Polarimetry (2. LF UK)

Definition
Polarimetry is an optical method used to measure chemical concentration in solution.

Principle
Light waves are electromagnetic (EM) waves. EM waves consist of perpendicular oscillating electric and magnetic fields; the direction of oscillation of both fields is random although they are at all times perpendicular to each other. Light waves which only oscillate in one direction are called polarized. Light can be polarised by passing it through a polarizing filter. Some substances can rotate the the polarisation of light ('optically active substances'), the higher the concentration and the higher the length of solution through which the light passes the larger the angle of rotation. Polarimeters consist of a sodium lamp which emits monochromatic yellow light which is passed through a polarizer. If this polarised light is passed through an optically active substance its plane of polarisation will be rotated, my measuring the angle of rotation one can calculate the concentration. The specific optical rotation [α] is the rotation per unit distance of cuvette and per unit concentration.



Importance In Clinical Medicine
The specific rotation is often used in pharmaceutical industry for the identification and purity control of chiral substances (which are always optically active). Of particular importance is the measurement of concentrations of 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 costs are low. In addition polarimetry is quite specific, because only a few substances rotate polarized light. However, the result may not be very accurate because the method involves the subjective assessment of brightness of light by the user.

How does it work?
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]= 0. If the compound is optical active, the plane of the light would be rotated. 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 rotation α depends on the specific rotation of the substance [α], the length of the tube and the concentration of the optical active compound (and frequency of light and temperature).

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.) Test Tube with solution 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

Measuring the specific rotation of a substance (D-Glucose solution). Specific optical rotation [α] is the rotation per unit distance of cuvette and per unit concentration.



 * 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

*Recommended relation 100α = [α] · l · c   c in g/100ml   α = measured angle of rotation l = cuvette length [α] = specific optical rotation

Measuring the concentration of an optically active substance
Enclosed 10cm long cuvette with D-Glucose with 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) to 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 are developed. Scientists are claiming that in the near future a microchip sized polarimeter will be invented, which will be a major development.

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