Electrophoresis Joule heat

Zone electrophoresis is defined as the differential migration of a molecule having a net charge through a medium under the influence of an electric field (1). This technique was first used in the 1930s, when it was discovered that moving boundary electrophoresis yielded incomplete separations of analytes (2). The separations were incomplete due to Joule heating within the system, which caused convection which was detrimental to the separation. 

It is important to bear in mind that an electrophoresis gel is an element in an electrical circuit and as such obeys the fundamental laws of electricity. Each gel has an intrinsic resistance, R, determined by the ionic strength of its buffer (R changes with time in discontinuous systems). When a voltage V is impressed across the gel, a current I flows through the gel and the external circuitry. Ohm s law relates these three quantities V = IR, where V is expressed in volts, I in amperes, and R in ohms. In addition, power P, in watts, is given by P = IV. The generation of Joule heat, H, is related to power by the mechanical equivalent of heat, 4.18 J/cal, so that H = (PI4.18) cal/sec. 

Fused silica capillaries are almost universally used in capillary electrophoresis. The inner diameter of fused silica capillaries varies from 20 to 200 pm, and the outer diameter varies from 150 to 360 pm. Selection of the capillary inner diameter is a compromise between resolution, sensitivity, and capacity. Best resolution is achieved by reducing the capillary diameter to maximize heat dissipation. Best sensitivity and sample load capacity are achieved with large internal diameters. A capillary internal diameter of 50 pm is optimal for most applications, but diameters of 75 to 100 pm may be needed for high sensitivity or for micropreparative applications. However, capillary diameters above 75 pm exhibit poor heat dissipation and may require use of low-conductivity buffers and low field strengths to avoid excessive Joule heating. 

Xuan, X., Hu, G., and Li, D. (2006). Joule heating effects on separation efficiency in capillary zone electrophoresis with an initial voltage ramp. Electrophoresis 27, 3171 — 3180. 

The recent introduction of non-aqueous media extends the applicability of CE. Different selectivity, enhanced efficiency, reduced analysis time, lower Joule heating, and better solubility or stability of some compounds in organic solvent than in water are the main reasons for the success of non-aqueous capillary electrophoresis (NACE). Several solvent properties must be considered in selecting the appropriate separation medium (see Chapter 2) dielectric constant, viscosity, dissociation constant, polarity, autoprotolysis constant, electrical conductivity, volatility, and solvation ability. Commonly used solvents in NACE separations include acetonitrile (ACN) short-chain alcohols such as methanol (MeOH), ethanol (EtOH), isopropanol (i-PrOH) amides [formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA)] and dimethylsulfoxide (DMSO). Since NACE—UV may present a lack of sensitivity due to the strong UV absorbance of some solvents at low wavelengths (e.g., formamides), the on-line coupling of NACE... 

Unlike capillary electrophoresis, wherein absorbance detection is probably the most commonly utilized technique, absorbance detection on lab-on-a-chip devices has seen only a handful of applications. This can be attributed to the extremely small microchannel depths evident on microchip devices, which are typically on the order of 10 pm. These extremely small channel depths result in absorbance pathlengths that seriously limit the sensitivity of absorbance-based techniques. The Collins group has shown, however, that by capitalizing on low conductivity non-aqueous buffer systems, microchannel depths can be increased to as much as 100 pm without seeing detrimental Joule heating effects that would otherwise compromise separation efficiencies in such a large cross-sectional microchannel [38],... 

According to Equation 6.6, the velocity of the EOF is directly proportional to the intensity of the applied electric held. However, in practice, nonlinear dependence of the EOF on the applied electric held is obtained as a result of Joule heat production, which causes the increase of the electrolyte temperature with consequent decrease of viscosity and variation of all other temperature-dependent parameters (protonic equilibrium, ion distribution in the double layer, etc.). The EOF can also be altered during a run by variations of the protonic concentration in the anodic and cathodic electrolyte solutions as a result of electrophoresis. This effect can be minimized by using electrolyte... 

Ohm s law States that the current, /, in a circuit is proportional to voltage, E, and inversely proportional to resistance, R I = E/R Ohm s law plot In capillary electrophoresis, a graph of current versus applied voltage. The graph deviates from a straight line when Joule heating becomes significant. 

Joule heating -of electrophoretic solution [ELECTROSEPARATIONS - ELECTROPHORESIS] (Vol 9)... 

Generation of Meat in Electric Fields. One of the practical problems encountered in electrophoresis is the generation of heat from resistive dissipation of energy in the electrophoretic medium. The generation of heat (Joule heating) is given by... 

CZE is high voltage, free-solution electrophoresis carried out in a capillary. The capillary is filled with the running electrolyte (a buffer solution), and the ionic analytes are separated on the basis of the differences in their electrophoretic mobilities. The favorable ratio of surface area to volume allows the dissipation of the Joule heat from the capillary and the application of high electric fields with rapid and efficient separations. Also, the anticonvective characteristic of the capillary limits the process of zone diffusion, maintaining the efficiency of separation without the need of further anticonvective media such as gels. 

The increase in temperature during a separation is observed to be due to Joule heating Ideally, the retention factor, k, should be independent of the applied voltage or current, but, actually, k does depend on the applied voltage. Although the temperature of the separation capillary is controlled with a Uquid coolant or a drculating airstream in most commercially available capillary electrophoresis (CE) instruments, the retention factor usually deaeases, almost linearly, with an increase in the velocity of the EOF or an increase in the current. However, the dependence is much less than that observed in a CE system without forced-cooling apparatus. 

Capillary electrophoretic methods including open-column zone electrophoresis, disc electrophoresis in gels, isotachophoresis and isoelectric focusing have received considerable attention from the analytical community over the last three or four years (80, 81, 82). In capillary zone electrophoresis (CZE), nanogram quantities of sample are placed in a silica capillary, 50 to 300 miaons in diameter and 50 to 100 cm long. Since the small dimensions of the capillary allow for efficient removal of Joule heat, electrical fields up to 350 V/cm can be applied. Under the influence of the field, sample components separate by zone electrophoresis while they are carried downstream by electro-osmosis. 

---