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Electrophoresis Power Supply EPS 3501 XL User Manual 18-1130-22 Edition AD
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5.6 Choosing run parameters
EPS 3501 XL is an automatic cross-over power supply that allows the user to set limits
for voltage, current and power. It is also possible to program linear voltage gradients.
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 x 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 x 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 of
which parameter is chosen as the 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.
For discontinuous buffer systems, the resistance will increase as the electrophoresis
proceeds due to a decrease in conductivity. Running at constant voltage will result in
decreasing current and power. Constant voltage will thus be “safe” in the respect that
the power will not increase and produce more and more heat. On the other hand, the
separation will slow down and impair resolution due to an increased time available for
diffusion. Running at constant power would give a faster electrophoresis and controlled
power, while running at constant current would, at the first sight, seem to be problematic
because of increasing voltage and power.
Operation 5
