Bio-Rad Gene Pulser Xcell™ Electroporation Systems - Manual (Page 52)

The decrease in voltage that occurs with a square wave pulse is inversely related to both the capaci-
tance of the instrument and the resistance of the sample. The fractional decrease in voltage at the end
of the pulse as a function of the initial voltage, V
0
, is termed droop and is given by the following equa-
tion:
Droop = (V
0
– V
t
) / V
0
.
Combining the two equations:
ln [1 / (1 – Droop)] = t / (R C).
In order for the pulse to most closely approximate a true square wave, droop must be minimized (i.e.,
V
t
= V
0
and V
0
- V
t
= 0). Experimentally, this is achieved by choosing the highest values for R and C.
For any given sample, R may be considered a constant. The Gene Pulser Xcell uses a 50 uF capacitor for
the high-voltage circuit and a 3275 uF capacitor for the low-voltage circuit. For each of these circuits,
C may also be considered a constant. Therefore, for the same sample, as pulse length increases,
droop also increases. However, increasing sample resistance reduces the droop at any given pulse
length. Increasing the sample resistance may be accomplished by (1) reducing the temperature of the
sample; (2) reducing the ionic concentration of the solution; and (3) reducing the volume of liquid in the
electroporation cuvette in the case of low resistance media. Never use the PC Module when performing
square wave pulsing because attaching a resistor from the PC Module in parallel with the sample will
result in increasing the droop of the pulse. Table 4.1 gives the pulse lengths corresponding to a given
droop as a function of resistance in the high voltage and low voltage ranges. For example, pulsing into
a 200-ohm load, the pulse length will be 0.510 msec with a 5% droop in the high voltage range and
33.4 msec with a 5% droop in the low voltage range.
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