The characterization of volatile molecular substances

Abstract

Diffusion for gaseous sources comprising more than one type of substance is examined to show how relative concentrations change with time and distance. The large variations which are predicted make nonsense of the popular assumption that odour or smell is an intrinsic property of the source material. However, some characterization of volatile chemical substances is needed. It is shown that this is possible by creating a uniform and stable atmosphere after the relapse of sufficient time by introducing the gas mixture into an enclosed space. In this investigation the situation is analysed for a spherical enclosure using Fourier analysis techniques for the long timescale behaviour and the Laplace transform for the short timescale solution. The measurement of odours via the response of sensor arrays within a spherical enclosure is considered and a proposal is made for utilizing such an enclosure in a definition of volatile molecular substances (analogous to biological ‘smell’). The conditions for optimum compatibility between an array of sensors and a set of calibrands are discussed and the practical means of effecting such measurements are considered in relation to known types of sensor. It is concluded that the quality of volatile molecular substances is definable and measurable down to very low gas concentrations in air: probably below 10 parts per billion for a wide range of gas mixtures unconstrained by such limitations associated with a biological nose such as toxicity, temperature and subjective evaluation.