Page 318 - Chiral Separation Techniques

310 12 Sub- and Supercritical Fluid Chromatography for Enantiomer Separations

with a hydrophobic cleft of the macrocycle may also play a role in chiral recogni-
tion.
The macrocyclic antibiotic-based CSPs have not been used extensively in SFC.
Two macrocyclic antibiotic CSPs, Chirobiotic T and Chirobiotic V, were included in
a study of various CSPs in SFC. At least partial resolution of approximately half of
the 44 test compounds could be obtained on these two CSPs in SFC [63]. A high
concentration of modifier was necessary to elute some of the analytes. Enantioreso-
lution of derivatized amino acids was also demonstrated in the same study. However,
a complex modifier comprised of methanol, water, and glycerol was required for
separations performed on the Chirobiotic T CSP. The separation of coumachlor
enantiomers on a vancomycin-based CSP (Chirobiotic V) in SFC is illustrated in
Fig. 12-5 [32].

Fig. 12-5. Separation of coumachlor enantiomers on a Chirobiotic V CSP. Chromatographic conditions:
–1
15 % methanol in carbon dioxide, 2.0 mL min , 20 MPa, 30 °C.

12.4.5 Other CSPs

Macaudière and co-workers performed a comparison of LC and SFC on a polymer
based-CSP (Chiralpak OT) [64]. The chromatographic behavior of this CSP seemed
to be quite different in SFC than in LC, although satisfactory separations were
achieved with both techniques. The chiral recognition mechanisms may be altered
by the nature (hexane-based or CO -based) of the eluent.
2
A CSP based on the adsorption of a chiral anthrylamine on porous graphitic car-
bon successfully resolved the enantiomers of tropic acid derivatives and anti-inflam-
matory agents in SFC [65]. The carbon-based CSP produced superior results when
compared to an analogous silica-based CSP. Occasional washing of the column was
necessary to remove highly retained substances.