Effect of Eluent pH on the Ionic and Molecular Forms of the Non-Steroidal Anti-Inflammatory Agents in Reversed-Phase High-Performance Liquid Chromatography

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Abstract
High-performance liquid chromatographic conditions for the best separation of some non-steroidal anti-inflammatory agents were described. The dependence of eluent pH on the ionic (protonated) and molecular (non-protonated) forms of analysed compounds have been investigated. This paper is the study of the retention behavior of some anti-inflammatory agents depending of the eluent pH. Some derivatives of phenol (acetaminophen, aspirin, salicylamide, phenacetin and salicylic acid), pyrazolidinedione (sulfinpyrazone, oxyphenbutazone, phenylbutazone and ketazone), amino-2-nicotinic acid (nixylic and niflumic acid) and amino-2-benzoic acid (mefenamic and flufenamic acid) were analysed with the isocratic RP-HPLC separation. The different variations of mobile phase methanol-water v/v, containing 1% acetic acid were performed in order to obtain the best conditions of the separation. For all analysed substances the pH, pKa and the fitted pKa values were calculated in different mixtures of methanol-water using the graphical method given by G. Charlot and B. Trémillon. The capacity factors (k′) and the separation factors (α) were calculated for all of them.

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