Capillary electrophoresis (CE) is a technique in which an electrophoretic separation takes place in a narrow-bore fused silica capillary.  The capillaries typically used in CE are commercially available at reasonable cost (about $4/meter.)  We use capillaries that range from 30 to 50 centimeters in length, 0.150 to 0.375 millimeters in outer diameter, and have a 0.010 to 0.075 millimeter diameter bore.
        In a CE separation,  the capillary is filled with buffer and each end is immersed in a vial of the same buffer. A sample of analyte is injected at one end, either by electrokinesis or by pressure, and a electric field of 100 to 700 volts/centimeter is applied across the capillary. As the analyte mixture migrates through the capillary due to the applied electric field (electrophoresis), differing electrophoretic mobilites drive each of the components into discrete bands. At the other end of the capillary each of the separated analytes is detected and quantified.
        Electrophoretic mobility is proportional to the charge of the molecule divided by its frictional coefficient. This is approximately equal to the charge to mass ratio of the molecule. So in general any molecules with differering charge to mass ratios can be separated by CE. 

      Imagine that the red and blue molecules you see in this diagram are two different proteins of approximately the same size and shape. Also imagine that in the pH of the buffer used in this capillary, the red protein has a net charge of +3 while the blue protein has a net charge of +1. At the right end of the capillary the two proteins are together immediately after injection (T1). After a 17 kilovolt field is applied across the capillary for some time (T2) the two proteins have separated. Notice that the red protein has migrated farther because it has a larger charge to mass ratio.