Thin layer chromatography
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Thin layer chromatography (TLC) is a widely-used chromatography technique used to separate chemical compounds. It involves a stationary phase consisting of a thin layer of adsorbent material, usually silica gel, alumina, or cellulose immobilised onto a flat, inert carrier sheet. A liquid phase consisting of the solution to be separated dissolved in a solvent is drawn through the plate via capillary action, separating the experimental solution. It can be used to determine the pigments a plant contains, to detect pesticides or insecticides in food, in forensics to analyze the dye composition of fibers, or to identify compounds present in a given substance, among other uses.
[edit] Plate preparation
TLC plates are made by mixing the adsorbent, such as silica gel, with a small amount of inert binder like calcium sulfate (gypsum) and water. This mixture is spread as a thick slurry on an inert carrier sheet, usually glass, thick aluminum foil, or plastic, and the resultant plate is dried and activated by heating in an oven. The thickness of the adsorbent layer is typically around 0.1–0.25 mm for analytical purposes and around 1–2 mm for preparative TLC.
[edit] Technique
The process is similar to paper chromatography with the advantage of faster runs, better separations, and the choice between different stationary phases. Because of its simplicity and speed TLC is often used for monitoring chemical reactions and for the qualitative analysis of reaction products.
A small spot of solution containing the sample is applied to a plate, about one centimeter from the base. The plate is then dipped in to a suitable solvent, such as ethanol or water, and placed in a sealed container. The solvent moves up the plate by capillary action and meets the sample mixture, which is dissolved and is carried up the plate by the solvent. Different compounds in the sample mixture travel at different rates due to differences in solubility in the solvent, and due to differences in their attraction to the stationary phase. Results also vary depending on the solvent used. For example, if the solvent was a 90:10 mixture of hexane to ethyl acetate, then the solvent is more nonpolar than it is polar. This means that when analyzing the TLC, the nonpolar parts will move further up the plate. The polar compounds, in contrast, will not move as much. The reverse is true when using a solvent that is more polar than non-polar (10:90 hexane to ethyl acetate). With these solvents, the polar compounds will move higher up the plate, while the non-polar compounds will not move as much.
[edit] Analysis
As the chemicals being separated may be colourless, several methods exist to visualize the spots:
- Often a small amount of a fluorescent compound, usually Manganese-activated Zinc Silicate, is added to the adsorbent that allows the visualization of spots under a blacklight(UV254). The adsorbent layer will thus fluoresce light green by itself, but spots of analyte quench this fluorescence.
- Iodine vapors are a general unspecific color reagent
- Specific color reagents exist into which the TLC plate is dipped or which are sprayed onto the plate
Once visible, the Rf value of each spot can be determined by dividing the distance travelled by the product by the total distance travelled by the solvent (the solvent front). These values depend on the solvent used, and the type of TLC plate, and are not physical constants.
