Journal of Neuroscience Methods, 12 (1984) 91 91 Elsevier

NSM 00431

Letter to the Editor

DIGITAL ZERO IS NOT ZERO

Jean-Pierre Raynauld

Centre de recherche en sciences neurologiques, Facult~ de Universit~ de Montreal, Case postale 6128, succursale A, Quebec, H3C 3J7, Canada.

m~decine, Montreal,

Keywords: leakage current - digital zero - intracellular recording amplifier

Investigators using the new $7000 system of W-P Instruments Inc. (I) should be aware that, when using the panel digital voltmeter to make the zero leakage current adjustments, residual currents as large as ± 5 picoampere can exist at the input of the model 7071 electrometer when using the green probe.

According to the procedure recommended by the manufacturer, the leakage adjustment is done using an internal 108 ohm resistor in con- jonction with the digital voltmeter set at 200 mv full scale. Since the least significant digit represents ± 1 my, even when the display reads zero~ the voltage at the input of the analog to digital converter used in the voltmeter can be anywhere between -0.5 mv to + 0.5 my. The leakage current associated to this voltage will be ± 5 X 10 -12 amp.

If the leakage current happens to be -5 X 10 -12 amp and the inves- tigator is using a 3 M KCL filled pipette, chloride will be injected at such a rate as to change the internal concentration of a cell 20 microns in diameter by 33% in less than 3 minutes, given the initial internal chloride concentration to be an estimated 3 mM.

To circumvent this problem, it is suggested to use an oscilloscope at a sensitivity high enough to insure that the current is adjusted at the minumum acceptable level. Another alternative is to raise the value of the internal resistor (located in a shielded box on the back of the front panel of the 7071 module) to 109 ohms. Still better, do

both.

Reference:

I) W-P Instruments, Inc., 375 Quinnipiac Avenue, New Haven, Connecti- cut 06513, U.S.A.

(Received August 31st, 1984) (Accepted September 12th, 1984)