non-target analysis and target suspect screening of environmental samples (Blum
et al. 2017; Veenaas and Haglund 2017).
5.4 Other Experimental Approaches for Enantioselective
Capillary Gas Chromatography
As described comprehensively in an earlier review (Hühnerfuss and Shah 2009), the
gas chromatographic isomer-selective separation of complex mixtures of environmental pollutants is still an analytical challenge today. Enantioselective separations
will at least double or even multiply this analytical challenge. One possible approach
that may significantly improve separation in such difficult situations may be
multidimensional gas chromatography and comprehensive GCxGC (see above),
another possibility may be the application of tandem gas chromatography, that is,
direct coupling of two capillary columns with different stationary phases in series.
However, the chance of finding an optimised tandem system for specific separation
requirements is rather small, if the column properties (e.g. phase, length, film
thickness) are selected by only “trial and error” approach. Jones and Purnell
(1990), therefore, developed a general theory, which allows optimisation of the
separation columns coupled in series, in particular, the calculation of the length of
combination necessary for the baseline separation of a compound mixture. The
theory was verified experimentally with complex mixtures, which could not be
separated by single columns of any length (Oehme and Baycan-Keller 2000).
The first to suggest the tandem combination of two columns with an achiral and a
chiral phase in order to enhance the separation of chiral environmental pollutants
were Oehme et al. (1994). They reported the simultaneous enantioselective separation of chlordanes, a nonachlor compound, and o,p
0 -DDT in environmental samples
using tandem capillary columns. Basically, the order isomer/enantiomer separation
can be chosen or vice versa, but Oehme et al. suggested the first arrangement because
of the retention time shift phenomenon which led to insufficient enantiomer separation (Oehme et al. 1994).
The drawback of the sequence isomer/enantiomer separation is the relatively high
temperature at which the compounds are transferred from the isomer-selective
column into the enantiomer-selective one. This may cause some deterioration of
enantiomer resolution which can be partly compensated by the reduction of the
upper column temperature applied. Furthermore, the theory of Purnell et al. requires
isothermal separation, which is not feasible for the separation of complex mixtures
of environmental pollutants such as chlorobornanes (i.e. Toxaphene
® ) showing
relatively small differences in their retention properties (Baycan-Keller and Oehme
1999). Therefore, the same research group (Oehme and Baycan-Keller 2000)
attempted to overcome this limitation of the theory by approximating isothermal
conditions by using a very fast heating ramp to reach the selected isothermal
temperature as quickly as possible. This change had no influence on the
5.4 Other Experimental Approaches for Enantioselective Capillary Gas Chromatography
81
et al. 2017; Veenaas and Haglund 2017).
5.4 Other Experimental Approaches for Enantioselective
Capillary Gas Chromatography
As described comprehensively in an earlier review (Hühnerfuss and Shah 2009), the
gas chromatographic isomer-selective separation of complex mixtures of environmental pollutants is still an analytical challenge today. Enantioselective separations
will at least double or even multiply this analytical challenge. One possible approach
that may significantly improve separation in such difficult situations may be
multidimensional gas chromatography and comprehensive GCxGC (see above),
another possibility may be the application of tandem gas chromatography, that is,
direct coupling of two capillary columns with different stationary phases in series.
However, the chance of finding an optimised tandem system for specific separation
requirements is rather small, if the column properties (e.g. phase, length, film
thickness) are selected by only “trial and error” approach. Jones and Purnell
(1990), therefore, developed a general theory, which allows optimisation of the
separation columns coupled in series, in particular, the calculation of the length of
combination necessary for the baseline separation of a compound mixture. The
theory was verified experimentally with complex mixtures, which could not be
separated by single columns of any length (Oehme and Baycan-Keller 2000).
The first to suggest the tandem combination of two columns with an achiral and a
chiral phase in order to enhance the separation of chiral environmental pollutants
were Oehme et al. (1994). They reported the simultaneous enantioselective separation of chlordanes, a nonachlor compound, and o,p
0 -DDT in environmental samples
using tandem capillary columns. Basically, the order isomer/enantiomer separation
can be chosen or vice versa, but Oehme et al. suggested the first arrangement because
of the retention time shift phenomenon which led to insufficient enantiomer separation (Oehme et al. 1994).
The drawback of the sequence isomer/enantiomer separation is the relatively high
temperature at which the compounds are transferred from the isomer-selective
column into the enantiomer-selective one. This may cause some deterioration of
enantiomer resolution which can be partly compensated by the reduction of the
upper column temperature applied. Furthermore, the theory of Purnell et al. requires
isothermal separation, which is not feasible for the separation of complex mixtures
of environmental pollutants such as chlorobornanes (i.e. Toxaphene
® ) showing
relatively small differences in their retention properties (Baycan-Keller and Oehme
1999). Therefore, the same research group (Oehme and Baycan-Keller 2000)
attempted to overcome this limitation of the theory by approximating isothermal
conditions by using a very fast heating ramp to reach the selected isothermal
temperature as quickly as possible. This change had no influence on the
5.4 Other Experimental Approaches for Enantioselective Capillary Gas Chromatography
81