Fundamental aspects of contactless conductivity detection for capillary electrophoresis. Part I: Frequency behavior and cell geometry

Abstract

A better understanding of the characteristics of the axial contactless conductivity cell could be obtained by carefully studying the effect of the cell geometry on its frequency behavior. A good fit between theoretical and experimental results shows that the axial contactless conductometric detector can effectively be described by the simplest possible equivalent circuitry consisting of a capacitor, resistor, and a second capacitor. The cell constant is largely defined by the length of the gap between the electrodes. The effective electrode size is thus not related to the dimensions of the real electrodes but more closely to the cross-sectional area of the internal diameter of the capillary. Typical experimental values of 20 MOmega and 0.1 pF were obtained for the resistance and capacitances, respectively, of a cell formed by a 2 mm gap between two 4 mm long electrodes fitted with a capillary of 50 microm ID. It could be shown that the diameter of the electrode is not critical and tight coupling of the electrodes to the outer wall of the capillary is not needed. The peak overshoot phenomenon, which has frequently been reported, is an artefact that can be minimized by optimizing the frequency for cell excitation. The frequency setting has to be optimized for each cell design, operational amplifier, electrolyte solution and capillary.

Comparison of Different Contactless Conductivity Detectors for the Determination of Small Inorganic Ions by Capillary Electrophoresis

“…Each solution was then consecutively injected six times into the CE-C 4 D systems and analytical parameters were calculated for each detector, which are summarized in Table 1. The settings of the excitation voltage, frequency, gain and other parameters were individually optimized to achieve best signal-to-noise characteristics [6] and are also summarized in Table 1. The relative standard deviation (RSD) values of migration times and peak areas are given for the 200 mM concentration level for six consecutive injections (n ¼ 6).…”

“…This presumably is due to Joule heating and not an immediate feature of the detectors. The peak distortions for the detector with the miniaturized cell which have become apparent for the high conductivity buffer must be due to the relatively low operating frequency that has to be employed for this detector [6]. The salient performance parameters for quantifications in both electrolyte solutions are summarized in Table 2.…”

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

“…It has been found for classical CE detectors with tubular and semitubular electrodes [13,14] that the measurable AC current only flows between certain parts of the electrode surfaces, called the effective surfaces that are closest to one another (effective electrode widths). There is no reason to assume that it is different with the present wire detector cells and the present model calculations actually confirm this assumption, indicating that only those parts of the electrode surfaces are active that are oriented against one another (they "see" one another), as represented by the shaded areas in Figs.…”

Determination of tobramycin in human serum by capillary electrophoresis with contactless conductivity detection

“…Furthermore, the method is inexpensive. Although a relatively recent introduction, detailed studies of the influence of the cell design and its operating parameters on the performance of the C 4 D have been carried out and its functioning is well understood [29][30][31][32][33]. Recent reviews, which give an overview of the reported applications of the C 4 D, are available [34,35].…”