The application of microwave-frequency spectrometry, permittivity and loss measurements to chemical analysis

The frequency dependence of the interaction between microwave radiation and matter gives chemical information concerning both the bulk and molecular structure of gases, solids and liquids. Such chemical analysis has been investigated over the last four decades, but has only recently become generally accessible through the introduction of low-cost, microwave solid-state devices. The theory is reviewed, for both pure and mixed substances, to show the link between the electrical and chemical approaches. The principles, hardware and chemical physics of microwave rotational spectroscopy for multicomponent gas analysis are discussed. Microwave permittivity measurements as applied to two component mixtures are described with particular reference to the determination of the moisture content of natural products. Remote sensing of hydro-meteors is attracting increasing attention, especially in regard to propagation of microwaves in the millimetre regions.