RETENTION BEHAVIOR OF IONIZABLE ISOMERS IN REVERSED-PHASE LIQUID-CHROMATOGRAPHY - A COMPARATIVE-STUDY OF POROUS GRAPHITIC CARBON AND OCTADECYL BONDED SILICA

The retention behaviors of 36 positional isomers of ionizable substitu ted benzene compounds have been compared on two different packing mate rials: porous graphitic carbon (PGC) and octadecyl bonded silica (ODS) using 35% aqueous acetonitrile as the mobile phase. The effect of the mobile phase pH on the solute retention was studied over a range of p H values from pH 2.0 to 7.0, The retention as a function of pH was mod eled using equations based on solute ionization, With PGC, the theoret ical equations fitted the observed retention data for each class of so lute, indicating that the retention mechanism was uniform over the who le pH range, However, with ODS, only the acidic solutes showed agreeme nt with the theoretical model; for the amine-containing compounds, ser ious deviations from the theory were observed, suggesting that strongl y acidic silanols gave added retention at low mobile phase pH. Overall , PGC demonstrated a higher selectivity toward positional isomers than ODS. This was attributed to the greater steric discriminating ability arising from the flat surface of the PGC compared with the more fluid nature of the ODS bonded phase.