![]() Basic Theoretical ConsiderationsĪ simplified electric equivalent circuit of a contactless impedance cell, permitting sufficiently detailed theoretical description of the cell operation, is depicted in Figure 1. Some tentative directions for the future are also outlined. The results obtained primarily in the authors' laboratories are critically discussed and assessed in the context of global research. The present paper deals with less traditional geometric arrangements of the detection cells and applications of contactless impedance detectors. High-frequency impedance detection, now mostly called “capacitively coupled contactless conductivity detection”, abbreviated as C 4D, is mostly employed for detection in capillary zone electrophoresis (CE) reviews of recent developments in this field can be found, e.g., in references. The universal detection character and the separation of the sensing elements from the test medium make the technique attractive for analyses of complex mixtures of similar analytes. Rapidly developing high-performance separations in the liquid phase, especially those on a microscale or on chips, require appropriate detection techniques The use of substantially lower frequencies of the applied signal has led to better definition of the output, which is more easily interpreted Technical developments have made measurements simple, flexible and cheap Renewed interest in these measurements emerged quite recently (during the nineteen nineties), primarily for several reasons: However, its analytical application was rare and mostly limited to high-frequency titrations and to some determinations of water in non-aqueous samples. This principle is, in fact, rather old for reviews see, e.g., references. A good example is the revitalization of high-frequency measurement of the electrical impedance of test systems. To meet these requirements, all imaginable measuring principles are being tested and, where possible, developed and applied. The rapidly growing demands on sensing, detection, and determination of a vast variety of compounds in wide ranges of test systems under very different conditions emphasize the need for intense methodological research. New directions for the use of these sensors in molecular biology and chemical reactors and some directions for future development are outlined. Examples are given of cell designs for continuous flow and flow-injection analyses and of detection systems for miniaturized liquid chromatography and capillary electrophoresis. The most important problems to be resolved in coupling these devices with flow-through measurements in the liquid phase are also discussed. There is a detailed discussion of the effect of the individual operational parameters (width and shape of the electrodes, detection gap, frequency and amplitude of the input signal) on the response of the detector. In addition to characterization of traditional types of impedance sensors, the article is concerned with the use of less common sensors, such as cells with wire electrodes or planar cells. The paper provides a critical discussion of the present state of the theory of high-frequency impedance sensors (now mostly called contactless impedance or conductivity sensors), the principal approaches employed in designing impedance flow-through cells and their operational parameters.
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