Optical Rotation by USP <781>

Polarimetry, the measurement of the rotation of light by certain substances, has been used since the early nineteenth century. Much of the original work using polarimetry was conducted using simple prisms; however, modern instruments use a variety of optical systems. By "sorting" light into a particular plane, we produce what is

known as "plane-polarized" light. Substances which are optically active, or chiral, will rotate this light in one direction or the other. Compounds which rotate light in a clockwise direction are said to be dextrorotatory, or (+)-isomers, while those which rotate light in a counterclockwise direction are levorotatory, or (-)-isomers.

Optical Rotation Polarimeter

Historically, polarimetry measures optical rotation by visually matching the intensity of split fields, using visible light from the sodium D-line at 589nm. The rotation is usually expressed as the "specific rotation," although some USP articles are expressed in terms of the "observed rotation." Specific rotation is calculated from the observed rotation based on the concentration, length of the measuring tube and water content of the material, and is generally measured in a specific solvent.

Today we use an automated system from Rudolph Research Analytical, the Autopol V. Features include:

0.001^o Arc Optical Resolution
Electronic cooling and heating from 15°-30°C
Six Standard Wavelengths: 365, 405, 436, 546, 589, and 633 nm
NIST Traceable Quartz Standard,
100 mm Polarimeter Cell,
Temperature Validation Cell and Built-In Sample Measurement Probe.

Polarimetry is a powerful tool to determine the active purity of raw materials such as vitamins, steroids,

and antibiotics, since for most chiral compounds, only one isomer has biological activity. The most common application is for sugar content. Methods for determination of sugar in chocolate, wines, jams, jellies, and flour are well documented, as well as lactose in milk products.

Polarimetry can also be used in products where a mixture of optically active substances exist, such as both "normal" sugar and invert sugar. In these cases, additional sample preparation can be used to measure both compounds.

For a quotation...

Autopol V

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