6
Pre-Rinsing Recommended
Some solutions (e.g. serum, protein-containing solutions,
and organic solvents) can leave a film on the inside tip
wall, resulting in an error larger than the tolerance speci-
fied. Since this film remains relatively constant in succes-
sive pipettings with the same tip, excellent precision may
be obtained by refilling the tip and using the refilled vol-
ume as the sample. Successive samples from this same
tip will exhibit good reproducibility (low variance).
Reverse Mode Pipetting
Another way of reducing error due to film retention is re-
verse mode pipetting; the operating sequence is reversed:
1. Mount a disposable tip on the pipette shaft.
2. Press the pushbutton fully to the second stop.
3. Immerse the tip in liquid and return the plunger slowly
to the full up position. Wait a moment for the liquid
column to reach equilibrium in the tip.
4. Wipe any excess liquid from the outside of the tip
without touching the orifice.
5. To dispense, rest the end of the tip against the vessel
wall and press the plunger to the first stop. Hold this
position a few seconds, or long enough for the liquid
column to reach equilibrium again.
6. Remove the tip from the receiving vessel without
blowing out the remaining liquid.
7. Return excess sample in the tip to the original sample
container, if desired. Discard the used tip.
Pipetting Liquids of Varying Density
Pipetman lets you compensate for solutions of density
much different from water, by setting the volume slightly
higher or lower than that required. The compensation
amount must be determined empirically.
E.g., if pipetting 10 µL of CsCl solution, you determine
that the volume delivered is actually 8.5 µL (
≥
5 samples).
Try changing the volume setting to 11.8 µL and repeat the
measurements. If the volumes delivered are still not close
enough to 10 µL, make another slight volume adjustment
until the measurements are as desired.
Very dense liquids may not be suitable for air displace-
ment pipetting. Use Microman® positive displacement
pipettes for these liquids.
pipetman rev I 9/13/00 3:12 PM Page 8
