RETENTION BEHAVIOR OF CARBOXYLIC-ACIDS AND THEIR QUATERNARY AMMONIUM ION-PAIRS IN REVERSED PHASE CHROMATOGRAPHY WITH ACETONITRILE AS ORGANIC MODIFIER IN THE MOBILE PHASE

The retention behavior of some carboxylic acids (benzoic acid, salicylic acid, acetylsalicylic acid, 3-hydroxybenzoic acid, and 4-hydroxybenzoic acid) in acidic form and as ion pairs has been studied in a reversed phase chromatographic system using LiChrosorb RP-8 (10 μm) as support and mixtures of acetonitrile and aqueous phosphate buffers as mobile phases. The influence of pH, acetonitrile content, and the quaternary alkylammonium counterions on retention has been investigated. Equilibrium constants based on models for distribution of acids and ion pairs to a hydrophobic stationary phase are presented. Considerable adsorption of acetonitrile to the solid phase has been observed and found to increase with increasing percentage of acetonitrile in the mobile phase resulting in a decrease of the void volume. © 1979, American Chemical Society. All rights reserved.