Test of AC Waveform Symmetry from the Power Source
The AC waveform provided by the power supply might
not be perfectly symmetric. It was noted that a few
hundredth of a voltage differential between the positive and
negative output from the power source (or a few percent of
the total area under the waveforms) could induce significant
errors in the present study. In the experiments of AC
frequency higher than 20 Hz, the voltage across HEM was
monitored by a DC voltmeter and a small DC offset was
applied if necessary to ensure that there was no net DC
across the HEM during AC iontophoresis. At lower AC
frequency, the voltmeter reading fluctuated between the two
AC polarities and monitoring the net DC across HEM during
AC iontophoresis with a DC voltmeter became difficult. In this
case, the leads of the power output were switched (reversed) in
the middle of selected iontophoresis experiments. The permeability
coefficient in the first part of the iontophoresis run
(before switching) was compared with that in the second part of
the iontophoresis run (after switching).