Ruth Kuldvee Merike Vaher Mihkel Koel Mihkel Kaljurand Institute of Chemistry, Department of Sciences, Tallinn Technical University, Tallinn, Estonia Heteroconjugation-based capillary electrophoretic separation of phenolic compounds in acetonitrile and propylene carbonate A mixture of methyl- and hydroxy-substituted phenols was separated by capillary elec- trophoresis in pure acetonitrile and propylene carbonate. Interactions between undis- sociated phenolic compounds and the background electrolytes were investigated. In the present work, benzyltriethylammonium chloride, tetrabutylammonium acetate, and two room temperature-molten salts, 1-butyl-3-methyl imidazolium trifluoroacetate and 1-butyl-3-methyl imidazolium heptafluorobutanoate, were used as background elec- trolytes. The formation of a negative complex between background electrolyte anion and neutral phenolic compound was observed and the formation constant calculated. The formation constants for anion-analyte complexes were approximately the same in propylene carbonate and in acetonitrile. In both solvents the formation constants were the highest for acetate and the lowest for trifluoroacetate. The separation of analytes was slightly influenced by the nature of the solvent: in acetonitrile the resolution be- tween peaks was higher for 1,3-dihydroxyphenol and 1,3,5-trihydroxyphenol, in pro- pylene carbonate 3-methylphenol and phenol were better separated. It was demon- strated that traces of water influence the mobilities of anion-phenol complexes in pro- pylene carbonate. Keywords: Acetonitrile / Capillary electrophoresis / Heteroconjugation / Phenol / Propylene car- bonate DOI 10.1002/elps.200305378 1 Introduction During recent years, the application of organic solvents in capillary electrophoresis (CE) as an alternative to aque- ous solutions has been constantly increasing. Non-aque- ous solvents can solubilize a large number of organic molecules which are insoluble in water, and thus expand the field of CE applications. On the other hand, organic solvents influence the interactions between the analytes and the components of background electrolyte (BGE), and therefore also the separation efficiency and selectiv- ity [1–3]. In nonprotogenic solvents like acetonitrile (ACN) and propylene carbonate (PC, 4-methyl-1,3-dioxolan-2- one) take place specific molecule-ion interactions which are rare in amphiprotic solvents like water. These inter- actions can offer additional separation mechanisms and alter the separation selectivity of the method. Heterocon- jugation formation between the BGE anion and neutral analyte is a well-known example of those interactions and has been investigated by few research groups in ACN medium [1–4]. Okada [4, 5, 9] was the first to apply the heteroconjugation phenomenon for capillary electro- phoretic separations of undissociated weak acids. He studied the heteroconjugation formation with BGE anions for phenols (Phs), carboxylic acids and alcohols in ACN. Miller et al. [6] separated selected chloro- and nitro- phenols in ACN medium, based on a combined effect of deprotonation and heteroconjugation. In a recent work of Porras et al. [8] the investigation of heteroconjugation- based separation of Ph and its methyl-substituted com- pounds was also performed in ACN. The influence of traces of water on the separation and mobilities of analytes was observed as well. Even though ACN is a favorable organic solvent for CE separations in terms of its low UV absorbance, low viscosity (allowing short analysis times) and availability at reasonable price and high quality, it also has several disadvantages. ACN is rather toxic and de- spite of its relatively high permittivity there is still a notice- able degree of ion association occuring in the solvent [10]. Correspondence: Dr. Ruth Kuldvee, Laboratory of Analytical Chemistry, Department of Chemistry, Tallinn Technical University, Ehitajate tee 5, Tallinn 19086, Estonia E-mail: ruth@chemnet.ee Fax: +372-620-20-20 Abbreviations: BMIFAc, 1-butyl-3-methyl imidazolium trifluoro- acetate; BMIHFBu, 1-butyl-3-methyl imidazolium heptafluoro- butanoate; BTEACl, benzyltriethylammonium chloride; FG, 1,3,5-trihydroxyphenol; 3MePh, 3-methylphenol; 34MePh, 3,4-dimethylphenol; 5MeR, 5-methyl-1,3-dihydroxyphenol; 25MeR, 2,5-dimethyl-1,3-dihydroxyphenol; PC, propylene car- bonate; Ph, phenol; R, 1,3-dihydroxyphenol; TBAAc, tetrabutyl- ammonium acetate Electrophoresis 2003, 24, 1627–1634 1627  2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0173-0835/03/1005–1627 $17.501.50/0 CE and CEC