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Product Focus

Product Focus: Polarimeters

Increasing the accuracy of optical rotation

Mike May, PhD

To measure the rotation of linearly polarized light going through an optically active or chiral molecule, researchers use a polarimeter. As Alex White, product specialist at Anton Paar USA (Ashland, VA), says, “There is now more demand for polarimeters in all industries that use or need to measure optically active samples, as polarimeters have the capability of being very accurate, precise, robust, and user-friendly.” He adds, “Anton Paar has seen significant growth in polarimeter sales, particularly in the pharmaceutical, food, and flavor and fragrance industries. This demand has in turn led to more requests from universities to use our polarimeters for research.”

This technology lets researchers explore materials in many ways. For instance, Tom Papandrea, vice president of quality and technical affairs at Excellentia International (Edison, NJ), says, “We are suppliers to the flavor and fragrance industry. Optical rotation is important to us because it can be a clue as to the source of a chemical substance and whether or not it is natural or synthetic.” He adds, “Many chemical entities occur in nature as either enantiomeric form, but natural substances usually have one predominant enantiomer, while synthetics are usually racemic mixtures. For example, natural mints that contain l-menthol, dill and caraway are predominantly d-carvone, but spearmint is primarily l-carvone.” Although he’s largely happy with existing technology, he says, “Many of the products we use in this industry are extremely expensive, so I guess it would be nice if accurate measurement could be made with less sample.”

Ensuring accuracy

The expanding use of polarimeters requires increasing accuracy. White points out: “Anton Paar has recently added new features to give the user the ability to know, almost absolutely, that a measurement was performed correctly.” As an example, Toolmaster, White says, “ensures that the correct cell was used for the selected method, as cell data is transmitted wirelessly, thus allowing the user to simply place the sample cell or quartz control plate in the instrument and hit ‘Start.’”

Some units keep getting more accurate. For example, Richard C. Spanier, director of sales and marketing at Rudolph Research Analytical (Hackettstown, NJ), says, “Our Autopol VI is accurate to the fourth decimal place, but it’s still very, very easy to use.” This company also offers temperature control in its polarimeters with its patented TempTrol Electronic Heating and Cooling System.

In some cases, accuracy is not as important as understanding. As Noah Radford, senior product specialist at ATAGO USA (Bellevue, WA), explains: “For teaching purposes, we often sell our semiautomatic polarimeter, the POLAX-2L. It hits a ‘sweet spot’ between antiquated manual units and advanced high-precision models. The user must judge the angle based on the light level before and after inserting a sample, but it gives a digital display of the angle of rotation.” He adds, “Lots of schools like this because it forces the student to apply some knowledge of the concept of polarimetry without forcing them to use a fully manual unit.”

As White says, “In the end, it’s all about improving instrument accuracy, repeatability, usability, traceability, and flexibility.”

For additional resources on polarimeters, including useful articles and a list of manufacturers, visit