Chromatographic behaviour of positional isomers on porous graphitic carbon

Retention characteristics of six sets of ionizable substituted benzene isomers have been measured on a porous graphitic carbon column using buffered aqueous eluents containing 35% acetonitrile. The elution orders varied with the different types of substituents on the benzene ring and were not readily predictable from the known structures of the isomers. Over the range of mobile phase between pH 2 to 9.4, the retention of the solutes was correlated directly with their degree of ionization, with the ionized form being least retained. An interpretation of these data was suggested based on the solute molecular orientation effect induced by the competing interactions of solute with adsorbent and solvent