Chromatographic separations of aromatic carboxylic acids.

The purpose of this review is to present methods of chromatographic analysis of aromatic carboxylic acids. The separation, identification and quantitative analysis of aromatic carboxylic acids are necessary because of their importance as non-steroid antiphlogistic drugs, semi-products of biosynthesis of aromatic amino-acids in plants (phenolic acids), metabolites of numerous toxic substances, drugs and catecholamines. HPLC separation of ionic samples tends to be more complicated than separation of non-ionic compounds. The review describes the dependence of the retention of ionic solutes on pH and solvent composition as well as on the ionic strength of a mobile phase. The application of the ion-suppressing RP-HPLC method using organic modifiers (aqueous buffer solutions) as eluents in aromatic carboxylic acid analysis is also presented. In more difficult cases of analysis the addition of an ion-pairing reagent, such as the quaternary alkylammonium ion, is necessary to obtain satisfactory separations. Hypotheses of ion-pair formation in reversed-phase systems as well as the influence of various agents on the separation of ionic solutes in IP-RP systems are explained. Examples of the application of ion-pair liquid chromatography to the analysis of aromatic carboxylic acids have also been reviewed. The principles and application of ion-exchange chromatography to the purification, isolation and less frequently, to chromatographic analysis are discussed. Polar adsorbents and polar bonded stationary phases are also widely used in carboxylic acid separation in normal-phase systems, mainly by TLC, often coupled with densitometry. The review also shows examples of separation of chiral benzoic acids and their derivatives in LC systems. The possibilities of application of gas chromatography preceded by derivatisation or pyrolysis of acidic compounds and applications of GC-MS and Py-GC-MS coupled methods in identification and quantitation of aromatic carboxylic acids is also reviewed.