noses ) are used for such purposes [826a]. 

The performance of common multisensor arrays is ultimately determined by the properties of their constituent parts. Key parameters such as number, type and specificity of the sensors determine whether a specific instrument is suitable for a given application. The selection of an appropriate set of chemical sensors is of utmost importance if electronic nose classifications are to be utilised to solve an analytical problem. As this requires time and effort, the applicability of solid-state sensor technology is often limited. The time saved compared with classic analytical methods is questionable, since analysis times of electronic nose systems are generally influenced more by the sampling method utilised than the sensor response time [185]. 

James, D., Scott, S.M., Ali, Z., O Hare, W.T. (2005) Review Chemical sensors for electronic nose systems. Microchim. Acta 149 1-17. 

With respect to the type of sensors that can be used in an electronic tongue, practically all the main families of chemical sensors have been used to form the sensor array, viz. potentiometric, voltammetric, resistive, gravimetric and optical, if main sensor families have to be quoted [11], Table 30.1 sketches a survey of different approaches that can be recorded when the specialized literature is inspected. Even hybrid systems have been proposed, mainly those combining potentiometric and voltammetric sensors [3,12], The combination of electronic noses and electronic tongues to improve detection or identification capabilities, in a sensor fusion approach, has also been proposed [13,14],... 

Electronic noses provide new possibilities for monitor the state of a cultivation non-in-vasively in real-time. The electronic nose uses an array of chemical gas sensors that monitors the off-gas from the bioreactor. By taking advantage of the off-gas components different affinities towards the sensors in the array it is possible with the help of pattern recognition methods to extract valuable information from the culture in a way similar to the human nose. For example, with artificial neural networks, metabolite and biomass concentration can be predicted, the fermentability of a medium before starting the fermentation estimated, and the growth and production stages of the culture visualized. In this review these and other recent results with electronic noses from monitoring microbial and cell cultures in bioreactors are described. 

Subsequently, other researchers developed the electronic nose idea with a variety of chemical gas sensor arrays using different pattern recognition techniques for improving the interpretation of responses [2-5]. 

A variety of chemical gas sensors are or could be used in electronic nose instruments. So far, successful results have been reached with conductive polymer (CP) sensors, metal oxide semiconductor (MOS) sensors, metal oxide semiconductor field effect transistor (MOSFET) sensors, quartz crystal microbalance (QCM) sensors, and infrared sensors. 

Fig. 2. The principle configuration of an electronic nose system where the analyte mixture is contacted with a chemical sensor array that produces raw data which subsequently are treated with a pattern recognition algorithm that delivers the predicted result...

The ethanol concentration in the medium of a Saccharomyces cerevisiae cultivation can be monitored from its content in the gas phase by directly recording the current from a chemical MOS sensor [28]. The accuracy of such a measurement was significantly improved by using an electronic nose with five sensors in the array and recognizing the response pattern with ANN [29, 30]. The sensors were a combination of MOS and MOSFET sensors selected from a PCA loading plot. Data sets from three cultivations were used to train the ANN. When the trained net was applied on new cultivations the ethanol was predicted with a mean square error (RMSE) of 4.6% compared to the off-line determined ethanol (Fig. 6). With only one sensor the RMSE was 18%. 

Electrochemical nose — (electronic nose e-nose or artificial nose ) is an array of chemical and/or - electrochemical sensors mimicking the physiological olfac-... 

Arrays of sensors have received a great deal of interest for many reasons. In the case of physical sensor devices, an array of sensors can provide vastly improved spatial resolution of the phenomenon being measured. By combining sensors of different types, a more comprehensive measurement can be made, perhaps over a much wider dynamic range of measurand values. In the case of chemical sen sors, arrays have grown in popularity as a means to enhance selectivity and versatility. Indeed, preliminary efforts in this direction have proved the concept of building an electronic nose based on the acoustic array sensor/pattem recognition approach [43,44]. 

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