5.5 Choosing run parameters
EPS 601 is an automatic cross-over power supply that allows the user to set limits for
voltage, current and power. During electrophoresis, only one of the parameters is limiting
at a time. The limiting parameter determines, together with the conductivity in the
electrophoresis system, the values for the other two parameters. Voltage, current, power
and conductivity are related by the following equations:
U = I / L
(1)
P = U × I
(2)
Where U = Voltage, I = Current, P = Power and L = Conductivity
Equation (1) is more familiar if the conductivity is replaced by the reciprocal resistance (R):
U = R × I
(Ohm’s law)
The electric field E, measured in V/cm, is the driving force behind electrophoresis.
E = U / d
where E = Electrical field strength, U = Voltage, d = distance
The electrical field strength is achieved by applying a voltage. The higher the voltage, the
faster the electrophoresis. Fast electrophoresis is beneficial since it counteracts diffusion.
The disadvantage of increasing the voltage too much is that most of the generated
electrical energy, the product of power and time, is transformed to heat. Therefore
cooling of electrophoresis equipment is recommended. Cooling will also reduce “smiling”
effects which are caused by mobility differentials across an electrophoresis gel resulting
from poor heat transfer. Since the cooling efficiency cannot be increased indefinitely, the
power should be limited when programming the power supply.
The parameter that should be chosen as the constant and thus control the
electrophoresis depends on the type of electrophoresis. In the case of homogeneous
buffers throughout the system (same electrode and gel buffer), the conductivity is
constant during the electrophoresis. If the conductivity is constant, the voltage will be
proportional to the current and the power to the square of the current, according to (1)
and (2). This means that the result of the electrophoresis will be the same, regardless
which parameter is chosen for constant. For historical and practical reasons, voltage
is most commonly used for regulation. Submarine gel electrophoresis of DNA/RNA
and pulsed field electrophoresis are usually run at constant voltage. SDS-PAGE using
continuous buffer systems is run at constant voltage or current.
20
Electrophoresis Power Supply EPS 601 User Manual 18-1130-19 Edition AD
5 Operation
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