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(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

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 dielectricconstant 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

of about three orders of magnitude. The thermal lens detector is, in fact, a special form of the refractive index detector and might, therefore, be considered a universal detector. Nevertheless, like other bulk property detectors, it can not be used with gradient elution or flow programming and has sensitivity that  is no better, if as good as, other refractive index detectors. The Dielectric Constant Detector The refractive index of a substance is a complementary property to the dielectric constant and in some circumstances is a direct function of it. For non-polar substances, the relationship between dielectric constant (e) and refractive index (n) is given by                                                 e  =  

cell. Both electrodes were made of stainless steel. The two cylinders were electrically isolated with a cylindrical flow path through the cell. The inner cylindrical electrodes were 1.26 cm in diameter and 0.625 cm long separated from the outer cylinder by about 0.009 cm. The linear dynamic range of the detector was reported to be 3.5 x 104 . The sensitivity was quoted as about 1 x 10-7 g/ml, which would be close to the theoretical limit for bulk property detectors. An example of the use of the dielectric constant detector to monitor a separation of triglycerides is shown in figure 23. Bulk property detectors have neither the sensitivity nor the linear dynamic range of solute property detectors and are less frequently used in modern LC analyses. None can be used satisfactorily with gradient elution, flow programming or temperature programming and so they restrict the choice of development. They do have certain unique areas of application, some of which have already been mentioned.

nbsp; The Dielectric Constant Detector In1961Winefordneretal.(40)described a GC detector that functioned on the change in dielectric constant of the carrier gas when a vapor or another gas was present. Like the katharometer, the detector responded to most gases and vapors providing a suitable carrier gas was chosen. The sensor took the form of a variable capacitor mounted in a special cell which allowed the column eluent to pass between its plates. The capacitor was made to constitute part of the "

such forces can be expressed as   where (ap) is the polarizability of the molecule, (vo) is the characteristic frequency of the molecule, (hp) is Planck's constant, and (l) is the distance between the molecules. The polarizability (a) is the crucial factor that controls the dispersive force acting on the molecule, which, for substances that have no dipoles, is given by    where (D) is the dielectric constant of the material and (nv) is the number of molecules per unit volume