Optimization of the high-frequency contactless conductivity detector for capillary electrophoresis

Abstract

Two constructions of the high-frequency contactless conductivity detector that are fitted to the specific demands of capillary zone electrophoresis are described. The axial arrangement of the electrodes of the conductivity cell with two cylindrical electrodes placed around the outer wall of the capillary column is used. We propose an equivalent electrical model of the axial contactless conductivity cell, which explains the features of its behavior including overshooting phenomena. We give the computer numerical solution of the model enabling simulation of real experimental runs. The role of many parameters can be evaluated in this way, such as the dimension of the separation channel, dimension of the electrodes, length of the gap between electrodes, influence of the shielding, etc. The conception of model allows its use for the optimization of the construction of the conductivity cell, either in the cylindrical format or in the microchip format. The ability of the high-frequency contactless conductivity detector is demonstrated on separation of inorganic ions.

Determination of the spectrum of low molecular mass organic acids in urine by capillary electrophoresis with contactless conductivity and ultraviolet photometric detection—An efficient tool for monitoring of inborn metabolic disorders

“…The C 4 D has been developed and assembled at the Department of Physical Chemistry, Charles University, Prague, Czech Republic [34]. It operates with a sine-wave signal at a frequency of 1.25 MHz and an effective voltage of 50 V. The C 4 D was placed in series with the photometric detector into a cassette containing the separation capillary; the cassette was thermostatted at 25 • C. The C 4 D cell and the detection window for the photometric detector light beam were positioned at a distance of 85 and 92 cm, respectively, from the capillary inlet.…”

A thin-layer contactless conductivity cell for detection in flowing liquids

“…The stray capacitance value in this thin-layer system has been estimated at 0.05 pF for which the best agreement between the calculated and experimental data has been obtained. The aluminum electrode thickness is very small so that the direct signal transfer between the electrodes along the foil surface is expected to be small and need not be minimized by placing an earthed shield between the electrodes, as is common with some C 4 D designs [26].…”

Thinly Insulated Wire Cells – A New Device for Sensitive Contactless Conductivity Detection in Flow Analyses

“…The curvature of the experimental dependence appears at the lowest studied concentrations and is manifested in a negative intercept of the regression straight line constructed over the linear part of the dependence. The curvature of the dependence at low conductances is attributed to parasitic signal transfer between the electrodes [16], whereas the curvature at higher conductances is caused by changes in the optimum frequency. The principal analytical parameters of the detectors tested are given in Table 1.…”

“…It can be seen that the optimum frequency shifts to higher values with increasing solution conductance and thus must be selected in dependence on the expected solution conductance [15]. It further follows from the experimental and calculated dependences of the response on the conductance given in Figure 5 for 100 and 300 kHz that the curvature at low conductances is not caused by mere parasitic signal transfer between the electrodes [16], but that is a complex function of this parasitic transfer and of the lack of optimization of the input signal frequency.…”