Thin-layer chromatography is a technique for separating a sample into its component parts. It's used to test for the presence of various materials, to monitor the rate and progress of a reaction or to determine the purity of a product. Filter paper impregnated with solvent is usually used to saturate the development chamber's air with solvent vapor so the stationary phase doesn't dry during the process.
Layers and Phases
The "thin layer" in thin-layer chromatography refers to an adsorbent matrix thinly painted onto a plate. An adsorbent is a material that attracts particles in a compound and makes them stick to the plate, such as alumina powder or silica gel. The plate itself is usually a very thin sheet of glass or plastic. This matrix-coated sheet is called the stationary phase. It's used to separate particles in the mobile phase.
Dissolution Is the Solution
The mobile phase is a solvent -- or combination of solvents, called a "solvent system" -- plus dissolved particles of your sample. A solvent is the liquid medium for a chemical reaction. In TLC the stationary matrix is slowly impregnated with various solvents, which dissolve samples placed on the matrix. The component materials separate as the solvent and dissolved sample travel up the sheet.
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Paper Placement, Air Impregnation
You place your filter paper in the sealed development chamber after the solvent but before the stationary phase. It absorbs the liquid in the solvent and provides more surface area for evaporation. More surface area means more and faster evaporation. More evaporation means more solvent vapor in the chamber's air, which is desirable.
Damp, Chemical-Filled Air
You want your chamber air thoroughly impregnated with solvent vapor in TLC because this keeps the stationary phase from drying out before the process is finished. The solvent evaporating from the paper saturates the chamber's air so it doesn't wick solvent off the stationary phase as quickly. If the stationary phase dries out prematurely, the components in the sample won't separate properly and your results will be incorrect. The stationary medium should remain dampened with solvent until you remove it from the chamber and dry it on purpose.