Liquid Chromatography Detectors - The Electrical Conductivity Detector > Page 87

The electrical system consists of a frequency generator (1,000-5,000 Hz) that provides an AC potential across the cell. The sensor is placed in one arm of a Wheatstone bridge as shown in figure 56.

If a single sensor is employed, it is situated in one arm of the bridge; if, however, a reference sensor is also used, then the two sensors are situated in opposite arms of the bridge. In either case the out–of–balance signal can be passed to a precision rectifier and the output either handled by an analog circuit or sampled directly by the computer.  Sometimes a variable resistance is situated in one of the other arms of the bridge and is used for zero adjustment to back-off any signal arising from ions contained by the mobile phase.

Figure 56 One Form of the Wheatstone Bridge Used with the Electrical Conductivity Detector


The out-of-balance signal is rectified and the DC signal either passed to a nonlinear amplifier or a computer data acquisition system. The non- linear amplifier is necessary to render the output linearly related to ion concentration. If the output is passed directly to the computer, then the linearization can be carried out with appropriate software and the chromatogram is presented on the printer.

An example of a separation monitored with a conductivity detector useing an ion supression technique (see book 15) is shown in figure 57.