Polarimetry Fundamentals – Definition – Biots Law
Polarimetry is a sensitive, nondestructive technique for measuring the optical activity exhibited by inorganic and organic compounds. A compound is considered to be optically active if linearly polarized light is rotated when passing through it. The amount of optical rotation is determined by the molecular structure and concentration of chiral molecules in the substance. Each optically active substance has its own specific rotation as defined in Biots law:
[α] = specific rotation, T = temperature,
λ = wavelength, α = optical rotation,
c = concentration in g/100ml, l = optical path length in dm.
Biots Law is named after Jean-Baptiste Biot and Félix Savart, who discovered this relationship in 1820. Historically, polarimetry was performed using an instrument where the extent of optical rotation is estimated by visual matching of the intensity of split fields. For this reason, the D-line of the sodium lamp at the visible wavelength of 589nm was most often employed. Specific rotation determined at the D-line is expressed by the symbol:
Specific Rotation Symbol
and much of the data available are expressed in this form. Use of lower wavelengths, such as those available with the mercury lamp lines isolated by means of filters of maximum transmittance at approximately 578, 546, 436, 405, and 365nm in a photoelectric polarimeter, have been found to provide advantages in sensitivity with a consequent reduction in the concentration of the test compound. In general, the observed optical rotation at 436nm is about double and at 365nm about three times that at 589nm. Reduction in the concentration of the solute required for measurement may sometimes be accomplished by conversion of the substance under test to one that has a significantly higher optical rotation. Optical rotation is also affected by the solvent used for the measurement, and this is always specified.
It is now common practice to use other light sources, such as xenon or tungsten halogen, with appropriate filters, because these may offer advantages of cost, long life, and broad wavelength emission range over traditional light sources
— USP<781> Download PDF
The polarimeter must be capable of giving readings to the nearest 0.01°. The scale is usually checked by means of certified quartz plates. The linearity of the scale may be checked by means of sucrose solutions.
— EP 2.2.7 Download PDF
The polarimetric method is a simple and accurate means for determination and investigation of structure in macro, semi-micro and micro analysis of expensive and non-duplicable samples. Polarimetry is employed in quality control, process control and research in the pharmaceutical, chemical, essential oil, flavor and food industries. It is so well established that the United States Pharmacopoeia and the Food & Drug Administration include polarimetric specifications for numerous substances.
Polarimetry – Industry Applications
Research applications for polarimetry are found in industry, research institutes and universities as a means of:
- Isolating and identifying unknowns crystallized from various solvents or separated by high performance liquid chromatography (HPLC).
- Evaluating and characterizing optically active compounds by measuring their specific rotation and comparing this value with the theoretical values found in literature.
- Investigating kinetic reactions by measuring optical rotation as a function of time.
- Monitoring changes in concentration of an optically active component in a reaction mixture, as in enzymatic cleavage.
- Analyzing molecular structure by plotting optical rotatory dispersion curves over a wide range of wavelengths.
- Distinguishing between optical isomers.
In each of these applications, the AUTOPOL Polarimeter offers up to six discrete wavelength selections to observe the effect of wavelength upon an optically active substance.
Using Polarimetry in Quality and Process Control Applications
Quality and process control applications, both in the laboratory or on-line in the factory, are found throughout the pharmaceutical, essential oil, flavor, food and chemical industries. A few examples are listed below.
Polarimetry determines product purity by measuring specific rotation and optical rotation of:
- Amino Acids.
- Amino Sugars.
Polarimetry for Flavor, Fragrance, and Essential Oil Industry
Utilizes polarimetry for incoming raw materials inspection of:
- Orange oil.
- Citric acid.
- Lavender oil.
- Spearmint oil.
- Glygeric acid.
- Lemon oil.
Polarimetry for Food Industry Applications
Polarimetry – Ensures product quality by measuring the concentration and purity of the following compounds in sugar based foods, cereals and syrups:
- Various starches.
- Natural monosaccharides.
Using Polarimetry in the Chemical Industry
Polarimetry – Analyzes optical rotation as a means of identifying and characterizing:
- Natural polymers.
- Synthetic polymers.