For my protocols, I'm supposed to electroporate at:
2.5 kV
1000 ohms for resistance
25 uF for capacitance
However, some electroporators appear only to allow adjustment of the voltage. Does it matter? How can I get similar conditions for the resistance and capacitance?
Adjustable settings of capacitance and resistance are important for adjusting the duration of the pulse. Normally, the voltage between the electrodes V decreases exponentially from the set value Vo (2.5 kV in your case) with a time constant given by the product of resistance R times capatcitance C : V = Vo*exp-(R*C). Thus, a 1000-ohm resistance with a 25-µF capacitance will give you a time constant of 25 ms. For E. coli, the usual settings are rather 2 kV, 200 ohms, 25 µF, giving a time constant of 5 ms. However, in order to increase the time constant, it would be preferable to increase the capacitance, since the conductivity (i.e. the inverse of resistance) to be taken into account is the total conductivity composed of the internal conductivity of the electroporator plus the conductivity of the cell suspension lying between the electrodes. Assuming for example that the resistance of the cell suspension is 500 ohms, when coupled to a resistance of 1000 ohms, it will yield a final resistance of 333,3 ohms, i.e. a 3-fold decrease of the time constant; coupled with a with a 200-ohm resistance, the final resistance will be 142,8 ohms, i.e. a 28.5 % decrease only of the time constant.
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