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Measuring optical activity means measuring the optical rotation of a compound using a polarimeter, a device which employs polarimetry.
But why is this technique important? In chemistry, polarimetry is essential in understanding chiral molecules, which are characterized by their ability to rotate a plane of polarized light.
A chiral substance is going to have the ability to slow light down, visually altering the direction of oscillation. The wavelength’s change in direction is then measured.
Measuring the change in direction of a wavelength is possible because light constantly oscillates across all possible planes perpendicular to its origin.
For instance, a light bulb will light up a room because every wave emitted vibrates in its own direction; up and down, side to side, and perpendicular to the direction the light wave is traveling. This is known as unpolarized light.
In the case of polarimetry, light is polarized using a polarizer, an optical filter that allows for a specific wavelength to pass through, all while blocking other wavelengths of light. Through this process, an undefined or mixed light beam becomes a well-defined one, traveling in a single direction.
Polarimeters analyze different chiral solutions and their specific rotations by introducing linear plane-polarized light through a small tube containing a liquid sample. If a compound in a solution is optically inactive, the polarized light will not change.
However, if the compound is an optically active substance, the plane of light will change. As the light travels through the liquid sample, it will interact with the chiral molecules. This interaction causes the light to turn slightly. As it reaches the other end of the tube, its angle of rotation will be different than how it entered.
Polarimeters are used in numerous settings, from the pharmaceuticals, chemicals, and medical industries to the food industry.
The pharmaceutical industry relies on them in tandem with high-performance liquid chromatography systems to measure concentration of optically active compounds found in pharmaceutical drugs; the chemical industry uses polarimeters for characterization and identification of various compounds; the medical field relies on polarimetry for research in enzymology and toxicology; and lastly, the food industry uses polarimeters for quality control of products at a number of stages, as well as the concentrations and purity of food products.
This device is made up of a number of components, some of the important ones will be covered here. Polarimeters also come in a wide variety of configurations. As automation has become increasingly available, semi-automatic and fully-automatic polarimeters have replaced manual ones, changing the way scientists study optical activity.
In general, two polaroid plates are used to polarize light in a polarimeter. Placed apart from one another, the lower plate is fixed, and is known as the polarizer. The higher plate can often be rotated, and is referred to as the analyzer. The polarizing plate, or filter, only allows light of a certain direction of polarization to enter, blocking all other directions of light. Having passed through the first plate, the light then passes through the second plate, so as to maintain the plane of oscillation and to observe the oscillations.
The light source of these devices can vary. Typically, an LED is used to produce a beam of ordinary light. Sodium or mercury discharge tubes may also be used as a more cost-effective alternative. These conve2401nient light sources produce monochromatic light which allows many interference effects to be made visible. Many brands employ LED as a lightsource due to their long lifespan.
These devices have two polarization filters positioned so that no light passes through. If an optically active sample is used in a circular polarimeter, the light will be redirected through the darkened filter. By repositioning the secondary filter, the amount of rotation from the solution can be observed and measured.
A semi automatic polarimeter consists of both manual and digital components. The manual zero-point calibration allows a user to adjust their optical field of view and eliminate any zero-point errors. The digital component usually includes a digital interface that allows the user to reset the polarimeter to its original calibration.
The benefits of a fully automatic device include the ability to perform a digital read-out at the press of a button. These devices yield accurate results within seconds and can provide continuous measurements. This device also includes the addition of a Faraday modulator, which creates an alternating magnetic field. This enhances the accuracy of detection by oscillating the plane of polarization, allowing the light to pass through the filters multiple times. A fully automatic polarimeter can also control the temperature of the solution inside the device, which reduces measuring errors. All results can be transferred directly to a computer or network for automatic processing.
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