Performing electrophoresis in small-diameter capillaries allows the use of very high electric fields because the small capillaries efficientlydissipate the heat that is produced. Increasing the electric fields produces very efficient separations and reduces separation times.
Capillaries are typically of 50 µm inner diameter and 0.5 to 1 m in length. The applied potential is 20 to 30 kV. Due to electroosmotic flow, all sample components migrate towards the negative electrode. A small volume of sample (10 nL) is injected at the positive end of the capillary and the separated components are detected near the negative end of the capillary. CE detection is similar to detectors in HPLC, and include absorbance, fluorescence, electrochemical, and mass spectrometry.
The capillary can also be filled with a gel, which eliminates the electroosmotic flow. Separation is accomplished as in conventional gel electrophoresis but the capillary allows higher resolution, greater sensitivity, and on-line detection.
Schematic of capillary electrophoresis
The surface of the silicate glass capillary contains negatively-charged functional groups that attract positively-charged counterions. The positively-charged ions migrate towards the negative electrode and carry solvent molecules in the same direction. This overall solvent movement is called electroosmotic flow. During a separation, uncharged molecules move at the same velocity as the electroosmotic flow (with very little separation). Positively-charged ions move faster and negatively-charged ions move slower.
Schematic of the double layer on the capillary surface