Mobile Phases:

The choice of the mobile phase is largely empirical but some general guidelines can be formulated. A mixture of organic solvents and water with the addition of acid, base, or complexing agent to optimize the solubility of the components of a mixture can be used. It may be emphasized that a large degree of trial and error is involved in the selection of the mobile phase. The polar solvents could themselves become strongly adsorbed; therefore, producing an undesirable condition of partition system.

The subsequent criteria might be adopted.

i) The developing solvent must be of the highest purity because even trace amounts of impurities may yield irreproducible results.

ii) Good separation of polar or ionic solutes can be achieved with a mixture of water and n-butanol. The criteria used for the range of developing solvent are the same as for column chromatography.

iii) The eluting power of solvents increases in the order of their polarities e.g. from hexane to acetone to alcohol to water. Since eluotropic series could be used for the selection of the best solvent or the solvent mixture for a sample.

iv) If the stationary phase is hydrophobic, cyclohexane, mixtures of benzene, and chloroform in different ratios provide satisfactory mobile phase.