In order to use polymers in a sensor device, microfabrication techniques are employed to form two electrodes separated by a gap of 10 to 20 μm. A given compound's affinity for a polymer and its effects on the polymer's conductivity are strongly influenced by the counter-ions and functional groups attached to the polymer backbone. The interaction affects the transfer of electrons along the polymer chain, that is to say, its conductivity. Changes in the conductivity of these materials occur as they are exposed to various types of chemicals, which bond with the polymer backbone. Here, the active material is a conducting polymer from such families as the polypyrroles, thiophenes, indoles or furans. Polymer sensors can also be used as electronic nose conductivity sensors. However, the low cost and wide availability of this type of sensor makes it the most widely used type of gas sensor. A further disadvantage is that the sensors are susceptible to poisoning by sulfur compounds present in the odorant mixture. To counteract this, signal processing algorithms are usually employed. The sensors also respond to water vapour, more specifically to humidity differences between the gas sample being analysed and a known reference gas used to initialise the sensor.Ī disadvantage of metal oxide sensors is that the baseline response is prone to drift over periods of hours to days. Sensor sensitivity ranges from 5 to 500 ppm. Selectivity can be improved by altering the operating temperature. Typically the active metal oxide sensor material is designed to enhance the response to specific odorants, such as carbon monoxide or ammonia. As a VOC passes over the doped oxide material, the resistance between the two metal contacts changes in proportion to the concentration of VOC. Micromachining is often used to thin the sensor substrate under the active material, so that power consumption and heat dissipation requirements are reduced. At these elevated temperatures, heat dissipation becomes a major consideration of the mechanical design of the sensing chamber. The doped semiconducting material with which the VOC's interact is deposited between two metal contacts over a resistive heating element, which operates at 200° C. Typical offerings include oxides of Sn, Zn, Ti, W and Ir, doped with a noble metal catalyst such as palladium or platinum. Of the two types, metal oxide semiconductors have been used more extensively in electronic nose instruments and are widely available commercially. There are two types of conductivity sensors that are used for electronic nose applications: metal oxide and polymer, both of which exhibit a change in resistance when exposed to volatile organic compounds (VOC's). Electronic nose sensors fall into several different categories: conductivity sensors piezoelectric sensors MOSFETS and optical sensors. Many commercial applications are available for doing this, which uses a variety of different sensors. The purpose of these applications is to try to associate a gas sample with some form of label. In the last few years there has been increasing interest in artificial olfaction applications, also termed “artificial nose applications”. The present invention relates to improvements in gas discharge spectroscopy.
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