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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

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

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

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

interest will remain in the stationary phase. As a simple example, if the solute of interest is strongly polar, and the neighboring substance dispersive, then the stationary phase must be polar (e.g., silica gel in LC, or perhaps,  polyethylene glycol in GC) and in LC the mobile phase would be made predominantly dispersive (e.g., n-heptane or methylene dichloride or mixtures of both) so that polar selectivity remains dominant in the stationary phase. Unfortunately, all substances are not simply polar or dispersive or ionic but can exhibit various combinations of all three interactive characteristics. It follows, that the stationary phase will also need to have the appropriate mixture of interactive properties to maximize selectivity and in contrast the interactive character of the mobile phase will need to oppositely balanced to ensure maximum selectivity still resides in the stationary phase. Under some circumstances the selectivity of the mobile phase can also be exploited to

ûC would indicates that the dispersive contributions by the stationary phase to solute retention were relatively minor. In contrast, the C8 ethylhexanoic acids are eluted just before the n-pentadecane exposing the very strong polar character of the phase. The isomeric dichlorobenzenes demonstrate a good selectivity for the spatial isomers and there is also an effective separation of the two enantiomers. The separation of the dichloro-benzenes again indicates that the presence of strong induced polar interactions between the aromatic nuclei of the solutes and the strong polar groups of the stationary phase. Similarly, the strong polar interactions that are seen to occur between the carboxyl group of the enantiomer that penetrates closest to the stationary phase, and the neighboring polar groups of the interactive site obviously provide the necessary selectivity to resolve it from the other, slightly more excluded, enantiomer.   The Analysis of Gasoline   Gasoline is a multi-