Electronic nose instrumentation

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]. 

Liquid samples can be collected from the bioreactor sampling port and introduced into the electronic nose instrument manually. More appealing in bioprocessing is to sample on-line. An electronic nose system monitors non-in-vasively by sampling from the off-gas port of the bioreactor [23]. The humidity... 

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 At sample injection the gas in the container is withdrawn by a suction pump placed in the electronic nose instrument.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. 

A brief summary of electronic nose instruments currently available is now presented, followed by a discussion of strategies that may be or are being used to reduce the detection threshold of sensor-based electronic noses and hence increase their suitability to the ultimate detection of explosive materials. Readers interested in further details on electronic noses are directed towards a comprehensive book published in 2003 [5] that... 

In 2003 there were at least 17 companies manufacturing and selling electronic nose instruments of various types and these companies are summarized in Table 3 together with the sensor technology employed and their application(s). 

As part of a project aimed at producing an electronic nose instrument that can be used in the field for odour detection and measurement, we have developed a fully portable prototype system that has been tested in the field. 

It has been a long term goal of many researchers to use instrumental means to replace some sensory functions. The use of sensory panels for quality control purposes presents many problems which may be minimized through the use of supplementary instrumental techniques. Over the years, gas chromatography and mass spectrometry have found limited application for this purpose. Recently an instrument generically called an "electronic nose" has been commercialized. This paper will present a brief overview of gas chromatographic and mass spectral techniques used to monitor flavor quality in foods but focus on the new electronic nose instruments. 

Future perspectives for the electronic nose research field are listed below. They concern both expected sensing and technical sensing and performance. Improvement of sensing performance of the instrument ... 

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]. 

Hodgins, D. (1997). The electronic nose Sensory array-based instruments that emulate the human nose. In "Techniques for Analyzing Food Aroma", (R. Marsili, Ed.), pp. 331-371. Marcel Dekker, Inc., New York. 

E. Kress-Rogers, Sensors for food flavour and freshness electronic noses, tongues and testers. In E. Kress-Rogers and C.J.B. Brimelow (Eds.), Instrumentation and Sensors for the Food Industry, 2nd ed., Woodhead Publishing Ltd., Cambridge, UK, 2001, pp. 553-622, Chap. 19. 

---