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Open circuit ground multiplexing?

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thekeystoneclimber:
Hi all.  I am experimenting with electrospray ionization.  The setup has a 4kV power supply connected to an electrode that is separated from a return lug (or ground plate) via an air gap, followed by an inline pico-ammeter and finally ground.  The way this works is that a small drop of solvent is applied to the electrode, the 4kV power supply is energized, and the solvent forms an aerosol electrospray "Taylor cone" between the electrode and return lug.  When this is occurring (it is not especially visible except via laser light) the inline pico-ammeter reads somewhere on the order of 20-100 nA resulting from the ion transfer. 

I would like to test a very large number of electrodes by systematically switching the pico-ammeter & ground between the various return lugs.  My first thought was to use a multiplexer, but I'm not sure if this is even possible, and I have a few concerns.  Specifically, iIs there a mux that can switch a ground reference (through the pico-ammeter) to an open (high impedance) circuit, and has a low enough on resistance as to not substantially effect the 20nA ion transfer current?

Also, every so often the air gap gets a little too small and turns into a 4kV "spark gap" which results in having to change a fuse in the pico-ammeter.  What would be the best practices with regards to circuit protection for preventing this scenario from frying the switching circuitry?

Thanks in advance!

Kleinstein:
20 nA is not yet a very small current, so one could get away without really special parts.

One possibility would be having a small capacitor (e.g. some 10 nF) for each electrode, a mux to select the electrode and a suitable amplifier to read the charge signals one at a time and reset the capacitor to zero. With large enough caps the voltage will not rise very high and should not effect the electric field too much.
The usual MUX chips (e.g. CD4051) are good for 8 inputs and maybe a few MUX could be combined to a single charge amplifier.
Likely this would need something like a small µC for the control and ADC.

For protection one could use clamping diodes and maybe extra resistors. 20 nA is still high enough to use diodes without much extra effort.
One may need external diodes and the MUX internal diode toward the supply only as a second stage.

duak:
Do I understand correctly that the multiplexed target electrodes' voltages are individually varied to selectively attract or not attract ions from the emitter electrode?



thekeystoneclimber:
Individually switched to ground.

duak:
OK then, so when the MUX channel is off, the electrode will pick up a charge from the ions and its voltage will float toward the emitter electrode's voltage thus reducing the flow of solvent?  What sort of speed is needed?

I can't think of too many semiconductor devices that would have the low leakage current and capacitance needed and yet stand off 4 KVDC.  Thermionic devices might but they are limited to electrons being the current carriers.

Wasn't something done with video camera tubes eg., vidicons and image orthicons that could read electrical charges on a surface with a scanning electron beam?

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