Supercritical fluid chromatography for enantiomer separation

Phinney KW, Sub- and supercritical fluid chromatography for enantiomer separations, in Chiral Separation Techniques A Practical Approach, (Suhramanian G, Ed.), p. 299, VCH Verlag, Weinheim, Germany (2001). 

ChenWenda, Reza Haghpanah, Arvind Rajendran, Mohammad Amanuflah, Optimization of isocratic supercritical fluid chromatography for enantiomer separation. Journal of Chromatography A 1218 (2011), p. 162-170. 

The specific development of a batch process is illustrated in the following example, namely the separation of the enantiomers of racemic trans-stilbene oxide (TSO) [28], For this example, supercritical fluid chromatography was particularly appropriate for the resolution. 

The racemate of 1,3,2-benzodithiazole 1-oxide 42 was separated by supercritical fluid chromatography on the (A j )-Whelk-( )l column with supercritical carbon dioxide containing 20% methanol as a mobile phase. Peak areas of enantiomers prior to and after the separation, used for the calculation of the enantiomerization barrier, were detected by computer-assisted peak deconvolution of peak clusters registered on chromatograms using computer software <2002CH1334>. 

Chiral Stationary Phases for GC and HPLC. Enantiomerically pure NEA has been used to prepare a variety of chiral stationary phases for liquid, gas, and supercritical fluid chromatography. These stationary phases are used to separate enantiomers without derivatization of the substrate with a chiral agent. 

TLC has also been used for the separation of diastereomeric derivatives of enantiomers, but this form of chromatography has not attained widespread use in indirect resolutions. Other chromatographic techniques, for example, supercritical fluid chromatography, capillary electrophoresis, countercurrent chromatography, etc., have not received much attention in indirect enantioseparation. 

The coated polysaccharide-based phases have mostly been used in normal phase conditions, but an increasing number of preparative applications have been reported in supercritical fluid chromatography [33] or reversed phase mode [34]. The broad applicability of the coated polysaccharide-based CSPs has made them very popular and they are now widely used for preparative separation of enantiomers and large-scale applications up to tonnes per year have been reported [35, 36]. The success of these CSPs is documented in numerous papers and these CSPs are the most used phases for analytical and preparative applications. 

There are two general approaches for the separation of enantiomers [1-4,28-32]. The direct method is based on the formation of transient diastereomer association complexes with a chiral selector immobilized in the stationary phase, or added to the mobile phase. The former approach requires the use of special stationary phases (section 10.4) while the later uses conventional stationary phases with special additives included in the mobile phase (section 10.5). When preparative applications are contemplated the use of immobilized chiral selectors is the more common approach. Method selection also depends on the choice of the separation mode. Table 10.2. While chiral stationary phases are the only choice for gas chromatography [16,28,33-38], and are used almost exclusively for supercritical fluid chromatography [39-43] and capillary electrochromatography [44-47], they also dominate the practice of liquid chromatography... 

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