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4 PRE-RINSING
Whenpipetting liquids of higher viscosity oe lower surface tension than water (e.g. sera or organic
solvents), a film of liquid is formed on the inside wall of the pipette tip. This film can create an
error. Since the film remains relatively constant in successive pipetting operationswith the same
tip, this error can be avoided by forming the film before transferring the first sample. This is done
by aspirating a sample and dispensing it back into the same vessel. Since the film is already
formed, all of the following samples will have better accuracy and repeatability.
This pre-rinsing operation should be repeated when the volume to be aspirated is changed or
when a new tip is used.
5 DENSE AND VISCOUS LIQUIDS
The pipette specifications of accuracy and precision are based on pipetting distilled water. The
handling of liquids with physical qualities of density, viscosity and surface tension differing
extremely from water may need a gravimetrically checked compensation of the volume setting.
Normally the degree of error resulting from heavy or viscous liquids is negligible if the pipetting is
done slowly and carefully. It is most important to give the liquids some time to follow the change of
pressure by holding the pipette tip in its position for at least 2 sec. after the aspiration and the
blow out stroke.
If in extreme cases this method of operation does not result in accurate values, a compensation
could be achieved as follows :
Weigh the liquid pipetted when the pipette is set to the nominal volume.
Then calculate the set-off from the nominal volume :
Corr val
nom val
weight of liquid
density of liquid
.
.
.
.
2
Check this operation once again and correct if necessary. Note the corrected value for further
pipetting the same kind of liquid.
