Explain in Detail Principles of TLC

TLC stands for Thin-Layer Chromatography, which is an analytical technique to separate and analyze components in a mixture.

Principles of TLC

The TLC plate usually uses aluminum oxide or silica gel as the stationary phase, and the mobile phase is a solvent that is chosen based on the properties of the target components in the mixture. The principles of TLC rely on the differential distribution of individual components between the stationary and mobile phases.

TLC uses a stationary phase in the form of a thin layer of polar adsorbent such as silica gel, alumina, or cellulose on a flat surface. A TLC plate can be a glass plate, a sheet of metal, or a plastic coated with a thin layer of the adsorbent. The adsorbent material contains polar functional groups that can interact with the components in the mixture.
The mobile phase is a liquid solvent or a mixture of solvents that can flow over the stationary phase. It is a liquid or a mixture of different liquids that can dissolve the component molecules and migrate over the stationary phase.

How does TLC Work?

A small amount of the mixture or compound is applied to a single point on the TLC plate.
This functions as the starting point just above the bottom of the TLC plate.
The mixture is loaded repeatedly on the same point with ample time in between to allow the mixture to dry.
The plate is then developed in the mobile phase.
It is placed in a developing chamber, which is usually a shallow pool of solvent.
The plate is carefully placed on the solvent in such a way that it touches the solvent just enough to allow the mobile phase to pass through.
The touching point should be just below the starting point where the compound was loaded.
As the solvent is drawn up through the plate via the capillary force, it moves over the mixture.
The mixture is separated into its components slowly.
Some compounds may stay behind while others move up along the solvent.
The movement of the compounds depends on their physical properties, molecular structure, functional groups, etc.
Compounds with similar physical properties to the mobile phase stay with the mobile phase longer and are carried to the farthest up the TLC plate.
The less soluble compounds and those having a higher affinity to the particles on the TLC plate stay behind.
This creates the separation of the compounds. Later, these compounds are scraped and separated via other methods. .

Some compounds are colored and are easy to spot. Other compounds that do not have a distinctive color are visualized using shining ultraviolet light or by spraying a reagent (e.g., ninhydrin) onto the plate. These reagents will react with the components. Some compounds are saturated with iodine vapour to be detected.

The ninhydrin method is common for amino acids, where the free amino and carboxyl group react with ninhydrin to a characteristic blue colour or pale yellow.

References

Unknown. (2021). Thin-layer chromatography. https://www.iitg.ac.in/biotech/BTechProtocols/Thin%20Layer%20Chromatography%20Protocol.pdf
Kumar, S., 1, Jyotirmayee, K., Sarangi, M., Department of Botany, Ravenshaw University, Cuttack – 75 3003, Odisha, India, & MITS School of Biotechnology, Bhubaneswar -75 1024, Odisha, India. (2013). Thin Layer Chromatography: A Tool of Biotechnology for Isolation of Bioactive Compounds from Medicinal Plants. In International Journal of Pharmaceutical Sciences Review and Research (Vol. 18, Issue 1, pp. 126–132). https://www.globalresearchonline.net/journalcontents/v18-1/18.pdf