ELSEVIER Journal of Chromatography A, 753 (1996) 253-260 JOURNALOF CHROMATOGRAPHY A Mixed-mode liquid chromatography of carboxylic acids and inorganic anions on a latex-based pellicular stationary phase 1 Gabriella Rrvrsz, Prter Haj6s*, Hortenzia Csiszfir Department of Analytical Chemistry, University of Veszpr#m, P.O. Box 158, Veszpr~m H-8201, Hungary Received 26 February 1996; revised 9 July 1996; accepted 17 July 1996 Abstract The retention behaviour of mono-, dicarboxylate and inorganic anions on latex-based pellicular anion-exchanger was investigated. This paper is focused on the possible retention mechanism providing a retention database on a variety of mobile phase compositions. It has been demonstrated that anions can interact with the cation-exchange layer on the highly crosslinked latex-based pellicular phase as well as with the anion-exchange sites. A hypothesis is proposed to explain a mixed-mode retention of the series of analytes. A combination of two mechanisms has been found responsible for the enhanced selectivity: ion-exchange and ion-exclusion. Applications include the separation of formate, acetate, propionate, lactate, pyruvate, oxalate, malonate, succinate, tartrate, fumarate, maleate, chloride, nitrate and sulphate with NaOH eluent using suppressed conductivity detection. The study effectively characterizes the behaviour of different analytes under elution conditions of practical importance. Keywords: Inorganic anions; Carboxylic acids 1. Introduction Both carboxylic acids and their carboxylate salts are widely used in chemistry and in commercial products [1-3]. In the pharmaceutical industry, organic acids are used as antioxidants, preservatives, acidifiers and drug adsorption modifiers. Various diseases have been correlated with increased con- centrations of organic acids in biological fluids. Organic acids are also fundamental components of fruits. Therefore a rapid and simple method for determining these compounds is required. Carboxylic acids are usually separated by three *Corresponding author. ' Paper presented at the International Ion Chromatography Sym- posium 1995, Dallas, TX, USA, 1-5 October 1995. different liquid chromatographic methods: anion-ex- change [4,5], ion-exclusion and reversed-phase chro- matography [6-8]. Traditional high-performance liq- uid chromatography methods use refractive index (RI), ultraviolet (UV) absorption and derivatization fluorescence techniques for the detection of the acids. The most common application is the ion- exclusion technique. It is especially attractive as an adjunct to ion-exchange chromatography since selec- tivities obtained by these methods are quite different. Strong inorganic acid anions are excluded from the resin phase in a single peak according to the Donnan principle and elute at the void volume. Weaker and protonated species existing largely in the molecular form are retained on the stationary phase by a combination of ion-exclusion and hydrophobic inter- actions. For low-molecular-mass components, ion- 0021-9673/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S0021-9673(96)00644-9