be predominantly dispersive and in LC the eluting mobile phase would be predominantly polar (cf. reverse phase chromatography). If the stationary phase is largely polar in character then the retention mechanism will be predominantly polar and in LC the eluting mobile phase would-be made dispersive (cf. normal phase chromatography). An example of dispersive and polar interactions is afforded by the separation of the gasoline sample on both a highly dispersive stationary phase, and a strongly polar stationary phase. The separations are shown in figure 2. GC gives a clear indication of the retentive character of the stationary phase as there are no significant interactions in the mobile phase. Gasoline has a relatively high proportion of aliphatic hydrocarbons which can only interact dispersively with any stationary phase. However, it has also a significant number of different aromatic hydrocarbons present which, as already has been discussed (book 1), can be polarized and

stationary phase must also be dispersive (a reversed phase) to promote dispersive interaction with the solutes and provide adequate retention and selectivity. If the solutes are strongly polar then a polarizable stationary phase (one containing aromatic rings or cyano groups) would be appropriate to separate the solutes by polar and induced polar interactions. If the solutes are weakly polar then a strong polar stationary phase would be required (such as silica gel) to separate the solute by polar interactions. The mobile phase must be chosen to complement the stationary phase so that the selected interactions are concentrated in the stationary phase. Thus, a reversed phase having strong dispersive interactions would be used with a strongly polar mobile phase (e.g., mixtures of methanol and water acetonitrile and water or tetrahydrofuran and water). In contrast, if the strongly polar silica gel is selected for the stationary phase then a strongly dispersive mobile phase would be

adjustment of the chromatographic conditions. It should be pointed out that multi-layer adsorption is quite feasible. The second layer of ethyl acetate might have another absorbed layer of ethyl acetate on it. However, any multi solvent layers that may be formed must be progressively more weakly held  and sparse in nature. Under such circumstances their, presence, if in fact real, will have little impact on solute retention     Solute Stationary Phase Interactions Mobile Phase Component Dispersive or Weakly Polar There are two types of interaction that can take place between a solute and an adsorbent surface. Firstly, the solute molecule can interact with the adsorbed solvent layer and rest on the top of it. This type of interaction is called sorption interaction and occurs when the molecular forces between the solute and the silica are relatively weak compared with the forces between the solvent molecules and the silica. The second type of interaction is

shown in figure 36. Two conditions are employed, the first used pure ethanol as the mobile phase, which is relatively dispersive, and in the second, a mobile phase that contains 90% of water which will be strongly polar. With pure ethanol there will be relatively strong dispersive interactions in the mobile phase which, in any event, will significantly exceed any dispersive interactions involved between the solute and the stationary phase. It follows that the dominant retentive forces will be polar or ionic in nature. In the second case, the mobile phase is predominantly water and thus provides very strong polar interactions with the solute but very weak dispersive interactions. It also follows, that the retention forces of the stationary phase, in this case, will be dominantly dispersive in nature. The two separations demonstrates a very useful flexibility of Vancomycin as a stationary phase. Adjusting the mobile phase composition, can be very effective for separating solutes

interaction, when the silica surface is covered with a layer of molecules from one or more different solvents. Such layers can impede the interaction of the solute directly with the stationary phase as in sorption. If the solute interacts strongly with the stationary phase, however, the solute molecule will replace a solvent molecule on the surface which will be accompanied by the release of a solvent molecule into the mobile phase. In the case of silica gel, if the solute is strongly polar, it may be capable of displacing either, or both, solvents from the surface. If the solute has an intermediate polarity, the solute molecules may displace the weaker solvent component but interact directly with the layer of the stronger solvent by sorption. In this way, both interactive processes would take place at the same time, sorption and displacement. Mobile Phase Component Polar Mobile phases consisting of mixtures of polar and dispersive solvents frequently produce surface bi-

that contain permanent dipoles and can exhibit polar interactions with other molecules are alcohols, esters, ethers, amines, amides, nitriles, etc.  The retentive characteristics of a polar stationary phase are displayed in the lower chromatogram in figure 2 and can be compared with the retentive characteristics of a dispersive phase shown in the chromatogram above. The polar stationary phase is a cyanopropyl polymer that exhibits relatively weak dispersive interactions but strong polar interactions. The aliphatic hydrocarbons, that are well retained by the dispersive stationary phase, are rapidly eluted on the polar phase but the aromatics are strongly retained and well resolved from one another. On the dispersive stationary phase, all the solutes are spread along the chromatogram, roughly in order of their increasing molecular weights. Finally, it has been shown that the polarizability of a substance containing no dipoles will, among other factors, determine the