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D-3
Appendix D
Pipetting Techniques
Minimizing Dilution Effect
An undesirable dilution sometimes occurs when an
aspirated sample replaces rinse or diluent material.
Some of the diluent fluid invariably clings to the
tubing walls, causing the unwanted dilution of the
sample. The part of the sample that enters the
tubing first is affected most because it is the closest
to the diluting liquid.
The corrective action you take depends on the
stringency of your requirements for sample purity
and precision.
• You can aspirate an air gap into the probe tip to
separate different fluids passing through the
tubing to reduce mixing.
• You can aspirate an amount in excess of the
required sample volume, dispensing the excess
sample, which is most subject to being diluted,
to waste.
In extreme cases, you can combine the methods
listed above and perform the following routine to
eliminate mixing of different liquids.
1 Move probe to sample tube.
2 Aspirate air gap.
3 Lower probe to sample level.
4 Aspirate small amount of sample - this is the
excess sample volume.
5 Lift probe.
6 Aspirate air gap.
7 Lower probe to sample level.
8 Aspirate sample amount.