Modern day polarimetry

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, says R Elangovan

Polarimetry is a technique for measuring the optical activity exhibited by inorganic and organic compounds. A compound is considered to be optically active if linearly polarised light is rotated when passing through it. The amount of optical rotation is determined by molecular structure and concentration of chiral molecules in the substance. Each optically active substance has its own specific rotation as defined in the Biot’s law.

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, flavour and food industries. It is so well established that the United States Pharmacopoeia (USP) and the Food & Drug Administration (FDA) have included polarimetric specification for numerous substances.

The one big industry which uses polarimeter very widely is pharmaceutical. The US Pharmacopoeia in their monograms gives an idea about the specifications.

To measure specific rotation between +19 and +24 degrees on certain samples, USP strongly recommends 200mm sample tube. US Pharmacopoeia in its earlier versions mentioned sodium D-line when the monogram was specifying analysis in 589nm. Even in that the stress was only on the sodium’s D-line and not on the sodium lamp itself. As the technology developed, there were better lamp sources including Tungsten-halogen, Xenon etc.

Particularly, Tungsten-halogen lamps were having a lot of advantages against traditional sources such as sodium. These include cost, life-time of the source and flexibility. The USP recognised these. In their National Formulary 18 in USP 23 the made a mention that “its now common practice to use other light sources, such as Xenon or Tungsten-halogen, with appropriate filters, because they may offer advantages of cost, long life …” . This has made many manufacturers to switch from traditional and costly sources to Tungsten-Halogen sources.

Optical rotation is dependent on the temperature of the sample. The Monogram if it does not specify any temperature it means that the sample should be maintained in 25 degree C (according to USP) and 20 degree C (according to BP). Traditionally, a water bath was used to circulate water around a jacketed cell, or the sample was poured into a test tube which was then inserted into a tub filled with ice.

Periodic measurement of temperature was taken. When the sample temperature reaches a degree lesser, the sample was poured into the sample tube and the readings were taken. Both these methods were costly, cumbersome and time consuming. To avoid these problems, there are also instruments in the market, which have incorporated electronic heating and cooling via Peltier effect. A laboratory daily grapples with a big problem and that is with regard to calibration of each of their equipment.

A polarimeter was always calibrated with suchrose solution, which has known values. To prepare suchrose solution one has to go through the process of drying the Suchrose, weighing it, preparing distilled water, measuring distilled water etc. In this process, there is a chance of error creeping in in every step and instruments like moisture oven, balance, RO system, micropipette etc being used.

In an audit, the laboratory has to spell out in clear terms as to how they are calibrating and validating each of these equipment. With a NIST traceable quartz control plate, which is known as a calibration tool of the first order, all the above problems are solved in one stroke.

As the technology grows there is an urgent need for a laboratory to seek methods and tools, which would enable them to do their day-to-day work more efficiently. Towards this they always embrace the latest technology offering.

The writer is with Agaram Industries. E-mail: elangovan@agaramindia.com