Effect of alkyl chain length of the stationary phase on retention and selectivity in reversed-phase liquid chromatography : Participation of solvent molecules in the stationary phase

Abstract

The retention behaviour of various hydrocarbons, including n-alkanes, polynuclear aromatic hydrocarbons (PAHs) and alicyclic compounds, were examined on alkylsilylated stationary phases with trimthylsilyl (C 1), octyldimethylsilyl (C 8) and octadecyldimethylsilyl (C 18) functionality. In general, a longer alkyl chain in the stationary phase provided a longer retention time. Although the retention of rigid planar aromatic hydrocarbons relative to non-polar aromatic compounds and alkanes was much larger on the C 18 phase than on the C 8 and C 1 phases in a methanolic mobile phase, the effect of chain length was much smaller in a mobile phase containing acetonitrile. A change in the organic solvent in the mobile phase from methanol to acetonitrile resulted in grossly different selectivities between aromatic hydrocarbons and the saturated hydrocarbons regardless of the rigidity or planarity in structure of the C 18 phase, in contrast with minor changes on the C 1 and C 8 phases. The increase of organic solvent content in the mobile phase resulted in much larger retention of rigid planar aromatic compounds relative to other aromatic compounds on the C 18 phase, whereas on the C 1 and C 8 phases this effect was smaller. The retention characteristics of the C 8 phase for the hydrocarbons were found to be relatively close to those of the C 1 phase. The results suggest that the effect of chain length is influenced by both the molecular structure of the solutes and the type of organic solvents used in the mobile phase.