HEKA Elektronik EPC 10 USB, Hardware Manual, Page: 53 / 81

7.3 Offset Compensation 47
Note: When applying the above rules for calculating the correction LJ, two sign inversions of the liquid
junction potential are effective for the standard liquid junction potential correction. First, the liquid
junction potential that was present during the reference measurement disappears during the experiment
(after seal formation). Second, according to Barry & Lynch, the potentials are defined with opposite
polarity as those for patch clamp experiments (bath vs. electrode instead of electrode vs. bath). Thus,
values in the table can be taken as they are and entered a such in the LJ control. If however, a liquid
junction potential appears during a measurement (e.g., during solution changes), then only one sign
inversion applies. In that case, the sign of the value in the table must be inverted before adding it to
the ”Correction Sum”.
In the following, some specific examples together with explanations will be given. In all these cases it is assumed
that the reference measurement is performed in standard saline.
Example 1: An outside out or whole cell measurement with normal saline in the pipette. In this case, LJ should be
set to zero. This is one of the few measurements which do not require any correction. It is quite unphysiological,
however.
Example 2: An outside out or whole cell measurement with KCl-based internal solution in the pipette. LJ should
be set to 3 mV (see table) in order to correct for the disappearance of a liquid junction potential between the KCl
containing pipette and the NaCl-based bath solution.
Example 3: An episode with low-chloride bath solution during the experiment of example 2. It is assumed that the
reference electrode in the bath includes a salt bridge such that the change in
Cl
−
concentration is not ”seen” by
the Ag-AgCl-wire. Nevertheless, a liquid junction potential will develop at the bath/salt-bridge interface, unless
a ”bleeding” KCl-bridge is used (see Neher, 1992). Similarly, a liquid junction potential will develop during local
microperfusion. Thus, the correction during the episode in low-chloride medium will be the sum of this liquid
junction potential and the correction of Example 2 (3 mV). Taking the value for a low
Cl
−
solution (e.g., sulfate
Ringer; see table), we arrive at a value of LJ = 3 + (
−
6) =
−
3 mV , which should be set during that part of the
experiment.
Note: The sulfate Ringer in this case is -6 mV (the inverse of the value in the table), because this
potential ”appears” during the measurement with inverted polarity to the convention of Barry & Lynch.
Example 4: An outside out or whole cell measurement with Cs-citrate-based internal solution. In this case, LJ
should be set to 12 mV (see table above).
Example 5: A cell-attached measurement with sulfate Ringer in the pipette. Two corrections apply:
1. the correction for the liquid junction potential during the reference measurement (6 mV, see table above)
2. the resting potential of the cell. We assume the latter to be -60 mV and therefore set LJ to -54 mV
In the cell-attached mode polarities of the amplifier readout are inverted, thus the amplifier will display the
”physiological” patch potential.
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