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Simpson, David A
Languages: English
Types: Unknown
Porous graphitic carbon has been developed as a high-performance liquid chromatography stationary phase over the past 30 years. The evolution of PGC as a stationary phase was motivated by the desire to find a substitute for reversed-phase silica gel based materials in areas where these materials are inadequate (e.g. extremes of pH). However, PGC possesses a number of chromatographic properties which are thus far largely unexplained and differ from traditional silica-based reversed-phase supports. The retention mechanisms of mono-substituted benzenes and biphenyls on porous graphitic carbon stationary phase were investigated using chromatographic and computational methods. The studies on a range of n-alkylbenzene analytes demonstrated that retention on PGC was found to be greatly influenced by hydrophobic parameters such as Hansch-Fujita and log P and that PGC has superior selectivity for isomers of amylbenzene in terms of its chromatographic retention properties when compared to octadecyl-silica (ODS). Molecular modelling of the alkylbenzene analytes indicated that the interaction between toluene and ethylbenzene and PGC was in a cofacial geometry whereas that between the longer chain alkylbenzene was of a face-edge (perpendicular) nature. This was confirmed by the relatively poor retention of highly branched amylbenzenes. Benzene derivatives demonstrated retention properties on PGC such that the logarithm of the retention factor (log kw) was found to be closely correlated with a combination of the Hansch-Fujita parameter and Lowest Unoccupied Molecular Orbital Energy (ELUMO) of the analytes. This was augmented by similar correlations between log kw and Hansch-Fujita, ELUMO and mean polarisability. Chromatographic studies of the benzene derivatives on PGC gave enhanced retention for polar and charged analytes and reduced retention for the alkyl substituted benzenes used in this study when compared with ODS. Preliminary semi-empirical calculations of the interaction between the analyte and the PGC stationary phase for benzene derivatives showed qualitative relationships between the energy of interaction and log kw for closely related benzene derivatives. The retention of mono-substituted biphenyl compounds was found to be greater on PGC than on ODS stationary phase, with the strongest retention found for highly conjugated species (such as 4-phenylcinnamic acid and 4-vinylbiphenyl). This observation supports the hypothesis that the presence of a planar moiety in a molecule imparts an increased retention when using PGC as the stationary phase. PGC was found to be more retentive for the separation of both polar and non-polar biphenyl derivatives. Semi-empirical calculations suggested that the ease with which an analyte could attain a planar geometry was an important factor influencing the retention of biphenyl derivatives on PGC.
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