Principles and Practice of Chromatography - Chromatography Applications > Liquid Chromatography Applications > Analysis of β-blockers > Page 95


Other than controlling the pH, the effect of the chosen buffer has little or no effect on chiral selectivity. This is verified by the chromatograms shown in figure 53. It is seen that virtually the same selectivity is obtained from all three buffers irrespective of the actual chemical nature of the buffers themselves. However, although the difference is exceedingly small, the slightly greater separation ratio obtained from the buffer containing triethylamine might reflect the relatively strong dispersive character of the ethyl groups in the buffer molecule. The enthalpic contributions to retention can be strongly dispersive, and/or strongly polar, or result from induced dipole interactivity. The aromatic rings will allow induced dipole interactions with the stationary phase and conversely, the strong polar groups on the stationary phase can induce dipole interaction with polarizable groups on the solute.

Figure 53 The Separations of the Isomers of Terbutaline Employing Different Buffer Solutions


The cavities are more shallow than those in the cyclodextrins and thus interactions are weaker however, this allows more rapid solute exchange between the phases, and thus higher column efficiencies. An example of the use of the stationary phase to separate the enantiomers of 3-methyl-5-phenylhydantoin is shown in figure 54. The separation is carried out under two conditions, 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 is strongly polar. Pure ethanol provides extremely strong dispersive interactions in the mobile phase relative to that of the aqueous solvent which will be significantly more dispersive than any interactions involved with the stationary phase.