Stationary phase solvent strength affected

Mobile phases are usually binary or ternary mixtures of solvents. Selectivity is affected mostly by mobile phase composition rather than strength, and peak shape and retention are both influenced by the addition of organic modifiers.101 Some compounds naturally have 77-donor or 77-acceptor groups and can be resolved directly. In many cases, however, introduction of 77-donating groups by derivatization steps is necessary. Figure 2.20 shows the proposed three-point interaction of 3-aminobenzo[a]pyrene, a polycyclic aromatic hydrocarbon (PAH), with a Pirkle-type stationary phase.111 Two possible interactions are illustrated, showing the best orientations for maximum interaction. 

In MLC, solutes are separated on the basis of their differential partitioning between bulk solvent and micelles in the mobile phase or surfactant-coated stationary phase. For water-insoluble species or for species strongly bound to micelles, partitioning can also occur via direct transfer between the micellar pseudophase and the modified stationary phase. Partition equilibria are affected by a variety of factors, such as the nature and concentration of surfactant and organic modifiers, temperature, ionic strength, and pH. 

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