Internal electrolyte temperatures for polymer and fused-silica capillaries used in capillary electrophoresis

Abstract

Polymers are important as materials for manufacturing microfluidic devices for electrodriven separations, in which Joule heating is an unavoidable phenomenon. Heating effects were investigated in polymer capillaries using a CE setup. This study is the first step toward the longer-term objective of the study of heating effects occurring in polymeric microfluidic devices. The thermal conductivity of polymers is much smaller than that of fused silica (FS), resulting in less efficient dissipation of heat in polymeric capillaries. This study used conductance measurements as a temperature probe to determine the mean electrolyte temperatures in CE capillaries of different materials. Values for mean electrolyte temperatures in capillaries made of New Generation FluoroPolymer (NGFP), poly-(methylmethacrylate) (PMMA), and poly(ether ether ketone) (PEEK) capillaries were compared with those obtained for FS capillaries. Extrapolation of plots of conductance versus power per unit length (P/L) to zero power was used to obtain conductance values free of Joule heating effects. The ratio of the measured conductance values at different power levels to the conductance at zero power was used to determine the mean temperature of the electrolyte. For each type of capillary material, it was found that the average increase in the mean temperature of the electrolyte (DeltaT(Mean)) was directly proportional to P/L and inversely proportional to the thermal conductivity (lambda) of the capillary material. At 7.5 W/m, values for DeltaT(Mean) for NGFP, PMMA, and PEEK were determined to be 36.6, 33.8, and 30.7 degrees C, respectively. Under identical conditions, DeltaT(Mean) for FS capillaries was 20.4 degrees C.

Determination of the surface heat‐transfer coefficient in CE

“…Conductance is a good choice for monitoring thermal effects in CE [13,17,30]. It has a strong temperature dependence (see Eq.…”

“…where p L is a constant equal to the mean temperature rise of the electrolyte caused by Joule heating at the rate of 1 W/m and may be calculated using the following equation: [17] for values of l). This treatment agrees with the classical approach to calculating the rise in temperature of the electrolyte adopted by Knox [12], Kok [18] and Porras et al [3], who assumed that a steady state is reached in which the rate at which heat is generated in the electrolyte is equal to the rate at which it is dissipated from the outer wall so that a constant temperature profile is achieved.…”

Joule heating effects and the experimental determination of temperature during CE

“…In 2005, Evenhuis et al [36] extended the work of Knox and McCormack [13] and of Burgi et al [32] who used the conductivity of the BGE as a temperature probe. Evenhuis et al found a linear relationship between the power per unit length and the conductance of the BGE.…”

“…Thermal conductivities of polymer materials are typically 5-10 times less than those of fusedsilica or borosilicate glass. On this basis, glass microchips are more attractive than their polymer counterparts for heat dissipation [36].…”

“…The mean rise in temperature of the electrolyte above the ambient temperature or temperature set by thermostat (DT Mean ) depends on the power dissipated per unit length (P/L) [32,36,45,46] (see Fig. 1).…”