A contactless capacitively coupled conductivity detector for capillary electrophoresis is introduced. The detector consists of two electrodes which are placed cylindrically around the outer polyimide coating of the fused-silica capillary with a detection gap of 2 mm. The electrodes form a cylindrical capacitor, and the electric conductivity of the solution in the gap between the electrodes is measured. A high audio or low ultrasonic frequency for coupling of the ac voltage is used in order to minimize the influence of reactance of the liquid. For an improved version of the detector, two syringe cannulas are used as the electrodes and the capillary is simply assembled into the tubing. This allows an easy placement of the detector on various positions along the capillary. The limit of detection of inorganic cations and anions is 200 ppb, as determined for sodium and chloride, respectively.

When water flows over glass which has been treated with the vapor of dimethyldichlorosilane and thus made water repellent, slipping on the boundary between the solid surface and the water takes place. This is is shown in capillary tubes of various diameters. The amount of slipping is small, but measurable. It disappears or becomes extremely small in case of turbulent flow.

Capillary electrophoresis and conductometry represent a combination of a high-resolution separation method with a sensitive detection principle for the analysis of ionic species. In this paper, results are reported that are obtained with a contactless conductivity detector. This device works without a galvanic contact of the electrolyte and the electrodes. The conductivity sensor is based on two metal tubes that act as cylindrical capacitors. These electrodes are both placed around a fused-silica capillary with a detection gap of 1 mm left in between. When a high audio or low ultrasonic oscillation frequency between 40 and 100 kHz is applied to one of the electrodes, a signal is produced as soon as an analyte zone with a different conductivity compared to the background electrolyte passes the detection gap. An amplifier and rectifier is connected to the other electrode where the signal is further processed. Limits of detection for lithium and fluoride are 4 and 13 ppb, respectively, with a linear range over 4 orders of magnitude from 90 ppb up to more than 1000 ppm for both anions and cations. Furthermore, it is demonstrated that for species with lower equivalent conductivities, such as organic ions, indirect conductivity detection is a sensitive alternative to indirect optical detection methods. Limits of detection of 50 ppb and below are obtained for organic acids.

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