Christopher J. Evenhuis 1 Vlastimil Hruska 2 Rosanne M. Guijt 1 Miroslav Macka 3 Bohuslav Gas ˇ 2 Philip J. Marriott 4 Paul R. Haddad 1 1 Australian Centre for Research on Separation Science, University of Tasmania, School of Chemistry, Hobart, Tasmania, Australia 2 Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University in Prague, Hlavova, Czech Republic 3 School of Chemical Sciences, Dublin City University, Glasnevin, Dublin, Ireland 4 Applied Chemistry, RMIT University, Melbourne, Australia Received May 11, 2007 Revised June 15, 2007 Accepted June 19, 2007 Research Article Reliable electrophoretic mobilities free from Joule heating effects using CE Ionic electrophoretic mobilities determined by means of CE experiments are sometimes different when compared to generally accepted values based on limiting ionic conductance measurements. While the effect of ionic strength on electrophoretic mobility has been long understood, the increase in the mobility that results from Joule heating (the resistive heat- ing that occurs when a current passes through an electrolyte) has been largely overlooked. In this work, a simple method for obtaining reliable and reproducible values of electro- phoretic mobility is described. The electrophoretic mobility is measured over a range of driving powers and the extrapolation to zero power dissipation is employed to eliminate the effect of Joule heating. These extrapolated values of electrophoretic mobility can then be used to calculate limiting ionic mobilities by making a correction for ionic strength; this somewhat complicated calculation is conveniently performed by using the freeware pro- gram PeakMaster 5. These straightforward procedures improve the agreement between experimentally determined and literature values of limiting ionic mobility by at least one order of magnitude. Using Tris-chromate BGE with a value of conductivity 0.34 S/m and ionic strength 59 mM at a modest dissipated power per unit length of 2.0 W/m, values of mobility for inorganic anions were increased by an average of 12.6% relative to their values free from the effects of Joule heating. These increases were accompanied by a reduction in mobilities due to the ionic strength effect, which was 11% for univalent and 28% for diva- lent inorganic ions compared to their limiting ionic mobilities. Additionally, it was possible to determine the limiting ionic mobility for a number of aromatic anions by using Peak- Master 5 to perform an ionic strength correction. A major significance of this work is in being able to use CE to obtain reliable and accurate values of electrophoretic mobilities with all its benefits, including understanding and interpretation of physicochemical phenomena and the ability to model and simulate such phenomena accurately. Keywords: Electrophoretic mobility / Ionic strength correction / Joule heating / Limiting ionic conductance / PeakMaster 5 DOI 10.1002/elps.200700343 Electrophoresis 2007, 28, 3759–3766 3759 1 Introduction In an electrolyte containing several species varying in size and charge, the limiting ionic mobility of the ith constituent with a relative charge z (u 0 i;z ) is a fundamental parameter related to the effective size and charge of the solvated ion [1]. Effectively, the limiting ionic mobility is the electrophoretic mobility (u i,z ) of a species free from the influence of coun- terions. Experimentally, it cannot be measured directly but can be determined by measuring u i,z in an electrolyte for a range of ionic strengths and extrapolating to zero ionic strength. Pragmatically, it is simpler to measure u i,z for a particular ionic strength and to calculate u 0 i;z by performing an ionic strength correction. The details of the process are described in detail later in the manuscript. The knowledge of u 0 i;z allows for instance simulation of electrophoretic processes and a direct correlation of the simulated and experimental results. Both theoretically and practically, it is also important that electrophoretic mobility of a charged species is directly associated with its limiting equivalent conductivity (l i,z ) and its diffusion coefficient (D i,z ) [2, 3]. Therefore, the ability to determine the limiting ionic mobilities accurately is essential. There have been two major approaches to determining limiting electrophoretic mobilities of ions, (i) based on measurements of conductivity [2, 3] and (ii) electrophoretic methods [4]. The first approach uses the relationship be- tween the limiting molar ionic conductivity, l 0 i;z , and the limiting ionic mobility, l 0 i;z ¼ Fu 0 i;z , where F is the Faraday Correspondence: Dr. Miroslav Macka, School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland E-mail: mirek.macka@dcu.ie Fax: 1353-1-700-5503  2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com