Electronic nose MOSFET

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. 

The sensors of the electronic nose are assembled in an array. The array is normally a small electronic unit that integrates the different sensors into a practical circuit card or another appropriate system that is easy to insert into the electronic nose instrument. If the array is to be used in a flow injection setup the unit also comprises a flow cell compartment with minimal volume. The system depicted in Fig. 2 shows how MOS and MOSFET arrays are integrated in a flow injection system [11]. Larger arrays can be integrated into silicon chips, as described for CP sensors where, for example an ASIC chip with 32 sensors has been fabricated with BiCMOS technology and having an area of 7 x 7 mm [18]. If the array is be inserted in the headspace volume of a bioreactor, the technical solution is a remote array probe that can be placed in a gas sample container [19]. 

A typical sensor array system interfaced to a bioreactor representative of the studies described in this paper is shown in Fig. 3. The bioreactor off-gas is conducted to a container by its overpressure. At sample injection the gas in the container is withdrawn by a suction pump placed in the electronic nose instrument. The sample gas passes the sensor array, which is distributed over three serially coupled units. In the example in Fig. 3 the first unit contains ten MOSFET sensors, the second six MOS sensors and the third unit an infrared sensor. After injection, a valve is switched to a reference carrier gas, taken from the ambient air or from a gas flask with a controlled composition. 

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%. 

Many methods have been used for the determination of the floral and geographical origin of honey, and these include analysis of pollen content, sensory analysis, volatile compounds, phenols, and markers [32], Some of these, however, are time and labor intensive and/or require technical personnel. With this in mind, Benedetti and colleagues have used an electronic nose equipped with 10 MOSFET s and 12 MOS sensors to generate a pattern of the volatile compounds present in 70 honey samples... 

Electronic noses The so-called electronic noses consist of chemical gas sensors that are able to monitor changes in the offgas composition of fermentation processes. The different sensors of electronic noses are based on conductive polymers (CP), metal oxide semiconductors (MOS), metal oxide semiconductor field effect transistors (MOSFET), or quartz crystal microbalance (QCM). CP-based sensors use the electrochemical properties of polymers like polypyrrole or polyindole. The absorbance of selected molecules of the off-gas into the polymer film causes changes in the sensors conductivity. MOS sensors possess an electrochemically active surface of metal oxides like tin oxide or copper oxide. The sensitivity... 

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