How does a polarimeter actually work?
Discovery of spectral polarimetry?
A polarimeter is a device for determining the polarisation direction of the light or the rotation of an optically active substance. The physicist Francois Jean Dominique Arago made a discovery on quartz, which was very important for polarimetry. Arago cut a quartz crystal perpendicular to the crystal axis and saw the rotation of linearly polarised light on the cut quartz crystal.
The scientist Jean Baptiste Biot observed that the rotation of the polarisation axis also exists in certain liquids and gases of organic substances. His conviction: the optical activity is in some way related to the structure of the chemical compound.
The chemist Louis Pasteur continued Biot’s work by studying the optical activity of crystal forms with several salts of tartaric and racemic acid with a polarimeter. He realized that there are four different isomers: the counter-clockwise D (–) 17, the clockwise L (+) 16, the so-called racemic and one called meso-tartaric acid 18. The meso shape does not rotate the plane of polarized light. With this realization, he laid the foundation for the development of polarimetry.
Polarisation, an important property of light
The polarisation of sunlight in the Earth’s atmosphere is invisible to humans. For some insects, such as grasshoppers, bees and ants, it is perceptible. Bees and other insects use it as a navigation aid, they change their direction of flight depending on the polarisation of the incident light.
A tropical butterfly species even reflects polarised light with its wings in certain patterns. This polarised “beacon” attracts females for mating. Polarisation phenomena are hidden from us humans. We have to use physical instruments to observe polarisation effects.
Polarising filter and the plane of oscillation of light
Light as it surrounds us is unpolarised. This means that each light wave oscillates randomly in one spatial direction. To convert unpolarised light into polarised light, polarising filters are used. So do polarimeters.
A polarising filter transmits only light in a certain polarisation direction and suppresses the light of other polarisation directions. Light which has passed through a polarising filter is called polarised.
When polarised light passes through a second filter placed parallel to the first filter, the plane of oscillation is maintained and light reaches the screen. If the second filter is rotated by 90°, it suppresses the polarised light beam; the screen behind it remains dark.
A personal experiment
Very nice the effect is to be seen when you turn a 3D glasses or a polarised sunglass in front of an LCD computer screen. At a certain rotation angle the transparency disappears. If the glasses are turned further, the review appears again. Just try it …
How to measure with polarimeters?
Optically active substances change the polarisation direction of the light wave. Optically active are chiral substances which have molecular systems whose asymmetry results are in handedness.
These chiral substances have a pair of nonsuperimposable mirror-image shapes. Examples of optically active media are sugar, lactic acid, quartz, calcite and sodium chlorate.
If a tube with no optically active substance is placed between two parallel polarisation filters, the oscillation plane is retained. Light will be visible on the screen at the end.
If the tube is filled with an optically active substance, e.g. B. a solution of granulated sugar a change becomes visible.
… The operation of an optically active substance has the following effect: the light at the back of the screen becomes much less visible. This happens because the oscillation plane of the light has been changed. The reason: sugar solution, as an optically active substance, has rotated the polarisation direction. The angle of rotation depends on the length of the path in the solution and on the concentration of the solution itself and it also depends on the type of sugar.
If the tube is filled with granulated sugar (sucrose), the angle of rotation is dextrorotatory (clockwise).
If the tube is filled with pure fructose, the angle of rotation is levorotatory (counter-clockwise).
Another example: clockwise - and counter-clockwise lactic acid
Lactic acids differ in the spatial arrangement of the OH group on the central carbon atom. The element symbol makes statements about the spatial arrangement, but not about the rotation angle clockwise (+) or counter-clockwise (–). This information is derived from (+) and (–).
An example: In the case of the clockwise L(+) lactic acid, the OH group is on the left (L for Greek laevo = left). In the case of counter-clockwise D(–) lactic acid, the OH group is on the right (D for Greek dextro = right).
Sauerkraut and natural dairy products are rich in clockwise rotating lactic acid and lactic acid cultures. Counter-clockwise turning lactic acids are produced by metabolic products of certain intestinal germs. Normally, our body is almost exclusively clockwise rotating lactic acid. While we have a specific enzyme for L (+) – lactic acid that causes rapid degradation, the counter-clockwise lactic acid is metabolised only slowly. For this reason, infants should not consume counter-clockwise lactic acid in the first twelve months of life. Their not quite mature intestine is overwhelmed with the degradation.
Importance for the pharmaceutical industry
Many major pharmacological agents are optically active, due to their chirality and are present as enantiomers. Enantiomers (comparable to the left and right hand) generally differ only slightly in their physical properties, such as density or refractive index. They can hardly be distinguished and separated in this way, even using HPLC (liquid chromatography method). All these factors make the differences in their ability to react with enzymes and the sensory characteristics, such as smell and taste or the physiological effect of a medicine, even more important.
An outstanding example here is thalidomide: The substance was in sleeping pills present as a racemate (enantiomeric ratio 1:1). One of the enantiomers had a teratogen effect, i.e. when taken, it caused malformations in pregnancy on the embryo. This demonstrates clearly and impressively that the highly selective analysis of optical rotation may have a key role in the pharmaceutical industry.
What exactly is measured by the polarimeter?
A polarimeter measures the direction and extent of the polarisation plane rotation. Depending on the type of instrument, the analyser is rotated manually or automatically until the maximum intensity of the light falls on the detector. A polarimeter is the entirety of light source, polariser, polarimeter tube, analyser and detector.
In the industrial field and in laboratories with a high number of measurements, digital polarimeters are used. Analyser and detector are automatically, electronically controlled in these devices. It`s short measuring time of a few seconds per measurement and a computer interface allow many series of measurements to be carried out.
What influences the measurement?
In the polarimetry the rotation angle gives information about the molecular structure, the purity and concentration of the optically active substance and sometimes you receive information about the solvent used.
The optical activity is also influenced by the 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 greater the angle of rotation.
In addition, the concentration of the substance and the type of solvent are important for the measurement. These variables are very important for the correct determination of the angle of rotation.