Thin-layer chromatography — common mistakes

In exam questions, always measure distances for $R_f$ calculations from the centre of the spot to the baseline. For the solvent front, measure from the highest point the solvent reached to the baseline. Be precise and use a ruler.

Different compounds have different polarities and solubilities. A compound's $R_f$ value depends on the balance between its adsorption to the polar stationary phase and its solubility in the mobile phase. A more polar compound will stick more tightly to the silica gel and have a lower $R_f$ value, while a less polar compound will travel further with the solvent and have a higher $R_f$ value (assuming a polar stationary phase and a less polar mobile phase).

No, an $R_f$ value can never be greater than 1. The $R_f$ is a ratio of the distance travelled by the spot to the distance travelled by the solvent front. A spot can never travel further than the solvent that is carrying it, so the numerator is always less than or equal to the denominator.

The choice of solvent (eluent) is critical. If the solvent is too polar, all compounds (polar and non-polar) will be highly soluble and travel with the solvent front, resulting in all spots having high $R_f$ values close to 1 and poor separation. If the solvent is too non-polar, no compounds will move from the baseline, resulting in $R_f$ values near 0. The ideal solvent provides good separation with a range of $R_f$ values across the plate.

TLC is primarily a qualitative technique, used for identifying components and checking purity. While the intensity and size of a spot can give a rough idea of its concentration, it is not a precise quantitative method. For accurate quantification, other techniques like High-Performance Liquid Chromatography (HPLC) are used.