Polarimetry

As polarized light passes through an optically active material the angle of rotation can be measured and
used to distinguish chemicals. This is especially useful for chiral substances. A chiral molecule is one that
is not superimposable on its mirror image. The common analogy, is your right hand and left hand which
are mirror images of each other but not superimposable. In chemistry, chiral chemicals are also called
enantiomers or optical isomers. Chemists have developed nomenclature for denoting stereoisomers as
R and S. A racemic mixture is one in which the equal amount of R and S enantiomers are present and
therefore cancel out the optical rotation of each other. In other words, a racemic mixture will have no
optical activity.

For obvious reasons, the identity of isomers are often more difficult to verify than the identity of non-
isomers. Polarimetry is one way chemists could use to determine how much of one optical isomer is
present compared to the other optical isomer. The FDA requires that for a chiral drug product of a single
enantiomer, control of the other enantiomer should be consider. Impurity tests as well as
specifications for the enantiomer should be established. Polarimetry is the most common method for satisfying this
requirement.

An instrument called a polarimeter is used to determine the angle of rotation of substances. Polarized
light waves propagates in one defined plane (called the plane of polarization). If the polarized
light hits an optically active compound, the plane is changed or rotated. Optical activity is additive
which means that the two compounds that rotate light in the same direction, will add to each other.  If
two compounds that rotate light in opposite directions the optical activity of one will be subtracted from
the other. For this reason racemic mixtures, mixtures with equal amounts of two enantiomers, will have
no optical activity, one enantiomer completely nullifies the other.