Capillary electrophoresis and contactless conductivity detection of ions in narrow inner diameter capillaries

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.