Dynamic aspects of electrophoresis and electroosmosis: A new fast method for measuring particle mobilities

Measuring the de electrophoretic mobility in a microelectrophoretic setup is traditionally performed at the stationary levels where the electroosmotic flow due to charge on the capillary wall is absent. Small deviations in the measuring position can give significant errors in the measured mobility, especially if the cell wall is highly charged. An electroosmotic streaming profile does not arise instantaneously upon switching on a field, but develops within a characteristic time, typically of the order of 1 s in a common capillary of 1 mm radius. From a theoretical study of the most pertinent relaxation times in electrophoresis and electroosmosis it is shown that, under the usual microelectrophoretic conditions, a colloidal particle responds much faster to an applied electric field than does the liquid inside the capillary. Therefore, It is possible to apply an alternating electric field with such a frequency that electroosmosis is suppressed, whereas the particles are still able to follow the field according to their de mobility. Measurements were performed with a commercial laser-Doppler microelectrophoretic device in an alternating electric field as a function of frequency and position in order to demonstrate this principle. This new method for measuring particle mobilities overcomes the problems associated with electroosmosis, e.g., finding the stationary levels. (C) 1997 Academic Press.