The Mechanism of Chromatographic Retention - Chiral Chromatography > Chiral Polysiloxane Stationary Phases > Page 77
There is considerable evidence supporting the concept of solute inclusion in the cyclodextrin cavity during chromatographic development including a number of NMR studies (23).
Cyclodextrins derivatize selectively and the reaction sites are distributed about a mean. The 2-OH and 6-OH groups are the most reactive while the 3-OH group is the least reactive. Armstrong et al. (24) obtained a plasma desorption mass spectrum for a mixture of O-(S)-2-hydroxypropyl-derivatized b-cyclodextrin which is shown in figure 40.
The number above each peak denotes the number of substituted hydroxy propyl groups per cyclodextrin moiety. It is seen that there is a (more or less) symmetrical distribution of substituents about a mean of 6 hydroxyl groups reacted per cyclodextrin structure. The minimum appears to be about 2 and the maximum about 12 substituents per moiety. This distribution resulting from substitution reaction, shows that the substituted cyclodextrin phases are not necessarily homogeneous substances. It follows, that their net chromatographic properties, including their chiral selectivity, will be the average effect of a number of differently substituted derivatives
Courtesy of Anal. Chem. [Ref. 10]
Figure 40. The Plasma Desorption Spectrum of a Mixture of O-(S)-2-Hydroxypropyl–Derivatized b-Cyclodextrin