Quoi de neuf, doc ?

Electrode calibration.

Once the electrodes are filled, the next thing is to calibrate them. For this we need a difference amplifier. That means either a double channel amplifier with the ability to substract channel 1 from channel 2, or two separate amplifiers with the correct specifications. The channel / amplifier for the "ionic" electrode should be of the "high impedance" type, such as those made for patch-clamp. On the other hand, the channel / amplifier for the standard electrode should just be ... standard i.e. any amplifier for extracellular recordings. The signals issuing from the amps have to be combined. This can be done through an analogic device. Alternatively, the amplifiers outputs could feed a computer which will make the substraction between channel/amplifier 1 and 2 i.e. (ion + voltage) minus (voltage alone). Of course, the two channels should be compensated for their individual capacitance before making the difference. For this, the injection of a voltage signal into the solution could be used.

The actual setup for the calibrationn of the electrodes is in fact a simplified recording setup. The photo shows that the probe holding the electrode and the vessel for changing solutions are protected by a Faraday cage. This is necessary due to the very high impedance of the "ionic" tip which then catch any interference caused by movements of static charges in the vicinity. The difference amplifier (on top of the cage) outputs are fed through a chart recorder (on the left).

Example of an electrode response to a change in solution
Upper trace : "ionic" side. Lower trace : reference side.

The solutions used for the calibration should have a range of concentrations which includes the one to measure. The calibration curve will be necessary to convert the recorded signals to concentrations.

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