By applying an electric field across the membrane, charged analytes are transported by electrokinetic migration from the sample solution into the acceptor solution, while non-ionic analytes and ions of opposite charge are withheld in the sample solution. The membrane can be in the form of a hollow fiber, as in the initial constructions, or a flat sheet, as in more recent developments. With modern flat sheet membranes, the volume of organic solvent is in the range of 8–10 microliters. The membrane can be about 10 mm wide and 0.1–0.3 mm thick. The organic solvent is often 2-nitrophenyl octyl ether (NPOE) for basic analytes and higher alcohols (hexanol, heptanol) for acidic analytes. Diallyl phthalate and dodecanitrile have also been proposed as alternatives to NPOE.
The transport of ions generates a current, depending on the applied voltage and several other factors, such as pH, added ionic carriers and different membrane solvents. Too high current can result in electrolysis of water, with gas generation and pH changes. Thus, in order to obtain a successful extraction, the current should be kept in the low microampere area. In addition, due to the formation of a boundary layer at the membrane surface, slowing the transportation across the membrane, a shaking mechanism is usually included.