Comparison of retention mechanisms of homologous series and triglycerides in non-aqueous reversed-phase liquid chromatography.

The retention behaviour of saturated homogeneous triglycerides was compared to that of single chain homologous compounds in reversed-phase liquid chromatography on silica-based octadecyl bonded phases with non-aqueous binary eluents containing chloroform. For both series, in all eluents and at all investigated temperatures, a discontinuity (1-6% slope change) in log k' vs. the carbon atom number, nc, of each solute hydrocarbonaceous chain was observed for a given critical number of carbon atoms, nc,crit approximately 12-13. Similar discontinuities were observed in delta H degrees and delta S degrees vs. nc for the same nc,crit value. These and other phenomena (existence of two convergence points for log k' vs. nc plotted at different eluent compositions; existence of two convergence temperatures in log k' vs. 1/T plotted for several members of a given solute series) reflect the mechanism of penetration of the solute aliphatic chains into the bonded layer. The eluotropic strength of acetonitrile-chloroform mobile phases was determined and seen to have a larger rate of variation with the solvent composition than that of some other non-aqueous binary eluents. The ratio of the slope of plots of log k' vs. nc curves for triglycerides and single chain homologous series is not a whole number but has a value lying between 2 and 3. This explains why triglyceride retentions cannot be predicted from retention data for the fatty acid methyl esters. More importantly, this indicates that the triglycerides may interact with the stationary phase using various conformations, one, two or three chains penetrating into the bonded layer. These conformations, in dynamic equilibrium with each other, contribute differently to the retention. This offers, in principle, the possibility to separate unsaturated triglycerides having the same number of carbon atoms and of double bonds but differing in the distribution of the unsaturations along the chains, if the double bonds located in a non-penetrating chain can selectively interact with a mobile phase component.