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 medicine 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. These results are relevant for tests such as Blood-Urea-Nitrogen-Test (gives info about kidneys and liver functions), Creatinine-Blood-Test (measures level of Creatinine), Transaminases-Test (Liver function), Hemogram- (checks for blood disorders) and Glycaemia-Test (concentration of glucose). Especially for the determination of fructose-, sucrose- and glucose-concentration the polarimetry is essential to diagnose diabetes. But not only for diagnosis polarimetry is of great importance, it is also indispensable for therapy-use to check regularly if a certain drug changes for example the glucose level in a patient. In pharmacy, and thus in medicine, there are tolerances for the fabrication for drugs which must not exceed specific rotation angles before consumed by the patient (e.g. the specific rotation of penicillin must be between -61° and -65°) and therefore must be reviewed frequently. Also in forensic medicine polarimetry is of great importance to determine an unknown substance by its rotation angle which might gives evidence of a persons death.

Theoretical Introduction
Light is a transverse wave (A transverse wave is a moving wave that consists of oscillations occurring perpendicular (or right angled) to the direction of energy transfer). The direction of propagation is perpendicular to the vectors of both the electric and magnetic field.

There are many theories to explain the propagation of light, however the most important ones are:

 

1) Planck's Radiation study of radiant Cavity (approx. 1900):

He assumed that the atoms that make up these cavity walls behave like tiny electromagnetic oscillators.

2)R. A. Millikan ( 1868 - 1953)

Nobel prize winner in 1923 for his study of the photoelectric effect.

a) Photoelectric effect is largely a surface phenomenon. (The photoelectric effect refers to the emission, or ejection, of electrons from the surface of, generally, a metal in response to incident light.)

b) Einstein's photon hypothesis meets the objections raised against the wave-theory interpretation of the photoelectric effect.

 

3)Seventeen centuries, Huygen's Principle and the law of refraction:

Using refraction and reflection, Huygen presented the light as a wave front. The application of Huygen's principle to refraction requires that if a light ray is bent toward the normal in passing from air to an optically dense medium then the speed of light in that optically dense medium 'glass' must be less than that in air. The theory is the same for the reflection of a plane wave.

 

4) Proposed theory in the past :

In the past some represented the light travelling as if the source was a machine-gun sending projectiles. The theory felt short of explaining the phenomenon of refraction and was dropped.

Polarization is based on the wave nature of light, it is a phenomenon of transforming unpolarized light into polarized light. When the direction of vibration of electric field vector takes place in all possible directions, then the light is said to be unpolarized, and after undergoing polarization, the light waves vibrate in one single plane.

These are a few ways to polarize light:-

1.       Polarization by transmission

2.       Polarization by reflection

3.       Polarization by refraction

4.       Polarization by scattering

 

Polarization can be done using polarizers. A polarizer is a machine that lets light of a certain polarization pass and blocks the rest of the light waves of other polarizations. Specific rotation is a fundamental property of chiral substances that is expressed as the angle to which the material causes polarized light to rotate at a particular temperature, wavelength and concentration.

Specific rotation is denoted by [α].

T- stands for the temperature

λ is the wavelength of light employed

c-concentration in grams per milliliter (for pure substances the density) or grams per 100 milliliters. The solvent (often ethanol, methanol, DSMO, acetone, water, etc.)

Specific rotation may also be expressed as degrees per mole of the substance where the conditions of measurement (i.e. solvent, light source, and path length) are also specified.

Optical rotation or optical activity is the rotation of the plane of polarization of linearly polarized light as it travels through certain materials. Optical rotation can only be observed in chiral objects or optically active substance. Optically active substances are those which can rotate plane-polarized light. Chirality is a property of a substance according to which it’s mirror image can’t be superimposed on it. Crystals are the most abundant naturally anisotropic occurring optically active substance. For example, the nicol prism is made up of two prisms of calcite cemented with Canada balsam. An "axis of anisotropy" is defined as the axis along which isotropy is broken. These substances also show the property of birefringence.(Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light.)

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
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   l = cuvette length -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