Reliable electrophoretic mobilities free from Joule heating effects using CE

Abstract

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 heating 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 electrophoretic 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 program 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 divalent 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 PeakMaster 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.

Investigating the formation and the properties of monoalkyl carbonates in aqueous medium using capillary electrophoresis with capacitively coupled contactless conductivity detection

“…Thus, an approach to obtain more reliable results is to use two detectors along the capillary and substitute the migration time by the difference of times between the second and the first detectors (Dt) and the distance L D by the distance between them (DL D ). The limiting values of mobilities were obtained as suggested by Evenhuis et al [29]. Taking into account that the pK a values of MACs are considerably below 7, we can conclude that they are essentially monocharged anions in the experiments.…”

Joule heating effects and the experimental determination of temperature during CE

“…It should be noted that the m ep determined from Eq. (37) Evenhuis et al [37] suggest that the variation of m ep with temperature cannot be understood by considering viscosity alone. They found that m ep increased linearly with temperature but the size of the increase depended on the species involved.…”

“…For temperature increases of up to 201C, m ep increases linearly with temperature [37], making it suitable for use as a temperature probe [13].…”

“…where subscript i refers to species i, t i is the migration time for species i and t EOF is the migration time for a neutral marker [37]. It should be noted that the m ep determined from Eq.…”

“…Most authors have assumed that electrical conductivity k increases linearly with absolute temperature [2,13,[32][33][34][35][36][37] as shown in the following equation:…”