Polarimetry (2. LF UK)

Definition
Polarimetry is an optical method which determines the concentration of a certain substance by measuring the rotation of the plane of linearly polarized light during the passage through this substance.

Principle
Light waves are electromagnetic waves which propagate in the room (perpendicular to the vectors of both the electric and magnetic field). The direction of oscillation of every single wave is differently and randomly distributed in all directions.

Lightwaves, which only oscillate in one direction are called polarized lightwaves.

Polarization of light can happen by reflexion, refraction and through a polarization filter.

For analysis of optical active substances (=substances which rotate/change the direction of polarized light), so called Polarimeters are used. These apparatuses consist of a sodium vapor lamp which beams out monochromatic light. This light is sent through a polarizer, which serves like a grid, only letting light through which oscillates in the direction of the grid.

If the analyzed substance has got the same "grid-direction" like the polarizer, the polarized light will pass through completely in the same direction like the light beam (substance is not optically active); if the analyzed substance is optically active, the direction of polarized light will be turned by a specific rotation angle specific for the substance, so that you will see a light beam which has got another direction than the "starting-beam".



Importance In Clinical Medicine
The specific rotation is often used in pharmaceutical industry for the identification and purity control of chiral substances. Of particular importance is the specification of the specific rotation for natural products, such as amino acids, terpenes and sugars, since the majority of these substances are optically active.

Advantages and disadvantages
For the polarimeter large sample volumes with high substance concentration are required. However, the sample used remains unchanged after measurement. Due to the low equipment complexity, the measurement is very simple and fast, which means that the costs remain very low. In addition to that the polarimetry is quite specific, because only a few substances rotate the polarized light. Yet, the result is not very accurate because the decision as to when the subfields of the half shadow device have the same brightness is very subjective. As a result, larger measurement errors can also occur.

How does it work?
Normal monochromatic light contains light that possesses oscillations of the electrical field in all possible planes perpendicular to the direction of propagation. When light is passed through a polarizer (i.e., Nicol prism, Polaroid film) only light oscillating in one plane will leave the polarizer. This linear polarized light can be described as a superposition of two counter-rotating components, which propagate with different velocities in an optical active medium. If one component interacts stronger than the other with a chiral molecule, it will slow down and therefore arrive later at the observer. The result is that the plane of the light appears to be rotated because the two vectors are not canceling each other anymore due to the phase shift. In a polarimeter, plane-polarized light is introduced to a tube containing a solution with the substance to be measured. If the substance is optical inactive, the plane of the polarized light will not change in orientation and the observer will read an angle of [α]= 0o. If the compound in the polarimetry cell was optical active, the plane of the light would be rotated on its way through the tube. The observed rotation is a result of the different components of the plane polarized light interacting differently with the chiral center. In order to observe the maximum brightness, the observer (person or instrument) will have to rotate the axis of the analyzer back, either clockwise or counterclockwise direction depending on the nature of the compound. For clockwise direction, the rotation (in degrees) is defined as positive ("+") and called dextrorotatory (from the Latin: dexter=right). In contrast, the counterclockwise direction is defined as negative ("-") and called levorotatory (from the Latin laevus=left). The observed specific rotation [α]obs depends on the length of the tube, the wavelength that it is used for the acquisition, the concentration of the optical active compound (enantiomer), and to a certain degree on the temperature as well.

Risks
There are no major risks associated with polarimetry; neither for the patient nor for the operator.

Ethical issues
Polarimetry does not raise any ethical issues

Construction of the Instrument
System diagram of the instrument



1.) Light Source (Sodium Light) 2.) Collector Lens 3.) Colour Filter 4.) Polarizer 5.) Half-wave Plate 6.) Teste Tube 7.) Polarization Analyzer 8.) Object Lens 9.) Eye Lens 10.) Magnifying Glass 11.) Dial Vernier 12.) Dial Rotary Hand-wheel 13.) Protective Plate

Manual Polarimeter; lateral view

Manual Polarimeter; superior view

Manual Polarimeter; superior view with testtube

Testtubes

Evaluation of specific rotation of a substance (D-Glucose) solution with a given concentration


*Recommended relation 100α = [α ] · l · c   c in g/100ml l = cuvette length
 * 1) Prepare the measuring equipment and the enclosed 20cm long cuvette with D-Glucose solution with concentration 10g/100ml of the solution.
 * 2) Insert the cuvette into the polarimeter
 * 3) Measure the optical rotation of the solution
 * 4) Repeat step 3.) five times
 * 5) Write the results in the report table
 * 6) Calculate average value and standard deviation
 * 7) Calculate specific optical rotation of D-glucose

Specific optical rotation
the change in orientation of monochromatic plane-polarized light, per unit distance–concentration of the product, as the light passes through a sample of a compound in solution

Evaluation of concentration of an optically active substance
Enclosed 10cm long cuvette with D-Glucose has an unknown concentration.


 * 1) Insert the cuvette into the polarimeter
 * 2) Measure the optical rotation of the solution
 * 3) Repeat step 2.) five times
 * 4) Write the results in the report table
 * 5) Calculate the average value of optical rotation [α ] and standard deviation σ
 * 6) Use your results and specific rotation value (from 1st part)
 * 7) Find the concentration

Future of Polarimetry
The future plans of polarimetry are to make polarimeters smaller, cheaper, mobile, simpler and more precise. Every year new advanced models get developed as further researches are conducted. Scientists are claiming that in the near future a microchip sized polarimeter will be invented, which will be a major development. The field of astro polarimetry is progressing quickly due to the need of an improved technology to make new discoveries which are more precise and informative. However, progress with polarimetry is very time consuming and difficult. Hence, big technological leaps are not expected to happen, rather slow and steady progress over time.

Reference list
1.) Rudolph Research Analytical; http://rudolphresearch.com/products/polarimeters/polarimetry-definitions/ 2.) Rudolph Research Analytical; http://www.chem.ucla.edu/~bacher/General/30BL/tips/Polarimetry.html 3.) ChemgaPedia; http://www.chemgapedia.de/vsengine/about/de/index.html 4.) Fun Man FUNG (Video); https://www.youtube.com/watch?v=T6zjiU_-91g 5.) TheSimpleChemics (Video); https://www.youtube.com/watch?v=l_G34WeJjgs&t=36s 6.) Andreas Jerrentrup; 1.ÄP Physik für Mediziner; 17th edition; 2006; ISBN: 978-3-13-114937-4