I am recently switching to WPI Duo-773 to measure extracellular diffusion of tetramethyl ammonium (TMA+, a small cation commonly used to measure the brain extracellular space) by the real-time TMA protocol of Nicholson & Phillips (1981). The TMA+ ion-selective microelectrode (ISM) was hand-made with two-barrel glass capillaries. The liquid ion resin I used is IE190 potassium ion resin from WPI (could be used to detecting TMA+ as well). The recording system was connected to the ground through a micropipette filled with 1M KCl in agarose. With the hand-made TMA+ ISM connected to the Duo-773, the amplifier detects the correct voltage response to logarithmic increases of standard TMA+ concentrations. However, when measuring the transient diffusion of TMA+ iontophoretically released through a nearby TMA+-filled glass pipette, Duo-773 got a flat signal (i.e., no voltage increases from the ISM barrel). I was pretty sure that the iontophoretic unit worked fine. So, I was guessing that it was a grounding problem (via the KCl- electrode). Does any know where I went wrong and how I could get the right voltage response during iontophoretic release of TMA+? Thanks in advance.
I can't comment on your application specifically, but I do know a bit about iontophoresis. Firstly, are you sure your iontophoretic electrode is close enough to your ion sensitive electrode? Iontophoresis typically only produces a very small volume where the ion is significantly enhanced.
Secondly, are you using any retaining current? The ion will diffuse out of the pipette if you don't have a retaining current (though too much retaining current is bad too).
Basically, you need a method to confirm your iontophoresis is working. If it was me, I would use Alexa Hydrazide, which I think is a positive ion at pH 7 (though you'd want to check that) and observe it under a fluorescent microscope. Though Rhodamine B might be cheaper. It is a positive species at pH 7.
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