thin+layer+chromatography

Thin-layer chromatography (TLC) is an analytical technique used to:
 * quickly and effectively separate and identify a small sample of organic compounds by polarity;
 * verify the existence of a certain compound;
 * calculate the retention factor (Rf) for any given compound.

This is not a large-scale separation technique but rather a micro-scale tool used to quickly ascertain the presence of a compound.

The most common material used in TLC is the silica gel sheet, a.k.a. TLC plate:



This is a thin sheet of plastic that is coated on one side with a solid silica layer referred to as the **solid phase** (stationary phase).



Small volumes of organic compounds (usually mixtures of such) are dissolved in a volatile solvent such as acetone and are transferred to the TLC plate; multiple samples are aligned horizontally along one edge, about 1.5 cm in:



The samples are transferred using a micropipet, or a glass capillary tube. These are usually hand-made and hold extremely small sample volumes.

Once the samples are placed on the TLC plate, the plate is then placed in a container that will immerse the bottom edge of the plate in the mobile phase. The mobile phase is usually a mixture of low- or non-polar organic solvents, designed to be absorbed via capillary action by the solid phase.

The mobile phase solvents will 'travel' up the TLC plate, dissolve and 'carry' the organic samples up the plate at rates dependent on their polarity. Since the solid phase is highly polar, any molecules that are polar will stick to the solid phase, slowing their progress. This means that polar molecules will not move far up the TLC plate, whereas nonpolar molecules will travel further.



When the solvent's visible progress (called the solvent front) reaches the top centimeter or so of the TLC plate, the plate is removed from the solvent and allowed to dry. This essentially locks the sample molecules into place on the plate.

Since most organic molecules are white or colorless, they will not be visible on the white solid phase. To identify the sample locations on the plate, it can either be immersed in a stain or placed under an ultraviolet lamp.



Once the spots have been identified and traced using a pencil, their distances traveled from the origin can be measured using a ruler. The solvent front is also measured from the origin. A ratio of these distances provides the retention factor (Rf) for each sample spot.





The retention factor is then a measure of a substance's polarity relative to other samples on the plate. It can only be compared to other samples that were run in exactly the same manner (same solid and mobile phase compositions).