Enantiomeric selection column packing

In recent years h.p.l.c. has become a valuable chromatographic tool for analytical and preparative scale work. In this latter area the separation of isomers (structural, diastereoisomeric, and enantiomeric) has been possible by the selection of appropriate column packing material and solvent systems. However, the equipment, operating costs, and column packing materials are more expensive than those in t.l.c., g.l.c. and conventional liquid-solid column chromatography. 

A unified theory recently proposed to explain the manner of sorption and the form of sorption isotherm in gas, liquid, and ion-exchange chromatography is presented in some detail. Selectivity in reversed-phase high-pressure liquid chromatography is explored at length. Several chapters deal with characterization of bonded phases, relationship of column-packing structure and performance, variability of reversed-phase packing materials, and the differences between silica-based reversed-phase and poly(styrene-divinylbenzene) columns. A short review is included to cover various approaches used in HPLC to achieve the desired selectivity for resolution of enantiomeric compounds. 

We conclude this survey with extended systems, where the assembled structures show the translational symmetry characteristic of the crystalline state. The packing of chiral units into a crystal lattice will inevitably involve some type of diastereo-selectivity, either homochiral or heterochiral, although this is fiequently not discussed in crystal stucture reports. If the associations are all homochiral, then an enantiomerically pure crystal will be obtained, and a solution of the racemate will yield a racemic mixture (see section 3). If, on the other hand, heterochiral association (often related by a centre of inversion or glide plane) is favoured, a racemic compound will crystallize. The double helicate [ 02(51)2] " crystallizes with a homochiral association of complexes along the helieal axis (Figure 38). The homochiral columns are then arranged in pseudohexagonal arrays with the ehirality... 

The first example show the separation of enantiomeric amides on a chiral stationary phase (4(a)) and on the same phase but in its liquid-crystalline state (4(b)) (first discovered by Lochmllller and Souter (5)). The enhanced selectivity of the latter is so great that a separation which required about 35,000 plates on a capillary column is reduced to a 100 plate packed column experiment. As a result the practical capacity is increased fom the picogram to the milligram range. 

Exactly the same process takes place as that in the Hurrel system but, in effect, the valving makes the columns appear to move instead of the packing. Part of the feed moves with the mobile phase and is collected by a small take-off flow in front of the feed port (B + solvent). The other, more retained portion of the sample, accumulates in a column on the other side of the feed port and is collected by a another small take-off flow behind the feed port (A + solvent). This particular system ideally, produces two products and thus lends itself specifically to the separation of enantiomeric pairs. However, for effective separation with high purity yields, the stationary phase capacity for the two enantiomers must be fairly large and thus the phase system must be carefully selected. The technique has been successfully used to isolate single enantiomer drugs [15-17]. 

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