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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.

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Gilson 215 Manuale Utente Pagina 111