Bio-Rad MicroPulser™ Electroporator - Manual (Page 12)

Fig. 4. Resistance of solutions of (A) NaCl and MgCl2 and of (B) buffers of NaPO4 at pH 6.1 and 7.3
and HEPES at pH 7.5. Resistance was measured in 0.2 cm cuvettes containing either 40 µl or 200 µl of
solution at room temperature.
Figures 4A and B show the effect of concentration of several biologically important ionic
solutions on sample resistance. Note that: (1) volume has a significant effect on sample
resistance—for ionic solutions, sample resistance is inversely proportional to the volume of
solution in the cuvette; (2) the resistance of a solution containing divalent ions is lower than
a solution containing the same concentration of monovalent ions; (3) the resistance of a
buffered solution is affected by its pH.
The addition of even small concentrations of ionic compounds significantly reduces the
resistance of the sample and may cause arcing. Residual salt from ethanol precipitation of
DNA should be reduced by washing the DNA pellet prior to dissolving it in either water or
Tris-EDTA. Table 1 shows that, although adding a solution of plasmid in 10 mM Tris,
pH 8.0–1 mM EDTA to water does reduce the sample resistance, this should not result in the
inability to electroporate a sample in the MicroPulser. DNA may be used directly from enzyme
reactions for transformation, but the final salt concentration in the electroporation sample
should be kept below ~5 meq for high voltage operation. Finally, ligation mixtures may be
used for transformation, but only in very low quantities or when the ionic strength is reduced
by dilution (Willson and Gough, 1988), dialysis (Heery and Dunican, 1989; Jacobs et al.,
1990), or ethanol precipitation (Böttger, 1988; Zabarovsky and Winberg, 1990).
Table 1. Resistance of Water in 0.2 cm Cuvettes To Which TE
Has Been Added
1 .
SAMPLE R
sample
R
sample
(40 µl volume) (200 µl volume)
Water > 6 x 10 5 > 6 x 10 5
Water + 1 µl TE > 6 x 10 5 35,000
Water + 5 µ TE 11,200 8,700
Water + 10 µl TE 4,850 4,700
1 The resistance of 0.2 cm cuvettes containing either 40 or 200 µl water and the indicated
volume of TE (10 mM Tris, pH 8.0, 1 mM EDTA) was measured at 1000 V.
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