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Low-Noise Recording
EPC9 Manual
81
holder, and the combination of pipette immersion and patch noise. The table below
shows the relative contributions calculated in this way for the “optimum” situation
just described.
The contributions to the variance
from the three sources are seen to
be comparable in size, and impro-
vements in the amplifier noise
level will not help very much,
unless corresponding improvements are made in the other noise sources. As it is,
rms noise values as low as those quoted here are obtained only with considerable
care. Some of the important considerations are outlined below.
As we mentioned in
Chapter
8. Patch-Clamp Setup
on page 67
, the unshielded holder
is greatly superior to the shielded one for low background noise. For low noise, the
holder must be made from a low-loss, hydrophobic plastic; polycarbonate is one of
the best, and plexiglas one of the worst materials. (For our purposes, low-loss
materials are those that show little dielectric relaxation in the frequency range of a
few kHz. Dielectric relaxation involves the reorientation of dipoles within the
material; since any dipoles will be in thermal motion, thermal reorientations in this
frequency range will result in current fluctuations coupled capacitively into the
pipette.)
It is very important that the pipette holder be kept clean and dry. Noise can be
coupled into the pipette from the thermal motion of ions in films of aqueous
solution, especially on the inside of the pipette. A good practice for low-noise work,
is to connect a valve to the pipette-suction line, and arrange for dry air or nitrogen to
flow into the suction line during the time while you change pipettes. This will dry
out any such aqueous films and keep the noise level low.
Films of aqueous solutions and dielectric relaxation are also serious problems with
pipette glass. Coating with Sylgard helps because it is hydrophobic and because it
has good dielectric properties. Also, its thickness helps to reduce the capacitance
between the pipette interior and the bath. This is mainly important because it
reduces the coupling of the glass's dielectric noise into the pipette interior. Clearly,
making thicker coatings (especially in the tip region) and coating closer to the tip will
reduce the pipette noise. The best glass type we know of is aluminosilicate; this glass
requires fairly high temperatures in pulling, and does not necessarily give the best
gigaseals; but its dielectric relaxation appears to be about a order of magnitude
smaller than soft glass.
Some improvement is probably to be gained by taking steps to prevent formation of
aqueous films on the back end of the pipette. It is a good idea to wipe the outside of
Noise Source
Contribution
rms Current
Amplifier
35 %
95 fA
Holder
21 %
73 fA
P Patch
44 %
105 fA