Retention of ortho‐ and para‐Positional Isomers of Some Model Solutes on Polar Bonded Stationary Phases in Different Eluent Systems by HPTLC

Abstract

The influence of a substituents' position on the retention of isomer pairs on polar bonded stationary phases has been examined. The pairs of ortho and para substituted phenols and anilines were chromatographed on aminopropyl, diol, and cyanopropyl layers by use of nonaqueous eluents consisting of n‐heptane and polar modifier (tetrahydrofuran, ethyl acetate, or 2‐propanol) or on cyanopropyl layer in RP systems with aqueous solutions of methanol or acetonitrile as eluents. Results obtained are presented graphically and as parameters of R M vs. log X (modifier concentration) relationships. The structural effects have been discussed. Stronger retention and higher slope values of para derivatives in comparison to ortho were ascertained for substituted phenols and anilines, which is typical for silica and magnesium silicate surfaces. Solutes with two strongly polar groups behave differently; in some cases, ortho isomers are adsorbed more strongly than para isomers on polar bonded stationary phases, in some eluents containing a high concentration of polar modifier in a non‐aqueous mobile phase. Similar retention behavior has been observed for ortho derivatives, which can interact with active alumina surface centers. The separation selectivity has been analysed by ΔR M values.