Figure 6. Polarization Modulation Under these conditions the refractive indexes (n_x) and (n_y) will differ and,

If the light passing through the block is oriented with its axis at 45 to the pressure axis, one of its components will travel faster through the medium than the other.

If the retardation is exactly () the emergent light beam will be right circularly polarized. In addition, if the retardation is the emergent beam will be left polarized. This process is an example of a general principle that was first described by Brewster and given the term photo elasticity. Electro-optic modulizers that are used in some modern liquid chromatography instruments operate in the same way. Returning to the CD spectrometer the selected left or right circularly polarized light is the passed through the sample cell and the intensity of the transmitted light measured by the sensor.

Chiral Measuring Devices

At present, there are no GC chiral detectors that have sufficient sensitivity for general analytical use. There are, however, some chiral LC detectors that can be used effectively in-line with LC columns. For details of LC chiral detectors see ref. (7,8). Chiral GC columns are employed with standard GC detectors (FID, Katharometer, Argon detectors etc.,) that, although very sensitive, are not chirally selective Before discussing molecular structures that result in chirality, the ways in which the enantiomeric content of a mixture is measured needs to be defined. There are three definitions of chiral content that are in common use and they are, Optical Purity, Enantiomeric Excess and Chromatographic Purity. These three measures of chiral content are described by the following equations,

Where (R) and (S) represent the proportion of the respective isomer in the mixture.

All three terms are in common use to describe purity but function enantiomeric excess is the one generally employed.