The two electrodes are separated from the test medium (usually liquid) by a dielectric layer and represent two capacitors characterized by coupling capacitance Ccpl; Ccpl depends predominantly on the thickness and permittivity of the dielectric layer. The electrical behaviour of the test medium appears as a parallel combination of the liquid resistance (Rliq) and the liquid capacitance (Cliq). Part of the electrical energy applied always strays from the test medium, passes along the surface of the dielectric or through its interior and appears as stray (parasitic) capacitance Cx, parallel to the main passageway. The parasitic effect of the stray capacitance is sometimes eliminated by placing a shielding foil between the electrodes [9,10].Figure 1.
A simplified scheme of the equivalent electric circuit for the contactless impedance cell with connections to the input high-frequency voltage source and the output signal meter. For discussion and symbols explanation see the text.The analytical signal is given by the cell impedance, Z, defined by the familiar general equation:Z=R+iX(1)where the real term, resistance R, is a function of the cell geometry and of the electrical conductivity of the test medium. The imaginary term, capacitance X, also depends on the geometric parameters and further on the relative permittivities of the test medium and of the dielectric, and on the angular frequency, ��, or ordinary frequency, f, of the input alternating signal (�� = 2��f); i is the imaginary unit.
It can be seen that the cell behavior depends on a number of experimental parameters and it should be emphasized that all these parameters affect one another, so that they must be considered together when studying the behavior of a particular cell under particular conditions. It is also evident that the set of experimental conditions determines whether the resistance term of Equation (1) predominates��this is the case of the contactless conductivity detection which is mostly used at present, or the capacitance is more important (dielectrometry).
The impedance of the electric GSK-3 equivalent circuit Cilengitide in Figure 1 can be calculated from Equation (2):Z=Z1Z2/(Z1+Z2)(2)where Z1 is the impedance of the bottom branch of equivalent circuit, which equals to:Z1=Rliq?(?i/��Cliq)Rliq?i/��Cliq?2i/��Ccpl(3)If Rliq (?i/��Cliq), the effect of the solution capacitance can be neglected, Equation (3) is simplified to Z1 = Rliq ? 2i/��Ccpl and the sensor works primarily as a conductivity detector. If Rliq (?i/��Cliq), the effect of the solution resistivity can be neglected, Equation (3) is simplified to Z1 = ?i/Cliq ? 2i/��Ccpl and the sensor works primarily as a dielectrometric detector.