Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > The Dielectric Constant Detector > Page 35
The Schering bridge is balanced by the iterative adjustment of (Ro) and (C'). At balance the following relationships will hold:
The resistance-component of the cell reduces the bridge sensitivity to changes in capacity and thus the plates should be well insulated to prevent conductivity through the mobile phase.
The capacity of the sensor can also be measured by making it one component of a resistance/capacity or an inductance/capacity oscillator. The frequency will depend, among other things, on the capacity of the sensor and, in turn, on the dielectric constant of the material between the plates. The frequency general can be heterodyned against a reference oscillator and the frequency difference will then be proportional to the change in capacity and hence the dielectric constant of the mobile phase.
Poppe and Kunysten (28) described a dielectric constant detector which included a reference cell for temperature compensation. The cell consisted of two stainless steel plates 2 cm x 1 cm x 1 mm separated by a gasket 50 mm thick. The two cells were identical and clamped back to back, sharing a common electrode.
The device was reported to have a sensitivity of 10-6 g/ml for chloroform (e = 4.81) in n-octane. As might be expected, it was found to be very sensitive to pressure changes in the cell (thought due to plate deformation) even when constant flow pumps were employed. The first dielectric constant detector became commercially available in 1979 (29) and was described by Benningfield and Mowery (30). Several applications were reported by Bade et al. (31). A diagram of the sensor is shown in figure 22.