What is a polarimeter?
Whereas regular (unpolarised) light includes waves that are inclined in any direction of space, polarised light is inclined at one defined angle.
Polarised light is ‘created’ by eliminating all light other than that which is inclined at one angle. To do this, a very fine meshed grating – a polarisation filter – is used to filter out all waves from the regular light that do not have the same inclination as the filter’s grating.
If the polarised light beam were then directed through a second grating, set at exactly 90 degrees to the first grating, no light would pass through it.
How does a polarimeter work?
Polarimetry measuring principle
However, optically active substances change the inclination of the light wave. So if an optically active substance is placed between the two filters, then the polarised beam would be rotated. A component of the light would then be able to pass through the second filter. Examples of optically active fluids are sugars, lactic acid, tartaric acid, and some biologically-active substances.
Polarimeters therefore measure the angle through which the ray of polarised light is diverted. Depending on the design of the device, the second filter is then rotated (manually or automatically) until no light falls on the detector. The amount of rotation and its direction (clockwise or anticlockwise) provides a powerful technique for measuring features of the optically active substance.
Polarimetry will provide information about the molecular structure, the concentration of the substance and sometimes information about the solvent used. The optical activity is also influenced by temperature, the wavelength of the light, and the length of the optical path – the longer the path of the light through an optically active substance, the larger the angle of rotation. These variables may be used to control the sensitivity of the instrument.
Wavelengths lower than 589 nm are available with mercury and deuterium lamps isolated through filters and may provide greater sensitivity. Generally, the observed optical rotation at 436 nm is approximately double and at 365nm about three times that at 589 nm. It is now common practice to use other light sources, such as xenon or tungsten halogen, with appropriate filters, because these light sources offer advantages of cost, long life, and broad wavelength emission range over traditional light sources.
Manual vs. automatic polarimeters
Although polarimetry is a mature technique, today’s instrumentation provide features and benefits that purely manual-optical systems do not. Busy labs processing multiple samples per day now have the option of automated data capture, variable wavelength and temperature, and (with modern instruments such as the Krüss P8000) up to 0.002 deg accuracy and 0.001 deg resolution. At this level of precision, process industries and formulators can set extremely narrow quality standards.
A traditional manual polarimeter requires a skilled operator and the process can take considerable time to perform.
Manual polarimetry still has its place, for low-throughput labs and for training, but for many modern industries, such as pharmaceutical, chemical, sugar, and food processing, an automatic polarimeter is the better choice and makes the process much quicker. Lab productivity is greatly improved, and the investment pays real dividends.
Features of automatic polarimeters
- Compatible with Good Laboratory Practice (GLP) and US FDA 21 CFR part 11.
- Accurate reading is provided in a few seconds or less.
- Operator-induced errors are eliminated.
- Measurement consistency is greatly improved – important for regulated laboratories.
- Touchscreen controls and clear digital readout
State of the art digital polarimeters are intended for use in FDA-regulated sectors due to their GLP compliance, integrated user management and full network support, for simple connection to the laboratory environment and a LIMS. Measuring time is reduced to around one second, regardless of the rotation angle of the sample. Continuous measurement is possible (for example for kinetic investigations – investigating kinetic reactions by measuring optical rotation as a function of time – or in HPLC use). The connection of a temperature-controlled gauge head permits extremely accurate measurement.
A digital polarimeter can be connected to a PC or linked to a network via an Ethernet interface. If there is internet access, remote maintenance and fault diagnosis are also possible.
Glossary of terms
Chirality – chiral substances have molecules which can take up different spatial arrangements (configuration isomerism). Optical activity is a property unique to chiral substances, for example 2-butanol, which exists as two mirror-image isomers, or enantiomers.
Enantiomers – see chirality
FDA (Food and Drug Administration) – an agency of the United States Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the regulation and supervision of food safety, tobacco products, medicines, and other related items.
US FDA 21 CFR part 11 deals with guidelines on electronic records and electronic signatures considered to be trustworthy, reliable and equivalent to paper records pertinent to drug makers, medical device manufacturers, biotech companies and other FDA-regulated industries.
GLP (Good Laboratory Practice) is a quality system concerned with the organisational processes and conditions under which non-clinical health and environmental safety studies are planned, performed, monitored, recorded, archived and reported. An internationally recognised definition of GLP can be found on the website for the Medicines and Healthcare products Regulatory Agency-UK.
HPLC – High-performance liquid chromatography (or high-pressure liquid chromatography) is a technique used to separate the components in a mixture, to identify each component, and to quantify each component.
Laboratory Information Management System (LIMS) – a software-based laboratory and information management system that offers a set of key features that support a modern laboratory’s operations, the workflow, data tracking and data exchange services needed to a laboratory’s use in regulated environments.
Optically active – the description given to materials which can bend polarised light – see also Chirality.