Polarimeters are scientific instruments used for measuring the rotation of the plane of polarized light as it passes through a sample of a compound which exhibits optical activity.
Many chemical compounds can exist in more than one optically active form. Each optically active form, or isomer, of a compound will be able to rotate the plane of polarized light by an amount characteristic of that isomer.
Polarimetry is a sensitive, nondestructive method for measuring the optical activity exhibited by these inorganic and organic compounds. The human eye cannot recognize whether light is polarized (light which consists of waves that vibrate only in one place), but the effects of polarization can be observed using polarizing filters.
Polarimetry measures the extent to which a substance interacts with plane-polarized light and whether it rotates plane-polarized light to the left, to the right, or not at all.
Description of Polarimeter. Image Credit: Molecular History Research Center
A substance is optically active if it rotates plane-polarized light to the left or to the right. To be optically active, a compound must have a chiral center - a carbon with four different groups attached. Depending on the orientation of these four different groups about the chiral carbon, the compound may rotate plane-polarized light to the left or to the right.
Compounds that do not rotate light at all lack a chiral center.
The number of degrees and the direction of rotation are measured to provide the observed rotation. This measurement must be corrected for the length of the cell used and the solution concentration. Comparing the corrected observed rotation to published values can aid in the identification of an unknown compound.
Because polarimeters use polarized light, the unpolarized light from the light source must first be polarized, and then passed through a sample cell. The analyzer is rotated until the zero transmission condition is reached, when the polarizer and analyzer transmission axes are at right angles to each other and no light passes through the instrument. This reading is noted on the graduated scale.
If an optical active substance is in a sample tube, the analyzer is rotated until the original dark condition is restored. The rotation is observed by looking through the analyzer, and is identified by a change in intensity of illumination. The new reading is marked on the scale and the difference between the two readings gives the amount in degrees by which the polarized light has been rotated by the optically active solution. Modern polarimeters determine rotation electronically or with lasers, and the results are presented on a digital screen within seconds.
Homemade polarimeter in use. Video credit: David Whyte/YouTube
Components and Types
Polarimeters have two polaroid plates mounted apart. The lower plate is generally fixed and known as the polarizer. The upper plate can be rotated and is known as the analyzer. Polarimeters have two Nicol prisms, a type of polarizer. This polarizer, a fixed prism, is used to produce a polarized beam of light. The analyzer is used to observe the polarized light that is produced, and can be rotated. The polarimeter uses a light source, usually a mercury or sodium discharge tube.
There are several different types of polarimeters.
- Circle polarimeters have two polarizing filters which are positioned so that no light passes through the combination. If an optically active sample is brought between these filters, light will pass again. By turning one of the filters to relocate the dark position, the amount of rotation introduced by the sample can be measured.
- Quartz wedge polarimeters use a quartz wedge instead of the rotating analyzer. The wedge is moved across the beam until the rotation induced by the sample is cancelled out.
Stokes Vector and Mueller Matrix Polarimeters
The Engineering360 SpecSearch database provides information about these types of polarimeters.
Light-measuring (Stokes vector) polarimeters measure the polarization properties of a beam of light. Stokes vector polarimeters derive four characteristics of an incident light beam, the Stokes vector elements, by measuring the flux of the beam upon passing through four unique polarization filters. The components of the four-element Stokes vector completely describe the polarization state of the incident light.
Completely polarized light may belong to one of two families, linear polarization or circular polarization. The orientation derived from the Stokes vector only applies to:
- linear polarized (horizontal or vertical)
- partially linear (i.e., elliptical) (horizontal or vertical)
- right-handed or left-handed circulatory polarized
Sample-measuring (Mueller matrix) polarimeters measure the polarization properties of a particular material.Mueller matrix polarimeters may be considered as a pair of Stokes vector polarimeters working together to determine the polarization properties of a particular material.
Mueller polarimeters perform a series of at least 16 measurements using unique generator and analyzer states to deduce the 4 x 4 Mueller matrix. As with the Stokes vector, the Mueller matrix describes completely the polarization properties of the sample. From this matrix, a number of optical properties of the sample may be deduced including diattenuation, retardance, and depolarization index. Diattenuation and retardance may further be resolved into linear and circular polarization.
When selecting polarimeters, buyers should note common variables in polarimeter design and selection. These include
Complete vs. Incomplete
- Complete polarimeters measure the full Stokes vector of an optical beam or measure the full Mueller matrix of a sample. Often, some characteristics can be neglected and the measurement of all Stokes or Muller elements is not necessary.
- Incomplete polarimeters measure a subset of characteristics and may be used when simplifying assumptions about the light wave or sample are appropriate.
Time-sequential vs. Simultaneous
- Time-sequential polarimeters take the series of flux measurement sequentially in time. Between each measurement, the polarization generator and analyzer are changed. They frequently use rotating polarization elements containing a set of analyzers as well as a single source and single detector. The advantage of this device is that only one polarimeter is needed, which makes setup and dismounting much easier. The disadvantage of sequential polarimetry is that changes in the radiation field cannot be measured.
- Simultaneous polarimeters take all of the necessary images at the same time. The advantage of simultaneous polarimetry is that temporally changing radiation fields (skylight after sunset or prior to sunrise) can also be measured. The disadvantage is that at least three polarimeters have to be handled simultaneously, which is often difficult and time consuming.
Monochromatic vs. Polychromatic
- Monochromatic polarimeters contain the same type of visual pigment
- Polychromatic polarimeters contain visual pigments with several colors.
Imaging Polarimeter vs. Non-imaging Polarimeter
- Imaging polarimeters measure a Stokes vector image or a Mueller matrix image from raw images collected from different analyzers through the polarimeter's focal point array detector. These devices are very sensitive to misalignment due to user or environmental factors such as source motion, polarimeter motion, and vibration.
- Non-imaging devices average polarization over an area of only a few degrees.
Manual (visual) vs. Electronic (automatic)
- Manual polarimeters require users to physically rotate the analyzer. The detector is the user's eye and the angle was marked on a scale that encircles the analyzer. This basic design is still used in the simplest polarimeters.
- Semi-automatic polarimeters require manual visual detection but use buttons to rotate the analyzer and offer digital displays.
- Fully-automatic polarimeters have automatic detection and analysis tools which give the user a digital readout.
- Spectral arc lamps - sodium or mercury can be designed for visible, ultraviolet (UV) and infrared (IR) wavelengths, and are suitable for stability of wavelength and long-term calibration.
- Incandescent lamps (tungsten) are less expensive than spectral arc lamps, but are limited to visible and IR radiation.
Industry, research institutes, and universities use polarimetery for:
- isolating and identifying unknowns crystallized from various solvents or separated by high performance liquid chromatography (HPLC).
- investigating kinetic reactions by measuring optical rotation as a function of time.
- monitoring changes in concentration of an optically active component in a reaction mixture, as in enzymatic cleavage.
- analyzing molecular structure by plotting optical rotatory dispersion curves over a wide range of wavelengths.
- distinguishing between optical isomers.
Quality and Process Control Applications
Quality and process control applications, both in the laboratory or on-line in the factory, are found throughout the pharmaceutical, essential oil, flavor, food and chemical industries. A few examples are listed below.
Pharmaceutical Industry - Polarimeters can be used to determine product purity by measuring specific rotation and optical rotation of compounds such as amino acids, antibiotics, tranquilizers and antibiotics.
For pharmaceutical testing, most work is done using 589 nm light (sodium D line). For electronic (automatic) polarimeters, total accuracies of 0.01° are possible.
Flavor, Fragrance, and Essential Oil Industry - Polarimetry is an important step for incoming raw materials (camphors, gums, natural acids and natural oils) inspection.
Food Industry - Polarimeters ensure product quality by measuring the concentration and purity of the following compounds in sugar-based foods, cereals and syrups: carbohydrates, sucrose, glucose, fructose, various starches, etc.)
Chemical Industry - Analyzing optical rotation as a means of identifying and characterizing chemicals, including bio, natural and synthetic polymers is an additional application for polarimeters.
ASTM International (formerly called the American Society for Testing and Materials (ASTM), maintains ASTM C1426 regarding the standard practices for verification and calibration of polarimeters. Other standards describe how to use polarimeters in standard tests (i.e. ASTM F218 for analyzing stress in glass). The Society of Manufacturing Engineers (SME) maintains SME EM930116 for the rapid contour measurement of composite structures utilizing a three-dimensional (3D) tracking interferometer.
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Horváth, Gábor, Varju, Dezsö. Polarized Light in Animal Vision: Polarization Patterns in Nature. Berlin: Springer, 2004. Print.
How It Works: Science and Technology. 3rd ed. Vol. 13. New York: Marshall Cavendish, 2003. Print.