Page 96 - Tandem Techniques

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is now mandatory that if a drug can exist with different chiral structures, the relative physiological
activity of the different enantiomers must be determined. It follows, that the separation and
identification of enantiomers is now a very important analytical problem. The chirality of a substance is
measured by its capacity for rotating polarized light

As discussed earlier, light consists of a sinusoidally changing electric field normal to, and in phase with,
a sinusoidally changing magnetic field. The plane of the electric vector in normal light, takes no
particular orientation, but in plane polarized light, the electric vector is either vertically or horizontally
polarized. If the electric vector transcribes an helical path, either to the right or left, the light is said to
be circularly polarized. A linearly polarized beam of light, can be considered to be the resultant of two
equal-intensity, in-phase components, one left and the other right circularly polarized, or of two
orthogonal linear components at ± 45°.

The differential absorption of these two ± 45° linear components in a medium is known as linear
dichroism; if there is a differential velocity between the two ± 45° linear components, when passing
through a medium (i.e. the refractive index of the medium to the two light components differ), then this
is known as linear birefringence. In an analogous manner, the difference in the adsorption
characteristics of a medium to left and right circularly polarized light is termed circular dichroism (CD)
and it follows, that the difference in refractive index of a substance to the two light components is called
optical rotary dispersion (ORD), sometimes reported as specific optical rotation.

CD spectra are usually measured as the differential absorption of left and right circularly polarized
light, i.e. (AL-AR) and is usually reported as the differential molar extinction coefficient (De), where

where (1) is the length of the cell,

and (c) is the morality of solute