TLC lab
Briefing
Thin-layer chromatography (TLC) is an extremely valuable analytical technique. It provides a rapid separation of compounds, and gives an indication of the number and nature of the components of a mixture. TLC can also be used to identify compounds by comparison with known samples, to check the purity of a compound, or to monitor the progress of a reaction, an extraction, or a purification procedure. This experiment will introduce you to the mechanics of TLC, and the chemical principles behind it. TLC is normally done on a small glass or plastic plate coated with a thin layer of a solid — the most common are silica (SiO2) or alumina (Al2O3). This is the stationary phase. The mobile phase is an organic solvent or solvent mixture. The sample mixture is applied near the bottom of the plate as a small spot, then placed in a jar containing a few ml of solvent. The solvent climbs up the plate by capillary action, carrying the sample mixture along with it. Each compound in the mixture moves at a different rate, depending on its solubility in the mobile phase and the strength of its absorption to the stationary phase. When the solvent gets near the top of the plate, it is allowed to evaporate, leaving behind the components of the mixture at various distances from the point of origin. The ratio of the distance compound moves to the distance the solvent moves is the Rf value (retention factor). This value is characteristic of the compound, the solvent, and the stationary phase. Silica and alumina are relatively polar stationary phases. Both have OH groups on their surfaces that interact strongly with polar compounds. Such compounds are adsorbed strongly and therefore move along the plate slowly, while non-polar compounds are absorbed only weakly and are therefore carried along the plate more quickly. Of course, solvent polarity also affects how fast compounds travel. Polar compounds are carried along quickly by polar solvents, but move slowly or
Thin-layer chromatography (TLC) is an extremely valuable analytical technique. It provides a rapid separation of compounds, and gives an indication of the number and nature of the components of a mixture. TLC can also be used to identify compounds by comparison with known samples, to check the purity of a compound, or to monitor the progress of a reaction, an extraction, or a purification procedure. This experiment will introduce you to the mechanics of TLC, and the chemical principles behind it. TLC is normally done on a small glass or plastic plate coated with a thin layer of a solid — the most common are silica (SiO2) or alumina (Al2O3). This is the stationary phase. The mobile phase is an organic solvent or solvent mixture. The sample mixture is applied near the bottom of the plate as a small spot, then placed in a jar containing a few ml of solvent. The solvent climbs up the plate by capillary action, carrying the sample mixture along with it. Each compound in the mixture moves at a different rate, depending on its solubility in the mobile phase and the strength of its absorption to the stationary phase. When the solvent gets near the top of the plate, it is allowed to evaporate, leaving behind the components of the mixture at various distances from the point of origin. The ratio of the distance compound moves to the distance the solvent moves is the Rf value (retention factor). This value is characteristic of the compound, the solvent, and the stationary phase. Silica and alumina are relatively polar stationary phases. Both have OH groups on their surfaces that interact strongly with polar compounds. Such compounds are adsorbed strongly and therefore move along the plate slowly, while non-polar compounds are absorbed only weakly and are therefore carried along the plate more quickly. Of course, solvent polarity also affects how fast compounds travel. Polar compounds are carried along quickly by polar solvents, but move slowly or