Page 76 - Chiral Separation Techniques

52 2 Method Development and Optimization of Enantiomeric Separations Using …

Fig. 2-18. Normal phase retention of the first eluted and second eluted enantiomer of mephenytoin on
–1
vancomycin CSP (250 × 4.6 mm). The flow rate was 1.0 mL min at ambient temperature (23 °C).

2.4.4 Optimization of Enantiomeric Separations in Normal Phase

Typical normal-phase operations involved combinations of alcohols and hexane or
heptane. In many cases, the addition of small amounts (< 0.1 %) of acid and/or base
is necessary to improve peak efficiency and selectivity. Usually, the concentration of
polar solvents such as alcohol determines the retention and selectivity (Fig. 2-18).
Since flow rate has no impact on selectivity (see Fig. 2-11), the most productive flow
–1
rate was determined to be 2 mL min . Ethanol normally gives the best efficiency
and resolution with reasonable back-pressures. It has been reported that halogenated
solvents have also been used successfully on these stationary phases as well as ace-
tonitrile, dioxane and methyl tert-butyl ether, or combinations of the these. The opti-
mization parameters under three different mobile phase modes on glycopeptide
CSPs are summarized in Table 2-7.