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Air vs. Positive Displacement Pipettes
Choose the best option, from pipette to tip
Both pipette types have a piston that moves in a cylinder or capillary. In air displacement pipettes, a certain volume of air remains between the piston and the liquid. In positive displacement pipetting, the piston is in direct contact with the liquid.
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Air displacement pipettes
Air displacement pipetting is highly accurate for standard pipetting applications. However, temperature and atmospheric pressure, as well as the specific gravity and viscosity of the solution, may affect the performance of air displacement pipettes.
Aspirating the liquid (steps 1–3)
- The piston moves to the appropriate position when the volume is set.
- When the operating button is pressed to the first stop, the piston expels the same volume of air as indicated on the volume setting.
- After immersing the tip into the liquid, the operating button is released. This creates a partial vacuum, and the specified volume of liquid is aspirated into the tip.
Dispensing the liquid (step 4)
- When the operating button is pressed to the first stop again, the air dispenses the liquid. To empty the tip completely, the operating button is pressed to the second stop (blow-out).
Positive displacement pipettes:
Positive displacement pipetting is based on direct contact of the piston with the liquid. The aspirated liquid amount depends on the dimensions of the cylinder or capillary and the movement distance of the piston. In positive displacement pipettes the tips contain both the cylinder/capillary and the piston.
- The piston moves down inside the tip to make direct contact with the sample
- The piston moves up to draw the sample into the tip.
- When dispensing, the piston descends and the selected volume is dispensed.
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