Properties of the Contactless Impedance Detector with Insulated Wire Electrodes Placed Inside the Flowing Liquid Stream
Abstract
Theoretical and experimental study of contactless permitivitty detector for detection of compounds in flowing liquids. The behavior of a flow cell, in which the impedance is measured between two insulated wire electrodes located within the stream of the test liquid, is described and compared with that of a semi-quantitative theoretical model.
A new approach is used to the monitoring of the impedance signal, based on connecting the cell as the input impedance of an operational amplifier in the differentiating circuit. A triangular AC voltage is fed to one of the electrodes and the other electrode is connected to the amplifier input.
The square-wave voltage at the amplifier output depends at low frequencies (from hundreds of Hz to several kHz) on the capacitive component of the overall impedance. The basic analytical parameters are in satisfactory agreement with the model and are comparable to those obtained with common contactless impedance detectors operating at higher frequencies of hundreds of kHz to a few MHz and monitoring primarily the conductance component of the impedance.
The system described offers new detection possibilities, mainly for nonionic analytes of varying polarities.