Page 208 - Chiral Separation Techniques

186 7 Chiral Derivatization Chromatography

7.2 Different Approaches for Derivatization
Chromatography

The educt, a racemate, is derivatized before the separation with an agent which
might be achiral or unichiral (Fig. 7-1), and afterwards is passed through a chro-
matographic system which is equipped with a stationary phase. This stationary phase
may also be achiral or unichiral in nature.

Fig. 7-1. General reaction scheme of chiral derivatization chromatography.

Derivatization techniques are divided into pre-column and post-column tech-
niques. Post-column derivatization is especially useful to enhance the detection of
compounds, whilst pre-column derivatization is the method of choice for enan-
tioseparations via derivatization.
Pre-column derivatization offers some general advantages:
every racemate with a functional group can be derivatized and separated;
nonreacted derivatization reagent can be removed before the separation;
the reaction can be used as a pre-purification step, if the reaction is selective for
one compound;
the derivatized compounds may show better chromatographic properties, e.g.
derivatization of free amino groups can reduce tailing, or the introduction of
derivatizing agents decrease detection limits; and
it has a high success rate compared to direct methods, which cannot guarantee a
separation.
Several strategies can be distinguished to achieve certain solutions, and these are
summarized in Table 7-1. The strategies each have their benefits in special cases.
The different approaches will be shown in the following examples.