low resistance grounding bridges in patch clamp

Miledi Lab qnguyen at darwin.bio.uci.edu
Sun Apr 30 20:34:31 EST 1995

In article <3nltvg$83n at saba.info.ucla.edu>,
   chrisdn at physci.lifesci.ucla.edu (christopher del negro) wrote:
>has anyone had trouble eliminating noise (line frequency) from patch
>clamp rigs attributable to faulty grounding with an agar bridge electrode?
>i have had line frequency problems for about 2 months and have tried many
>forms of grounding and only one was successful in eliminating line freq. 
>noise: a simple Ag/AgCl pellet (low res.) in the chamber. it seemed all 
>agar bridges were too high resistance and allowed noise to enter the set-
>up. i have recently constructed large diameter bridges which are a bit
>unwieldy in a small chamber such as mine but which apparently have low  
>enough resistance to sufficiently ground the preparation and worked well 
>for two experiments so far. has anyone else had such a problem?

	Dear Mr. Del Negro,

My suggestions could sound quite obvious to you, but when I make agar bridge
I try to make them as short and as wide as possible to reduce the series
resistance; they are made with heat-shaped, thin-walled glass tubes (you
can get almost a 2mm inner diameter with thin-walled glass tubes). Another
trick involves using 1M or 3M KCl to make the agar solution. You can also
insert a piece of Ag/AgCl wire inside your bridge to lower the overall
resistance. Under most circumstances, the resistance of your bridge should
be around a few kOhms, or less if you are careful. Therefore, unless your
briges are really long, I am not too sure if the interference problems can
be solely attributed to the bridges. Do you have long perfusion lines? They
can be the source of many problems, especially if the material used in them
is good as picking RF interference (a good brand to use is Tygon). If these
problems persist, I would suggest to shield your set-up in a Faraday cage,
front included (this is a nuisance, of course), or to move it in the room
away from potential RF sources, especially if your set-up is close to a good
RF source (elevator, computer screen, etc...).

	Hope this helps,

Quoc Thang NGUYEN
Laboratory of Cellular and Molecular Neurobiology
Dept. of Psychobiology
University of California, Irvine
Irvine CA92717 USA
Tel: (714) 824-4730
Fax: (714) 824-3522

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