The choice of a good solvent system for the mobile phase is highly dependent on the nature of the compound that is analysed. Often a few trial TLC plates, in different ratios of polar and non-polar solvent systems, are run to achieve a desired Rf range of 0.3 - 0.7. In addition to effectively separating the samples of interest, a good solvent system moves all compounds from the baseline but does not migrate compounds to the solvent front. Although some inferences can be made in relation to the polarity of the samples with the polarity of the solvent, the particular ratio and choice of solvent system can be only truly determined experimentally. It is important to bear in mind that even small additions of polar solvent can result in large changes in separation.
The most polar solvent system, will have a greater attraction to the stationary phase than the samples. Therefore, the samples will be carried all the way to the solvent front. Whereas, the least polar solvent system will have no interaction with the stationary phase, leaving the compounds at the baseline.