Modelling of migration behaviour of inorganic anions in ion-exchange capillary electrochromatography A theoretical model to explain the observed mobility of inorganic anions in capillary electrochromatography (CEC) using ion-exchange (IE) stationary phases has been de- rived. The model divides contributions to the observed mobility of an analyte ion into capillary electrophoretic (CE) and IE components. The CE component includes the influence of varying the ionic strength of the background electrolyte on the electropho- retic mobility of the analyte, while the IE component accounts for the variation in reten- tion of the analyte ion caused by changing the composition of the background electro- lyte. The model was verified using a mixture of UV-absorbing inorganic ions in electrolytes of differing eluotropic strength in both packed and open-tubular CEC sys- tems, with excellent agreement (r 2 > 0.98) for both systems. Values of constants in the model equation determined by nonlinear regression were used to estimate the relative strengths of the interactions of different analytes with the stationary phase and these were found to agree well with elution orders observed in conventional IE chromatogra- phy. Keywords: Ion exchange / Electrochromatography / Packed column / Open-tubular column / Modelling / Optimisation / Inorganic anions EL 4203 Michael C. Breadmore 1 Emily F. Hilder 1 Miroslav Macka 1 Nebojsa Avdalovic 2 Paul R. Haddad 1 1 University of Tasmania, Hobart, Tasmania, Australia 2 Dionex Corporation, Sunnyvale, CA, USA 1 Introduction 1.1 General Inorganic anions are routinely analysed by one of two techniques: ion chromatography (IC) and capillary elec- trophoresis (CE). These methods differ greatly when it comes to analysis time, detection sensitivity, and separa- tion selectivity [1]. In IC, analytes are separated predomi- nantly by electrostatic interactions with a charged func- tional group on the stationary phase whereas in CE separation is based on the charge-to-size ratio of the ana- lyte and its resulting electrophoretic mobility. Combination of both ion-exchange (IE) interactions and electrophoretic migration can be accomplished in capillary electrochro- matography (CEC) using an IE stationary phase. This approach offers the potential to provide separation selec- tivity that is a combination of IC and CE and also the means to manipulate separation selectivity in order to accomplish a desired separation. Such manipulation of selectivity can be best achieved through the use of a migration model which allows analyte mobilities to be pre- dicted for a wide range of experimental conditions and it is the development of such a model which is addressed in this study. Two methods for the introduction of IE interactions into a CE system are the use of capillaries packed with conven- tional IE stationary phases (silica- or polymer-based) or the use of IE pseudostationary phases. IE pseudostation- ary phases were pioneered in the early works of Terabe and Isemura [2, 3], who used a soluble IE polymer, and Kaneta et al. [4], who used a charged surfactant as the pseudostationary phase. However, these methods suf- fered from the constraints of a restricted time window in which to accommodate the migration times of the ana- lytes and the potentially problematic need to add compo- nents to the background electrolyte (BGE). The alterna- tive is to use IE stationary phases and these can be classified as open-tubular systems (denoted here as IE open-tubular capillary electrochromatography or IE- OTCEC), packed stationary phases (denoted as packed column IE-capillary electrochromatography or IE-CEC), and continuous-bed or monolithic stationary phases. The separation of inorganic anions by either IE-CEC or IE-OTCEC has received relatively little attention, espe- cially with regard to the separation selectivity that can be obtained in such systems. IE-CEC was used by Li et al. [5] to introduce IE interactions into CE with resulting changes in separation selectivity for iodide, iodate, and perrhenate. Pressure-assisted IE-CEC has been used for Correspondence: Prof. Paul Haddad, School of Chemistry, Uni- versity of Tasmania, GPO Box 252-75, Hobart, TAS 7001, Aus- tralia E-mail: paul.haddad@utas.edu.au Fax: +61-3-6226 2858 Abbreviations: IC, ion chromatography; IE, ion-exchange; OT, open tubular; SAX, strong anion-exchange Electrophoresis 2001, 22, 503±510 503  WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001 0173-0835/00/0303-503 $17.50+.50/0 CE and CEC