Normal-phase high-performance liquid chromatographic separations of positional isomers of substituted benzoic acids...

The separation of positional isomers of several substituted benzoic acids was studied using two different columns, i.e. amine and beta-cyclodextrin bonded silicas, at appropriate normal-phase conditions. Although some other columns do separate substituted benzoic acids, particularly the octadecylsilica column in a reversed-phase mode, the present separation under simple normal-phase, isocratic conditions is unique. In particular, the retention order of these acids for the amino bonded-phase column can be roughly predicted using the pKa values of the analytes. On the other hand, due to the strong interaction between substituted benzoic acids and the beta-cyclodextrin bonded phase, a small amount of acetic acid has to be added into the mobile phase to overcome band broadening and tailing problems. Possible retention mechanisms are also discussed.