Page 312 - Chiral Separation Techniques

304 12 Sub- and Supercritical Fluid Chromatography for Enantiomer Separations

tions between the two techniques. The occasional failures of SFC do not diminish
the unique advantages of sub- and supercritical eluents for enantiomeric separations.

12.3.1 Increased Efficiency

The efficiency of many CSPs increases dramatically when liquid eluents are
replaced with sub- or supercritical fluids. During a comparison of LC and SFC per-
formed with a Chiralcel OD CSP, Lynam and Nicolas reported that the number of
theoretical plates obtained was three to five times higher in SFC than in LC [26]. The
separation of metoprolol enantiomers by LC and SFC on a Chiralcel OD CSP is
illustrated in Fig. 12-2. Although impressive selectivity is achieved by both tech-
niques, resolution is higher in SFC (R = 12.7) than in LC (R = 4.8), and the higher
s s
flowrate in SFC reduces the analysis time. The increased efficiency of SFC also
improves peak symmetry.

Fig. 12-2. Separation of metoprolol enantiomers by LC and SFC on a Chiralcel OD CSP. Chromato-
–1
graphic conditions for LC: 20 % 2-propanol in hexane, with 0.1 % diethylamine, 0.5 mL min .
Chromatographic conditions for SFC: 20 % methanol with 0.5 % isopropylamine in carbon dioxide,
–1
2.0 mL min , 15 MPa, 30 °C.
The difference in resolution between LC and SFC can be significant enough to
turn a marginal LC separation into a viable chromatographic method in SFC. String-
ham et al. reported that packed column SFC yielded satisfactory chiral analysis