Hello.
What are the possible causes for a very high 'time constant' when using a Bio-Rad Gene Pulser to electroporate cells? I'm getting values around 11-13 with cells or even when only sorbitol is added to the cuvette. I know that the time constant should be around 5-6.
My settings are: 1.5 kV, 25 μF, 2mm gap cuvettes, resistance high range 600 ohms.
Help is appreciated. Thank you.
The time constant of the discharge of a capacitor through a resistance is the product of the resistance (ohms) time the capacity of the capacitor. In your case, the capacitor of the electroporator is connected in parallel to the cuvette and to the external resistance. Theoretically, if the cuvette had no conductivity, i.e. infinite resistance, the total resistance through which the capacitor (25 µF) is discharged would be that of the external resistance (600 ohm). thjus, the maximal time constant would be 600 ohm x 25 µF or 15 ms. Since the conductivity of the cuvette is not quite zero, the total resistance is somewhat lower, hence the values of 11-13 ms. If you want to decrease the time constant to 5-6, you should choose a smaller external resistance, e.g. 300 ohms or a smaller capacity, e.g. 12.5 µF
Hi Pedro Gonçalves
We had a similar problem a few months ago and we spent a lot of time until we noticed that a part that connect the circuit when you close the lid of the electroporation device was broken. To check if the machine is working properly, try to measure the resistance of a salt solution (i.e. 4M NaCl). If you obtain the same value than with an empty cuvette, shake the electroporation device and if you hear a piece moving inside you can suspect that the circuit is broken.
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