Effect of solvent strength on the chromatographic behavior of certain organic acids on a multmodal phenylpropanolamine bonded silica column

Abstract

The solvent of the injected sample significantly influenced the chromatographic behavior of acidic compounds on a phenylpropanolamine bonded silica column. The retention of the tested compounds, including phenol, L-ascorbic acid, benzoic acid, acetylsalicylic acid and salicylic acid, was manipulated by modifying the pH of the solution within a rage of 2.5-6.5 and the proportion of organic modifier in the mobile phase in isocratic mode. The mechanism of interaction at each pH was identified by changing the ratios of acetonitrile in the mobile phase and the solvent strength was studied. Four tested substances, i.e. phenol, benzoic acid, acetylsalicylic acid and salicylic acid showed the same effect, that is an increase in pH until reach their pKa resulted increase in the effect of solvent strength in accordance with the observation that their retention at high pH is governed predominantly by hydrophobic interaction comparing to low pH and hydrophobicity decreases when pH are higher than the pKa due to ionized form of all these organic acids except L-ascorbic acid, its retention at low pH is based mainly on ionic interaction. The effect of injected solvent strength on the multimodal stationary phase is the same as that on a reversed-phase column only when the hydrophobic interaction is dominant i.e. the number of theoretical plates of each compound distinctly depends on the strength of the injected solvent and reaches a constant level at the same strength as the mobile phase. Meanwhile the effect of the injected solvent at pH to which the ionic interaction is predominant is the same as that on an ion-exchange column.