The Mechanism of Chromatographic Retention - Chiral Chromatography > Chiral Polysiloxane Stationary Phases > Page 72

 

Vancomycin is a very stable chiral stationary phase, has a relatively high sample capacity, and can be covalently bonded by multiple linkages to the silica gel surface. It can be used with mobile phases with a high water content, as a reversed phase, or with a high solvent content, as a largely polar stationary phase. For example, when used as a reversed phase THF–water mixtures are very effective mobile phases. Conversely, when used as a polar stationary phase, n-hexane–ethanol mixtures are often employed. Vancomycin has a number of ionizing groups and thus can be used over a range of different pH values (pH 4.0 to 7.0) and exhibit a wide range of retention characteristics and chiral selectivities. Ammonium nitrate, triethyl-ammonium acetate and sodium citrate buffers have all been used satisfactorily with this stationary phase.

The effect of the chosen buffer has little or no effect on chiral selectivity only on pH control. An example of the use of Vancomycinas a stationary phase to separate the enantiomers of 3-methyl-5-phenylhydantoin 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 that depend largely on dispersive interactions (hydrophobic) to provide retention and selectivity or, alternatively, achieve the separation by exploiting ionic or polar interactions (hydrophilic).The fifth group contains thecyclodextrinbasedmaterials(which are the same as those used in GC) In LC, the cyclodextrin stationary phases used in LC are bonded to a support such as silica and are prepared using similar techniques to those for making reverse phases.