of the cyclodextrins the interior of the cavity is hydrophobic and favours the
selective inclusion and trapping of non-polar guest compounds. However, gas
chromatographic efficiency in terms of the number of theoretical plates is poor
with unmodified cyclodextrins irrespective of whether they are used as solids
(gas–solid chromatography) or in formamide solution. Improved peak shapes were
achieved when per-O-methylated cyclodextrins were used in capillary columns
(Juvancz et al. 1988), though the disadvantage of these phases was their
unfavourably high melting point. As a consequence, different experimental
approaches were used to lower the melting point. For example, a follow-up study
(Venema and Tolsma 1989) proved that per-O-methylated β-cyclodextrin behaves as
a supercooled liquid after conditioning at 200
C, and afterwards the columns could
be used as low as 76
C. Schurig and co-workers diluted the cyclodextrin derivative
in a polysiloxane, for example, OV-1701 (Schurig and Nowotny 1988). Most of
these derivatives are liquid at room temperature, highly stable, soluble in non-polar
solvents and with high enantioselectivity towards many chiral compounds. Subsequently, a considerable number of hydrophobic cyclodextrin derivatives was prepared and evaluated. For an extensive review on these early achievements the reader
is referred to Armstrong et al. (1990), Marker and Ballschmiter (1996) and Vetter
and Schurig (1997). The development of these latter chiral stationary phases represents a landmark in the enantioselective analysis of chiral environmental pollutants,
because they allowed the enantiomer separation also of hydrophobic compounds,
such as α-hexachlorocyclohexane, chlordane, heptachlor, atropisomeric
polychlorinated biphenyls, DDT derivatives, chlorobornanes and many other
organo-halogenated chiral contaminants including brominated and fluorinated substances (Badea et al. 2016). Also, non-halogenated chiral compounds have been
investigated. Here, mainly currently used pesticides and other agricultural aids are in
the focus of the analytical chemists (Ulrich et al. 2012).
5.2 Enantioselective Multidimensional Capillary Gas
Chromatography (MDGC)
A multidimensional “heart-cut” GC (MDGC) technique was introduced as an
enhanced high-resolution separation technique for the elucidation of complex mixtures (Kennedy et al. 1990; Reich et al. 2000; Marriott et al. 2003a; Bordajandi et al.
2005). With respect to environmental pollutants, Duinker et al. aimed at an effective
separation of PCB congeners which at that time could not be separated on a single
non-polar (5%-phenyl)-methylpolysiloxane capillary or comparable GC columns by
conventional methods (Duinker et al. 1988; Schulz et al. 1989). Due to the insufficient selectivity of the electron capture detector (ECD), the high-resolution power of
MDGC compensates for the separation limitation.
The MDGC technique involves the application of two columns of different
polarities in series, each in a separate temperature-controlled oven. The eluate of
5.2 Enantioselective Multidimensional Capillary Gas Chromatography (MDGC)
79
selective inclusion and trapping of non-polar guest compounds. However, gas
chromatographic efficiency in terms of the number of theoretical plates is poor
with unmodified cyclodextrins irrespective of whether they are used as solids
(gas–solid chromatography) or in formamide solution. Improved peak shapes were
achieved when per-O-methylated cyclodextrins were used in capillary columns
(Juvancz et al. 1988), though the disadvantage of these phases was their
unfavourably high melting point. As a consequence, different experimental
approaches were used to lower the melting point. For example, a follow-up study
(Venema and Tolsma 1989) proved that per-O-methylated β-cyclodextrin behaves as
a supercooled liquid after conditioning at 200
C, and afterwards the columns could
be used as low as 76
C. Schurig and co-workers diluted the cyclodextrin derivative
in a polysiloxane, for example, OV-1701 (Schurig and Nowotny 1988). Most of
these derivatives are liquid at room temperature, highly stable, soluble in non-polar
solvents and with high enantioselectivity towards many chiral compounds. Subsequently, a considerable number of hydrophobic cyclodextrin derivatives was prepared and evaluated. For an extensive review on these early achievements the reader
is referred to Armstrong et al. (1990), Marker and Ballschmiter (1996) and Vetter
and Schurig (1997). The development of these latter chiral stationary phases represents a landmark in the enantioselective analysis of chiral environmental pollutants,
because they allowed the enantiomer separation also of hydrophobic compounds,
such as α-hexachlorocyclohexane, chlordane, heptachlor, atropisomeric
polychlorinated biphenyls, DDT derivatives, chlorobornanes and many other
organo-halogenated chiral contaminants including brominated and fluorinated substances (Badea et al. 2016). Also, non-halogenated chiral compounds have been
investigated. Here, mainly currently used pesticides and other agricultural aids are in
the focus of the analytical chemists (Ulrich et al. 2012).
5.2 Enantioselective Multidimensional Capillary Gas
Chromatography (MDGC)
A multidimensional “heart-cut” GC (MDGC) technique was introduced as an
enhanced high-resolution separation technique for the elucidation of complex mixtures (Kennedy et al. 1990; Reich et al. 2000; Marriott et al. 2003a; Bordajandi et al.
2005). With respect to environmental pollutants, Duinker et al. aimed at an effective
separation of PCB congeners which at that time could not be separated on a single
non-polar (5%-phenyl)-methylpolysiloxane capillary or comparable GC columns by
conventional methods (Duinker et al. 1988; Schulz et al. 1989). Due to the insufficient selectivity of the electron capture detector (ECD), the high-resolution power of
MDGC compensates for the separation limitation.
The MDGC technique involves the application of two columns of different
polarities in series, each in a separate temperature-controlled oven. The eluate of
5.2 Enantioselective Multidimensional Capillary Gas Chromatography (MDGC)
79