disadvantages. Therefore, many experts conclude (Stevenson et al. 1988;
Subramanian 2001, 2007; Schurig and Ciogli 2013) that enantiomer derivatisation
into diastereomeric complexes is associated with additional method errors mainly
associated with complexation efficiency, chromatographic disturbances, elevated
LOD etc. Therefore, direct methods should be preferred for a robust and reliable
determination of enantiomeric distribution profiles.
3.4 Direct Methods
As described in an earlier review (Hühnerfuss and Shah 2009), the direct resolution
of enantiomers is achieved by using enantiomer-selective chromatographic techniques mainly based on a chiral stationary phase (CSP) that implies selective
interactions between the enantiomers of the respective analytes and a so-called chiral
selector. This chiral selector must associate preferentially with one of the enantiomers of the enantiomeric mixture to be separated. The difference in stability between
these complexes leads to a difference in retention time, where the enantiomer that
forms the less stable complex with the CSP will be eluted first. This implies that the
diastereomeric adsorbates must differ adequately in free energy. The chiral selector
may be part of the stationary phase or, alternatively, present as a mobile-phase
additive, in which case an achiral stationary phase can be used (Aboul-Enein and
Ali 2003; Subramanian 2007; Schurig and Ciogli 2013; Scriba 2013). Many enantiomeric mixtures were separated on conventional achiral LC columns by adding an
appropriate chiral compound to the mobile phase. Additives such as
camphorsulphonic acid (Iorio et al. 1991; Shin et al. 2000; Bielejewska et al.
2005) and quinine (Bicker et al. 2004; Qing et al. 2014; Wu et al. 2014; Lajko
et al. 2015; Ianni et al. 2016; Calderon and Lammerhofer 2017) have also been
successfully used.
The direct application of α-, β-, and γ-cyclodextrins in solution has also been
reported (Aboul-Enein and Ali 2003; Toda 2004; Subramanian 2007; Garrison et al.
2011; Ates et al. 2013; Schurig and Ciogli 2013; Scriba 2013). Advantages of this
technique are as follows:
• Less expensive conventional ASP such as silica or C 18 -silica (ODS) may be
applied.
• There is a wide variety of possible additives.
• The selectivity of chiral additives is sometimes extending or complementary to
those observed on the available chiral phases.
However, some disadvantages have to be mentioned:
• Many additives are costly, have limited application lifetime, or must be
synthesised individually shortly before application. In addition, their mode of
operation is complex and often associated with a variable performance (variable
yields).
36
3 Enantiomer-Selective High- and Ultra- High-Performance Liquid Chromatography
Subramanian 2001, 2007; Schurig and Ciogli 2013) that enantiomer derivatisation
into diastereomeric complexes is associated with additional method errors mainly
associated with complexation efficiency, chromatographic disturbances, elevated
LOD etc. Therefore, direct methods should be preferred for a robust and reliable
determination of enantiomeric distribution profiles.
3.4 Direct Methods
As described in an earlier review (Hühnerfuss and Shah 2009), the direct resolution
of enantiomers is achieved by using enantiomer-selective chromatographic techniques mainly based on a chiral stationary phase (CSP) that implies selective
interactions between the enantiomers of the respective analytes and a so-called chiral
selector. This chiral selector must associate preferentially with one of the enantiomers of the enantiomeric mixture to be separated. The difference in stability between
these complexes leads to a difference in retention time, where the enantiomer that
forms the less stable complex with the CSP will be eluted first. This implies that the
diastereomeric adsorbates must differ adequately in free energy. The chiral selector
may be part of the stationary phase or, alternatively, present as a mobile-phase
additive, in which case an achiral stationary phase can be used (Aboul-Enein and
Ali 2003; Subramanian 2007; Schurig and Ciogli 2013; Scriba 2013). Many enantiomeric mixtures were separated on conventional achiral LC columns by adding an
appropriate chiral compound to the mobile phase. Additives such as
camphorsulphonic acid (Iorio et al. 1991; Shin et al. 2000; Bielejewska et al.
2005) and quinine (Bicker et al. 2004; Qing et al. 2014; Wu et al. 2014; Lajko
et al. 2015; Ianni et al. 2016; Calderon and Lammerhofer 2017) have also been
successfully used.
The direct application of α-, β-, and γ-cyclodextrins in solution has also been
reported (Aboul-Enein and Ali 2003; Toda 2004; Subramanian 2007; Garrison et al.
2011; Ates et al. 2013; Schurig and Ciogli 2013; Scriba 2013). Advantages of this
technique are as follows:
• Less expensive conventional ASP such as silica or C 18 -silica (ODS) may be
applied.
• There is a wide variety of possible additives.
• The selectivity of chiral additives is sometimes extending or complementary to
those observed on the available chiral phases.
However, some disadvantages have to be mentioned:
• Many additives are costly, have limited application lifetime, or must be
synthesised individually shortly before application. In addition, their mode of
operation is complex and often associated with a variable performance (variable
yields).
36
3 Enantiomer-Selective High- and Ultra- High-Performance Liquid Chromatography