Joule heating, capillary

The narrow bore of the capillary column and the relative thickness of the capillary s walls are important. When an electric field is applied to a capillary containing a conductive medium, such as a buffer solution, current flows through the capillary. This current leads to Joule heating, the extent of which is proportional to the capillary s radius and the magnitude of the electric field. Joule heating is a problem because it changes the buffer solution s viscosity, with the solution at the center of the... 

A high electric field inside the capillary generates heat. This so-called Joule heat can destroy the efficiency of separation. An electric field corresponds to a well defined current value for a specific electrolyte system. An ohm plot can be constructed for each electrolyte system by measuring the current at increasing electric fields. For an ideal system, the ohm plot is a... 

The main bottleneck in the further development of CEC is related with the state of the art of the column manufacturing processes and the robustness of the columns/instrumentation. Moreover, evidence to demonstrate reproducibility of separations from column to column still has to be established. The formation of bubbles in the capillaries due to the Joule heating and variations in EOF velocity on passing from the stationary phase through the frit and into the open tube is still very challenging in packed column CEC. A way to overcome this problem is to use monolithic columns or apply open tubular CEC [108]. Currently, many efforts are placed in improving column technology and in the development of chip-CEC [115] as an attractive option for lab-on-a-chip separations. 

As the name implies, CE separates sample components within the lumen of a narrow-bore capillary (20 to 150 pm) filled with a buffered electrolyte. High electric fields (hundreds of volts/centimeters in practice, but sometimes in excess of 1000 V/cm) can be used in CE because the capillary contains a small volume of electrolyte and a relatively large surface area to dissipate the heat generated by the electric current (Joule heat). High-voltage applications result in reduced analysis time and therefore less diffusion. 

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. 

Increase diameter of capillary Increased detector signal Increase of current and Joule heating effects, decrease in linearity of signal... 

CE instruments are thermostated to dissipate excessive Joule heat. Generally that covers only the main part of the capillary, and not, e.g., the autosampler with the buffer and sample vials. In some instruments, it is difficult to control the autosampler temperature due to the near presence of extraneous heating sources such as the detector lamp. Also, some labs... 

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],... 

The Joule heat generation in a packed capillary column is given by the following expression, similar to Equation 6.19 ... 

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