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Evaluation of reversible interconversion in comprehensive two-dimensional gas chromatography using enantioselective columns in first and second dimensions A

Kröger, Sabrina, Wong, Yong Foo, Chin, Sung-Tong, Grant, Jacob, Lupton, David, Marriott, Philip J.
Journal of chromatography 2015 v.1404 pp. 104-114
cis-trans isomers, comprehensive two-dimensional gas chromatography, enantiomers, enantioselectivity, gas chromatography, isomerization, molecular weight, ovens, oximes, polyethylene glycol, temperature
The reversible molecular interconversion behaviour of a synthesised oxime (2-phenylpropanaldehyde oxime; (C6H5)CH(CH3)CHN(OH)) was investigated by both, single dimensional gas chromatography (1D GC) and comprehensive two-dimensional gas chromatography (GC×GC). Previous studies on small molecular weight oximes were extended to this larger aromatic oxime (molar mass 149.19gmol⁻¹) with interest in the extent of interconversion, enantioselective resolution, and retention time. On a polyethylene glycol (PEG; wax-type) column, a characteristic interconversion zone between two antipodes of E and Z isomers was formed by molecules which have undergone isomerisation on the column (E⇌Z). The extent of interconversion was investigated by varying chromatographic conditions (oven temperature and carrier flow rate) to understand the nature of the behaviour observed. The extent of interconversion was negligible in both enantioselective and methyl-phenylpolysiloxane phase-columns, correlating with the low polarity of the stationary phase. In order to obtain isomerisation along with enantio-resolution, a wax-type and an enantioselective column were coupled in either enantioselective-wax or wax-enantioselective order. The most appropriate column arrangement was selected for study by using a GC×GC experiment with either a wax-phase or phenyl-methylpolysiloxane phase as ²D column. In addition to evaluation of these fast elution columns, a long narrow-bore enantioselective column (10m) was introduced as ²D, providing an enantioselective–PEG (coupled-column ensemble: ¹D1+¹D2)×enantioselective (²D) column combination. In this instance, the ¹D1 enantioselective column provides enantiomeric separation of the corresponding enantiomers ((R) and (S)) of (E)- and (Z)-2-phenylpropanaldehyde oxime, followed by E/Z isomerisation in the coupled ¹D2 PEG (reactor) column. The resulting chromatographic interconversion region was modulated and separated into either E/Z isomers (achiral ²D column) or into the respective (R) and (S) enantiomers of the E/Z isomers when using a ²D enantioselective column. With this arrangement, the isomers underneath the broad interconversion plateau in 1D elution profiles, including the enantiomers, could be resolved, illuminating salient features and understanding of the molecular reversible process of the interconverting molecules during the chromatographic elution. The two-dimensional patterns (contour plots), resulting from the combination of interconversion process and chiral separation, are discussed phenomenologically.