Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > The Dielectric Constant Detector > Page 34

The more polar the substance, the larger is its dielectric constant. In normal chromatography (as opposed to reversed phase chromatography) the mobile phase is normally less polar than the solutes being eluted. Thus, the presence of a solute in the mobile phase will increase the dielectric constant of the mobile phase. Conversely, in reversed phase chromatography the solute is usually less polar than the solvent and the dielectric constant of the mobile phase is reduced by the presence of a solute. Thus. a device situated at the end of the column which responds to changes in dielectric constant would act as a chromatography detector. The sensor often takes the form of a cylindrical or parallel plate condenser. The volume of the sensor must be as small as possible to minimize dispersion. In addition, as the sensitivity of the device is proportional to the electrical capacity of the sensor, the capacitor plates must be very close together.

A suitable circuit for use in dielectric constant measurement is an electrical "bridge", the detector cell being situated in one arm of the bridge. If the sensor cell has a capacity greater than 100 pF, then a Wein bridge can be used; however such a cell may well have a fairly large volume. For smaller capacity cells, the Schering bridge is more appropriate and a diagram of a Schering bridge is shown in figure 21.

Figure 21 The Schering Bridge for the Measurement of Small Capacities

No capacitor is ideal, all will have some inductance and resistance in addition to its capacity. In fact, because the plates of the capacitor are situated in the mobile phase, if uninsulated, it is very likely to have a significant resistance component. The current though the resistive component of a conductor is in phase with the applied voltage and the capacity component lags the applied voltage by 90o. Thus, there are two components to be balanced before the output of the bridge (across (D)) can be used to monitor the elution of a solute.