Methods for characterization of selectivity in reversed-phase liquid chromatography : IV. Retention behaviour of oligomeric series

Abstract

The retention behaviour of oligomeric series in reversed-phase systems with respect to the number of repeat structural units and the mobile phase composition can be described by the same set of equations as for homologous series, but the constants of these equations are strongly dependent on the structure of both the repeat structural unit and the structural residue in the molecule. The separation selectivity for the individual oligomers is determined mainly by the size (molar volume) and polarity (interaction index) of the repeat units and of the structural residue, but it also depends on the type and concentration of the organic solvent in the mobile phase. If the repeat structural unit increases both the size and polarity of the oligomer molecules, the selectivity may vary as a function of the structural parameters of the structural residue and of the mobile phase composition, and the retention may even decrease with increasing number of repeat structural units in a given series. This behaviour can be explained, characterized and even predicted (at least qualitatively) from the structural constants V x, I x, on the basis of the interaction indices model. This retention behaviour was investigated and verified for oligostyrenes, oligoethylene glycols and oligoethylene glycol nonylphenyl ethers on various C 18 and C 8 columns in mobile phases containing methanol, 1,4-dioxane or 2-propanol in water. The effectiveness of gradient elution for the separation of oligomeric series depends strongly on their structure. The presence of cetyltrimethylammonium bromide (CTAB) in the mobile phase has only a minor influence on the separation of non-sulphated oligoethylene glycol nonylphenyl ethers, but it leads to an increase in the retention of sulphated ethers, with a separation selectivity similar to that of non-sulphated ethers. The individual oligomers in mixtures containing sulphated and non-sulphated surfactants of this type can be separated in a single run using CTAB-containing mobile phases.