Odours — The Stimulus for an Electronic Nose

Odorants form a large collection of organic molecules with a molecular mass in the range 18–300 Daltons. There is typically a single polar group in an odorant molecule although there are some notable exceptions to this rule. In laboratory investigations on both the human sense of smell and in electronic noses, it is convenient to use pure single odorants as the odour source. In many cases, the odour profile of these odorants is modified by levels of impurities which would be of no consequence in areas of chemistry other than osmochemistry. General trends in the relationship between odour type and molecular structure are known, but the underlying quantitative structure-odour relationships are poorly understood. In addition to the odour note, the other key properties are the threshold value and the intensity. Both of these properties are used in the design of functional smells, such as the perfuming of washing products, and both are expected to be important for the behaviour of electronic noses. Natural smells, especially perfumes and flavours, are extremely complex mixtures of odorants. The interactions between the responses to the individual odorants which give rise to the complexity of the subjective sensation are not well understood. However, in some notable cases, key impact flavour molecules play the dominant role in the generation of flavours. In addition to the problem of complexity, natural smells are frequently labile. Many key odorants undergo oxidative degradation to produce a variety of off-odours. Finally, this chapter discusses the olfactory system and the industrial need for an instrument that is capable of detecting odours.