Liquid Chromatography - Chiral Stationary Phases
From the results in figure 48, benzene does not appear to interact with methanol associated with water or water itself but solely with methanol. The linear curve is obtained with zero intercept confirming the validity of this dependence. In fact, the methanol associated with water plays no significant part in competing for the benzene against the dispersive interactions of the n-hexadecane.
Chiral Stationary Phases
There are basically five general types of chiral stationary phase in common use in LC. The first is the protein based stationary phase. These stationary phases usually take the form of natural proteins bonded to a silica matrix. As they are proteins, they contain a large number of chiral centers and are known to interact strongly with small analytes exhibiting strong chiral selectivity. There are specific interactive sites that provide chiral selectivity, but there are many more sites that only contribute to general retention. These other sites can be deactivated by mobile phase additives (e.g. octylamine) which reduces the overall retention and increases the chiral selectivity. The second type consists of relatively small molecular weight chiral substances bonded to silica 9 Pirkle (37). Each bonded group has a limited number of chiral centers available but, due to their small size, there can be a large number of groups bonded to the silica (as opposed to much larger complex chiral moieties). It follows, that a relatively high probability is maintained of the solute interacting with a chiral center. The advantage of the Pirkle chiral phases is that, as the overall interacting molecule is small, the solutes are not strongly retained and thus the chiral selectivity becomes the dominant factor. The third type is based on polymers of cellulose and amylose which were developed by Okamato (38). These are derivatized to link appropriate interactive groups to the cellulose polymer which is then coated onto a silica support. The fourth type is based on the macrocyclic glycopeptides introduced by Armstrong (39). These are materials that also contain a large number of chiral centers, together with molecular cavities in which solute molecules can enter and interact with neighboring groups. The spatial character of the solute will determine the degree of entry and consequently the proximity of interaction which, in turn, will determine the energy of interaction and the magnitude of the retention. Finally, the fifth group contains the cyclodextrin based materials that control retention in a similar manner to that previously described for GC. In LC, the cyclodextrin stationary phases are bonded to a support such as silica and are prepared using similar techniques to those for making reverse phases. The more recent and most effective stationary phases are without doubt those based on the macrocyclic glycopeptides and the cyclodextrins.