Page 74 - Chiral Separation Techniques

50 2 Method Development and Optimization of Enantiomeric Separations Using …

In reversed phase, many chiral separations (excluding amino acids) on glycopep-
tide CSPs are achieved at relatively low percentages of organic modifiers (10–20 %);
hence, the screening conditions are set up this way. Alcohols generally require
higher concentrations than acetonitrile or THF for comparable retention times.
Empirically, THF works best with vancomycin in terms of broad selectivity and high
efficiency, while methanol works best for teicoplanin and ristocetin A. Nevertheless,
the final choice and percentage of organic modifier is eventually determined by the
nature of the analyte and should be optimized accordingly.

Fig. 2-16. The effect of organic modifiers on the resolution of fluoxetine enantiomers on vancomycin
–1
CSP (250 × 4.6 mm). The flow rate was 1.0 mL min at ambient temperature (23 °C). (Courtesy of Scott
Sharpe, Eli Lilly & Co.)

This observation is important in the study of the chiral recognition mechanism in
this system. This may be a practical matter when determining the trace amount of
one enantiomer in the presence of its dominant antipode. The smaller peak is always
desired to be eluted first for best quantitation.