Ways of Estimating the Hold-up Time of a Gas Chromatographic System

Different ways of estimating the hold-up time of a system in gas chromatography are considered. It is shown that the hold-up time found from aerodynamic dependences proves to be minimal, but does not allow for the contribution from extracolumnar effects. The hold-up time, determined experimentally from the retention time of an unretainable sorbate (methane), proves to be maximal and depends on the intensity of sorbate-stationary phase interaction. The hold-up time found from the purely empiric correlation between the logarithm of the net retention time of homologs and the number of carbon atoms is intermediate between the theoretical and experimental values. It is shown that the correlation between the number of carbon atoms in a homological series and the relative retention time of sorbates (rather than their net retention time) is grounded thermodynamically. Corresponding correlations are proposed that allow estimates not only of the hold-up time of a system but also of such parameters as the phase ratio and the change in the free energy of a methylene unit during sorption on the stationary phase.