Chiral separation, direct using CMPAs

On the other hand, the direct chromatographic approach involves the use of the chiral selector either in the mobile phase, a so-called chiral mobile phase additive (CMPA), or in the stationary phase [i.e., the chiral stationary phase (CSP)]. In the latter case, the chiral selector is chemically bonded or coated or allowed to absorb onto a suitable solid support. Of course chiral selectors still can be used as CMPAs, but the approach is a very expensive one owing to the high amount of chiral selector required for the preparation of the mobile phase, and the large amount of costly chiral selector that is wasted (since there is very little chance of recovering this compound). Moreover, this approach is not successftd in the preparative separation of the enantiomers. 

Chiral mobile phase additives (CMPAs) are generally used to perform direct chiral separation in thin-layer chromatography (TLC). This mode offers the advantages of flexibility and low cost as compared to the equivalent chiral stationary phase (CSP). Also, the lack of a wide range of CSPs in TLC resulted in CMPAs becoming a commonly employed approach for enantiomeric separations. 

The chromatographic methods are considered to be most useful for chiral separations. Enantiomers can be separated by two methods (a) indirect method that utilizes derivatizing agents and (b) direct method that uses chiral stationary phases (CSPs) or chiral mobile phase additives (CMPAs) [49-56]. 

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