Page 38 - Chiral Separation Techniques

1.4 Enantioselective Membranes 13

1.3.3 Gas Chromatography

Gas chromatography (GC) has also been used for preparative purposes, but is
restricted to relatively volatile racemates such as anesthetics, pheromones or
monoterpenes and, therefore, very few applications are reported. Nevertheless, in the
cases to which GC may be applied, it could be considered as an economical alterna-
tive to HPLC. Most of the resolutions of enantiomers were performed on cyclodex-
trin-derived CSPs [109, 144–153], and only on very few occasions were other chiral
selectors used [153].
One of the latest resolutions of the anesthetic enflurane (8) has been performed by
preparative GC on a γ-cyclodextrin CSP, the process later being scaled-up via SMB
[109] (Fig. 1-4). This is the first GC-SMB separation described.

Fig. 1-4. Resolution of enflurane by GC.

1.4 Enantioselective Membranes

Membrane-based separation techniques constitute nowadays well-established pro-
cess methods for industrial treatments of fluids. Like SMB, membrane-based sepa-
rations can be performed in continuous mode. In the field of preparative-scale enan-
tiodiscrimination, much effort has been invested in this subject due its high potential
[154, 155]. (Chapter 5 of this book is devoted to the subject, and further discusses
the advantages and applications of membrane technologies.)
The first successful chiral resolutions through enantioselective membranes have
been published recently, but few cases are applicable to the preparative scale, mainly
due to mechanical and technical limitations. Low flow rates, saturation of the chiral
selectors and loss of enantioselectivity with time are some of the common problems
encountered and that should be solved in the near future.
Enantioselective transport processes can be achieved either with solid or liquid
membranes (Fig. 1-5). In this latter case, the liquid membrane can be supported by
a porous rigid structure, or it can simply be an immiscible liquid phase between two
solutions with the same character (aqueous or nonaqueous), origin and destination