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Theory

Electrophoresis units are simple electrical circuits that work following Ohm's law:

V= IR

where V is electrical field measured in volts, V is proportional to current (I) measured in miliamps times resistance (R) measured in ohms. Given an amount of V, a constant amount of current flows through the elements of the circuit. The total applied voltage decreases across any element as its resistance increases. Resistance is inversely proportional to the cross sectional area of any element and to the buffer's ionic strength, so sample separation can be modified by varying those parameters. For instance, a decrease in gel thickness or buffer's ionic strength will increase resistance and thus the voltage gradient across the gel, increasing the mobility of the sample.

Electrophoresis technique is based on the property of charged molecules of migrating in an electrical field. Biological macromolecules, like proteins or nucleic acids, have a certain electrical charge due to groups capable of dissociating electrolytically. Those charged molecules will therefore migrate in an electrical field in a velocity and direction depending on their electrophoretic mobility.

Particles will migrate according to the voltage density ( V cm^-1) of the gel. The velocity of migration (cm sec^-1) of a particle at 1 V cm^-1 is called electrophoresis mobility and is expressed as cm^-2V^-1sec^-1, depending its sign on the net charge of the particle. Mobility is also influenced by molecular parameters like the electrical charge and frictional coefficient. This last parameter is dependent on the shape and molecular weight of the molecule.

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