In capillary electrophoresis we inject the sample into a buffered solution retained within a capillary tube. When an electric field is applied across the capillary tube, the sample’s components migrate as the result of two types of action: electrophoretic mobility, νep, and electroosmotic mobility, νeof. Electrophoretic mobility is the solute’s response to the applied electrical field. The other contribution to a solute’s migration is electroosmotic flow, which occurs when the buffer moves through the capillary in response to the applied electrical field.
As shown here, a solute’s total mobility, νtot, is a sum of its electrophoretic mobility and its electroosmotic mobility.
Cations elute first because they have a positive electrophoretic velocity, νep, with small, highly charged cations eluting before larger cations of lower charge. Anions elute last because their negative electrophoretic velocity partially offsets the electroosmotic flow velocity; smaller, highly charged anions having the longest elution time. All neutral species elute together with a velocity equal to the electroosmotic flow.
