Thin Layer Chromatography is a type of liquid chromatography, although when the phrase liquid chromatography is used, most often it is in the context of High Performance Liquid Chromatography. However, TLC makes use of a liquid mobile phase and falls under this classification. A sound understanding of the more basic TLC will greatly aid in understanding both HPLC and column chromatography.

STATIONARY AND MOBILE PHASES

The silica coating on a
3" X 1" glass slide is the simple stationary phase of our common organic chemistry TLC lab experiment. Different types of silica are used in stationary phases across all chromatography types. Its widespread use can be attributed to both its inertness and its porous structure. The silica gel in TLC is mounted on the slide, or solid support.

A number of different solvents can be employed as mobile phases for a TLC experiment. Because a mobile phase must be able to transport an analyte, inorganic solvents only work with inorganic materials, and organic solvents can work with either organic or inorganic materials. Confinement of a TLC plate in a chamber which has its headspace (the air in the chamber) saturated with solvent vapor allows for elution of a sample by capillary action. The solvent simply rises up the slide and brings the analyte with it. Solvents are not strictly one and only one compound. Different solvents can be mixed in varying ratios to achieve a good separation.

Determining the optimum solvent mixture for your TLC experiment can be challenging, as there are no steadfast rules governing this procedure. It is almost entirely a matter of building experience through trial and error. However, understanding how chromatography works can make your guesswork a bit more educated.

While we have discussed a number of different interactions that occur between the mobile phase, the stationary phase, and the accompaning analyte, the factor that is most important here is polarity. Recall that the analyte exists in an equilibrium between the stationary and mobile phases. If there exists a greater polar attraction between the solvent and the analyte than the silica and the anaylte, then the analyte will spend more time traveling in the mobile phase along the plate than it will being attached to the stationary phase. Thus, the greater the polarity of a solvent, the greater the elution. However, this is all relative to the polarity of the analyte itself. If the analyte has a greater polarity than the solvent, it will remain more easily attached to the silica. Thus, different analytes will behave differently to the polarity of a solvent, depending on their own polarities. By experimenting with solvent mixtures and keeping in mind this idea of polarity, you can achieve a desirable separation.