Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > The Dielectric Constant Detector > Page 33

The cell volume can be as little as a few microliters and, thus, would be suitable for use with microbore columns. A sensitivity of 10-6 AU has been claimed for the detector and a linear dynamic range 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  =  n2

For semi-polar substances or mixtures of semi-polar substances and non-polar substances the Lorentz-Lorenz equation applies

However, for polar substances and mixtures of polar and semi-polar substances the relationship breaks down and no simple functions describe dielectric constant in terms of refractive index.