Microwave resonators for highly sensitive compositional analysis of solvents in microcapillary systems

The ability to precisely analyse the composition of liquid mixtures by non-contact techniques in both static and flow situations is extremely desirable for a diversity of industrial, analytical and quality control procedures. Microwave resonators allow very accurate and sensitive characterisation of the dielectric properties of polar liquids due to the strong interaction of the latter with microwave electric fields. They have the useful dual role of both exact characterisation of the complex permittivity of a dielectric sample when it is inserted within a region of high electric field of the resonator, and effective volumetric heating of the same sample if its dielectric loss is large enough to permit heating. They offer tremendous potential for investigation of very small amounts of polar solvents in non-polar hosts. In this regard they are superior to other traditional composition analysis techniques such as liquid chromatography, gas chromatography and mass spectrometry in the speed of analysis ( 1 s), non destructive nature and scope for miniaturisation of the resonator size to suit the system under test. For minute sample volumes, the resonator perturbation technique is extensively used for dielectric measurements on polar liquids. In this project, it has been employed for highly sensitive compositional analysis of two-component dielectric mixtures contained in microcapillary segments. The primary evaluation system used here was mixtures of acetonitrile and toluene, chosen because of the large difference in their molecular electric dipole moments. The results obtained from this first system provided the inspiration to assess mixtures made of acetonitrile and water, which are much more closely matched in terms of their electric dipole moments. Three different types of resonators namely hairpin resonator, split ring resonator and sapphire dielectric resonator were used to analyse both the aforementioned solution systems. The results show very sensitive characterisation and are in close agreement with the theoretical predictions governing perturbation of resonators by dielectric samples. In the last phase of this research, a miniaturised sapphire dielectric resonator was designed and fabricated that provided added enhancement in measured sensitivity of both evaluation mixtures