Effects of Ionic Liquid as Additive and the pH of the Mobile Phase on the Retention Factors of Amino Benzoic Acids in RP-HPLC

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As an organic salt, ionic liquids are widely used as new solvent media. In this paper, three positional isomers, such as o-amino benzoic acid, m-amino benzoic acid, and p-amino benzoic acid are separated with four different ionic liquids as additives to the mobile phase using reversed-phase (RP) high-performance liquid chromatography (HPLC). Amino benzoic acids are biologically active substances; the p-isomer is present in a group of water-soluble vitamins and is widely known as a sunscreen agent. The ionic liquids used are 1-butyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium methylsulfate, and 1-octyl-3methylimidazolium methylsulfate. The effects of the length of the alkyl group on the imidazolium ring and its counterion, the concentrations of the ionic liquid, and the effect of the pH of the mobile phase on the retention factor of the amino benzoic acid isomers are studied. Separation with the ionic liquid in the eluent was better than the separation without the ionic liquid. The pH mainly affected the retention and elution order of the solutes in RP-HPLC.
Introduction
Unlike traditional salts, ionic liquids are liquids composed of relatively large organic cations and inorganic or organic anions (1). Ionic liquidshave some unique properties,such as negligible vapor pressure, good thermal stability, tunable viscosity, strong polarity,and miscibility with water and organic solvents, as well as good extractabilityfor various organic compounds and metal ions (2). Ionic liquids have been applied to analytical chemistry (2),catalysis (3,4) and biocatalysis (5), synthesis (6), and separation science (7–16). When ionic liquids are diluted or immobilized on a stationary support, they may not possess all the properties of the ordinary ionic liquids, and in some cases, they may keep several kinds of intermolecular interactions. For this
reason, ionic liquids can be useful for chromatographic separations(1,15,17). Recently, numerous amino and benzene compounds have been developed as medical and biological agents. In particular, amino compounds have resulted in many effective drugs currently in clinical and preventive use, and newer compounds with an expanded spectrum of activity are presently in continuous development. Amino benzoic acids are biologically active substances, and thep-isomer is includedin a group of water-soluble vitamins (18,19). These vitamins play important roles in numerous biological processes. p-Amino benzoic acid is an essential nutrient for some bacteria and is sometimes called Vitamin Bx. However, p-amino benzoic acid is not essential for humans, and itsactivitydiffers from other B vitamins. This acid istaken successfully in vitamin supplements. Although humans lack the ability to synthesize folic acid from p-amino benzoic acid, it is sometimes marketed as an essential nutrient under the premise that it can stimulate intestinal bacteria. Also, some sulfa drugs are chemically similar to p-amino benzoic acid, and their antibacterial activity is due to their ability to interfere with the molecular utilization of bacteria. In addition, its most widely known use is as a sunscreen. Taking it orally will not protect from the sun: the sunscreen function is purely a matter of pamino benzoic acid acting as a dye that absorbs UV light. An interdisciplinary approach to their chemistry is currently directed towards medicinal chemistry, supramolecular chemistry, and biomedical applications. Data on the chromatographic behaviors of biological activity compounds, which is one of the most important fields in modern chemistry, gives useful information for biochemistry and combinatorial and medicinal chemistry. In addition, although modern liquid chromatography is a powerful separation method, the separation of isomers remains difficult. Furthermore, from the point of view of the adsorption theory of substances by surfaces, studying the chromatographic behavior of isomers has practical applications and theoretical interest. Oneexample of the  separationof positional isomers of substituted benzoic acids with amine and β-cyclodextrin bonded-phase
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Abstract
Effects of Ionic Liquid as Additive and the pH of the Mobile Phase on the Retention Factors of Amino Benzoic Acids in RP-HPLC
J. Zheng, Y. Polyakova, andK.H. Row* Center for Advanced Bioseparation Technology, Department of Chemical Engineering, Inha University, Incheon 402-751, South Korea
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Journal of Chromatographic Science, Vol. 45, May/June 2007
* Author to whom correspondence should be addressed: emailrowkho@inha.ac.kr.
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