• Type 4: Interactions between the analyte and CSP are induced by inclusion into
chiral cavities within a CSP of relatively low molecular mass, that is, not a highmolecular weight polymer. This effect was first demonstrated by Cram and
co-workers (Cram 1988; Cram et al. 1999) using a chiral crown ether bound to
silica gel. Commercially available chiral crown ether phases such as Diacel’s
Crown-Pak are still finding applications today (Cho et al. 2009; Lee et al. 2011,
2012; Nemeth et al. 2015; Agneeswari et al. 2016; Chen and Shi 2017), However,
during the past 10 years, scientific attention has increasingly focused upon the use
of bound cyclodextrins for this type of CSP (Schurig and Mayer 2001; Badea
et al. 2016; Chen and Shi 2017), especially for environmental applications
(Asami and Imura 2006; Karasek et al. 2007; Garrison et al. 2008; Naude and
Rohwer 2012; Caballo et al. 2013; Kania-Korwel and Lehmler 2013; Lee et al.
2016). The various moieties used to functionalise cyclodextrins can alter their
enantioselectivity greatly, thereby expanding their overall usefulness (Huang
et al. 2014; Yao et al. 2014; Zhou et al. 2015, 2016; Yang et al. 2017; Lin et al.
2018). For example, aromatic functionalised cyclodextrins can be used in the
normal-phase mode as π-complex chiral stationary phase or in the reversed-phase
mode, where inclusion complexation dominates (Lai et al. 2011; Tang et al. 2016;
Ma et al. 2017). Thus, completely different types of chiral molecules are resolved
in each mode.
A different experimental approach enhances the usefulness of native cyclodextrin
stationary phases and produces unusual enantioselectivities: In this approach, the
inclusion complexation is suppressed by using a non-hydrogen-bonding, polar
organic solvent, such as acetonitrile, as the main component of the mobile phase
(Schumacher et al. 2003; Si Ahmed et al. 2007; Zhang et al. 2008; Yao et al. 2014;
Lin et al. 2018). Although this technique is sometimes called the “polar organic
mode”, it is related most closely to normal-phase separations. The acetonitrile tends
to occupy the cyclodextrin cavity. It also accentuates hydrogen bonding between the
hydroxyl groups on the chiral analyte. The addition of a hydrogen-bonding solvent,
such as methanol, can decrease the retention of highly retained compounds. Adding
very small amounts of glacial acetic acid and triethylamine controls the protonation
of the analyte and enhances the enantioselectivity (Beesley and Scott 1998;
Gasparrini et al. 2001; Aboul-Enein and Ali 2003; Francotte 2017). In this mode,
the analyte presumably resides on top of the cyclodextrin selector—more or less a
lid—in such a way that hydrogen bonding is maximised. Using this technique, often
chiral compounds can be resolved containing two hydrogen-bonding groups (one of
which should be in α- or β-position to the chiral centre) and a bulky group, such as an
aromatic ring. Furthermore, this technique is excellent for preparative separations.
According to Armstrong (Armstrong et al. 1986, 1990), the polar organic mode
offers some practical advantages versus the traditional normal-phase mode, which
uses n-hexane or n-heptane and n-propanol solvent systems. First, most compounds
that exist as hydrochloride salts can be dissolved and separated in the polar organic
mobile phase, but not with non-polar organic solvents. Second, effluent from a
reversed-phase column can be switched directly onto a chiral column when using
3.5 The Evolution of Chiral Stationary Phases for Liquid Chromatography
41
chiral cavities within a CSP of relatively low molecular mass, that is, not a highmolecular weight polymer. This effect was first demonstrated by Cram and
co-workers (Cram 1988; Cram et al. 1999) using a chiral crown ether bound to
silica gel. Commercially available chiral crown ether phases such as Diacel’s
Crown-Pak are still finding applications today (Cho et al. 2009; Lee et al. 2011,
2012; Nemeth et al. 2015; Agneeswari et al. 2016; Chen and Shi 2017), However,
during the past 10 years, scientific attention has increasingly focused upon the use
of bound cyclodextrins for this type of CSP (Schurig and Mayer 2001; Badea
et al. 2016; Chen and Shi 2017), especially for environmental applications
(Asami and Imura 2006; Karasek et al. 2007; Garrison et al. 2008; Naude and
Rohwer 2012; Caballo et al. 2013; Kania-Korwel and Lehmler 2013; Lee et al.
2016). The various moieties used to functionalise cyclodextrins can alter their
enantioselectivity greatly, thereby expanding their overall usefulness (Huang
et al. 2014; Yao et al. 2014; Zhou et al. 2015, 2016; Yang et al. 2017; Lin et al.
2018). For example, aromatic functionalised cyclodextrins can be used in the
normal-phase mode as π-complex chiral stationary phase or in the reversed-phase
mode, where inclusion complexation dominates (Lai et al. 2011; Tang et al. 2016;
Ma et al. 2017). Thus, completely different types of chiral molecules are resolved
in each mode.
A different experimental approach enhances the usefulness of native cyclodextrin
stationary phases and produces unusual enantioselectivities: In this approach, the
inclusion complexation is suppressed by using a non-hydrogen-bonding, polar
organic solvent, such as acetonitrile, as the main component of the mobile phase
(Schumacher et al. 2003; Si Ahmed et al. 2007; Zhang et al. 2008; Yao et al. 2014;
Lin et al. 2018). Although this technique is sometimes called the “polar organic
mode”, it is related most closely to normal-phase separations. The acetonitrile tends
to occupy the cyclodextrin cavity. It also accentuates hydrogen bonding between the
hydroxyl groups on the chiral analyte. The addition of a hydrogen-bonding solvent,
such as methanol, can decrease the retention of highly retained compounds. Adding
very small amounts of glacial acetic acid and triethylamine controls the protonation
of the analyte and enhances the enantioselectivity (Beesley and Scott 1998;
Gasparrini et al. 2001; Aboul-Enein and Ali 2003; Francotte 2017). In this mode,
the analyte presumably resides on top of the cyclodextrin selector—more or less a
lid—in such a way that hydrogen bonding is maximised. Using this technique, often
chiral compounds can be resolved containing two hydrogen-bonding groups (one of
which should be in α- or β-position to the chiral centre) and a bulky group, such as an
aromatic ring. Furthermore, this technique is excellent for preparative separations.
According to Armstrong (Armstrong et al. 1986, 1990), the polar organic mode
offers some practical advantages versus the traditional normal-phase mode, which
uses n-hexane or n-heptane and n-propanol solvent systems. First, most compounds
that exist as hydrochloride salts can be dissolved and separated in the polar organic
mobile phase, but not with non-polar organic solvents. Second, effluent from a
reversed-phase column can be switched directly onto a chiral column when using
3.5 The Evolution of Chiral Stationary Phases for Liquid Chromatography
41