Retention relationships for aromatic hydrocarbons eluted from capped and uncapped octadecyl silica gels

Abstract

Retention characteristics of 54 aromatic hydrocarbons have been measured on three reversed-phase packing materials, ODS Hypersil (a fully capped octadecyl bonded reversed-phase silica gel), uncapped ODS Hypersil (an intermediate in the production of ODS Hypersil), and LiChrosorb RP-18, using three different eluent compositions, 90:10, 80:20, and 70:30 methanol-water (by weight). For all nine combinations of packing material and eluent log k′ shows a linear dependence on carbon number for the n-alkylbenzenes and for those polymethylbenzenes in which all methyl groups are ortho to each other. The gradients of the plots are essentially identical for ODS Hypersil and LiChrosorb RP-18, but are slightly lower for uncapped ODS Hypersil. For compounds with the same carbon number, those with higher hydrogen numbers are more retained. Examination of the correlation of log k′ for all 54 solutes with carbon number, hydrogen number and connectivity index shows that none of these parameters provides a satisfactory universal correlation. For isomeric alkylbenzenes, retention is significantly affected by the number of alkyl groups, by the structure of the alkyl groups and by the arrangement of the groups on the benzene ring. It is noted specifically that ortho-substituted alkylbenzenes have lower retention than isomers where the groups are not adjacent. This is termed the “reverse ortho effect”. Branching of alkyl substituents reduces retention. While the chromatographic behaviour of the 54 solutes on capped and uncapped ODS Hypersil is broadly similar, the retention of the alkylbenzenes is slightly increased by capping while the retention of polynuclear aromatics is slightly reduced by capping. LiChrosorb behaves similarly to uncapped ODS Hypersil except that its retention is much greater.