Recently, homochiral metal–organic frameworks (MOF)-based CSPs were identified and reported as suitable CSPs in gas chromatography. A recent review
summarises the application range of this type of chromatographic separation technique (Peluso et al. 2014). The authors report on the versatile applicability of
homochiral metal–organic frames, organo-metal hybrid materials. The interactions
and retention properties are organic-bridging ligand-based CSPs and allow focused
separation by strategically combining the inherent properties of the selected metal
and the organic ligand. Especially, the development of homochiral MOFs provides
new selective properties for the enantioselectivity of chromatographic separation.
The combination of chiral ligands (molecular chirality) with the structural chirality
of the metal frame, for example, helicity, as combined homochiral properties of the
material mimics, thus, the complex chirality of proteins and peptides and provides a
platform for new CSPs with a potential for multi-compound separation.
The combination of such metal–organic frames with well-established CSPs, for
example, cyclodextrin, is also reported to enhance the enantiomer separation of a
variety of chiral organic substances (Yang et al. 2017). The authors tested capillary
column MOF coated with [Cd(LTP)2]n, permethylated β-cyclodextrin (CD) and
sodium chloride (A), and MOF [Cd(LTP)2]n (B) and permethylated β-CD (C).
Based on this study they concluded that the combination of MOF and CD enhanced
the enantiomer separation of the standard compounds investigated significantly
(Yang et al. 2017).
5.1 The Evolution of Chiral Stationary Phases for Capillary
Gas Chromatography
Gil-Av et al. were the first to report a successful separation of N-trifluoroacetylated
amino acid esters by capillary gas chromatography applying acylated amino acid and
dipeptide esters as chiral stationary phases in glass capillary columns (Charles-Sigler
and Gil-Av 1966; Gil-Av et al. 1966a, b). Under such experimental conditions, Denantiomers eluted prior to L-enantiomers from stationary phases containing or
consisting of L-amino acids. The separation performance of these early phases was
largely attributed to hydrogen bonding interaction between the enantiomers of the
sample and the chiral stationary phase, where the diastereomeric association complexes between molecules of equal configuration were assumed to be more stable,
leading to longer retention times than in the case of unequal configuration. Particularly, large separation factors for amino acids were observed in the case of the
dipeptide derivative N-TFA-L-valyl-L-valine cyclohexyl ester. However, a serious
disadvantage of this phase was the fact that the maximum temperature of operation
was limited to maximum 110
C only. The range of application could be extended by
introducing other amino acids, for example, L-leucine, L-aspartic acid, or L-phenylalanine, into the dipeptide phases (König et al. 1970; König and Nicholson 1975).
Another type of chiral stationary phases suggested by Gil-Av and Feibush (Feibush
76
5 Enantiomer-Selective High-Resolution Gas Chromatography (esHRGC)
summarises the application range of this type of chromatographic separation technique (Peluso et al. 2014). The authors report on the versatile applicability of
homochiral metal–organic frames, organo-metal hybrid materials. The interactions
and retention properties are organic-bridging ligand-based CSPs and allow focused
separation by strategically combining the inherent properties of the selected metal
and the organic ligand. Especially, the development of homochiral MOFs provides
new selective properties for the enantioselectivity of chromatographic separation.
The combination of chiral ligands (molecular chirality) with the structural chirality
of the metal frame, for example, helicity, as combined homochiral properties of the
material mimics, thus, the complex chirality of proteins and peptides and provides a
platform for new CSPs with a potential for multi-compound separation.
The combination of such metal–organic frames with well-established CSPs, for
example, cyclodextrin, is also reported to enhance the enantiomer separation of a
variety of chiral organic substances (Yang et al. 2017). The authors tested capillary
column MOF coated with [Cd(LTP)2]n, permethylated β-cyclodextrin (CD) and
sodium chloride (A), and MOF [Cd(LTP)2]n (B) and permethylated β-CD (C).
Based on this study they concluded that the combination of MOF and CD enhanced
the enantiomer separation of the standard compounds investigated significantly
(Yang et al. 2017).
5.1 The Evolution of Chiral Stationary Phases for Capillary
Gas Chromatography
Gil-Av et al. were the first to report a successful separation of N-trifluoroacetylated
amino acid esters by capillary gas chromatography applying acylated amino acid and
dipeptide esters as chiral stationary phases in glass capillary columns (Charles-Sigler
and Gil-Av 1966; Gil-Av et al. 1966a, b). Under such experimental conditions, Denantiomers eluted prior to L-enantiomers from stationary phases containing or
consisting of L-amino acids. The separation performance of these early phases was
largely attributed to hydrogen bonding interaction between the enantiomers of the
sample and the chiral stationary phase, where the diastereomeric association complexes between molecules of equal configuration were assumed to be more stable,
leading to longer retention times than in the case of unequal configuration. Particularly, large separation factors for amino acids were observed in the case of the
dipeptide derivative N-TFA-L-valyl-L-valine cyclohexyl ester. However, a serious
disadvantage of this phase was the fact that the maximum temperature of operation
was limited to maximum 110
C only. The range of application could be extended by
introducing other amino acids, for example, L-leucine, L-aspartic acid, or L-phenylalanine, into the dipeptide phases (König et al. 1970; König and Nicholson 1975).
Another type of chiral stationary phases suggested by Gil-Av and Feibush (Feibush
76
5 Enantiomer-Selective High-Resolution Gas Chromatography (esHRGC)