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

Capillary electrophoresis and conductometry represent a combination of a hi gh-resolution separation method with a sensitive detection principle for th e analysis of ionic species. In this paper, results are reported that are o btained 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. The se electrodes are both placed around a fused-silica capillary with a detect ion gap of 1 mm left in between. When a high audio or low ultrasonic oscill ation 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 conductiv ity compared to the background electrolyte passes the detection gap. An amp lifier and rectifier is connected to the other electrode where the signal i s 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, i t is demonstrated that for species with lower equivalent conductivities, su ch as organic ions, indirect conductivity detection is a sensitive alternat ive to indirect optical detection methods. Limits of detection of 50 ppb an d below are obtained for organic acids.