Appendix
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.
1Move probe to sample tube.
2Aspirate air gap.
3Lower probe to sample level.
4Aspirate small amount of sample - this is the excess sample volume.
5Lift probe.
6Aspirate air gap.
7Lower probe to sample level.
8Aspirate sample amount.
D
Pipetting Techniques