Pipettes working with an air-cushion principle are most commonly used in labs around the world and highly accurate for most pipetting applications. However, the precision and repeatability of the pipetting results can be affected when working with liquids of different viscosity, volatility, surface tension or density than water. In these cases and also when the liquids are foaming, positive displacement pipettes simplify the pipetting process and secure accurate results. In addition, positive displacement pipettes eliminate the risk of cross-contamination and protect the end user and the instrument from hazardous  liquids. Read in the following how to cope with these liquids, even if you only have an air cushion pipette at hand.

Air Cushion Principle

Common pipettes work with the air cushion principle. An air cushion separates the piston from the liquid. The air cushion will expand or shrink when humidity or temperature changes. This influences the pipetted liquid volume. Eppendorf air cushion pipettes (Research plus^®, Reference^®2, Xplorer^® and epMotion^®) have especially small air cushions to reduce the error risk. In addition, they provide the possibility to easily readjust to special liquids with a separate user adjustment. Change back to manufacturer adjustment is possible in less than a minute.

Positive displacement principle

In pipetting systems working along the positive displacement principle, the piston is inside the tip, similar to a syringe. The piston is in direct contact with the liquid, without an air cushion. The pipetted sample volume cannot be influenced by changes of the air cushion volume. Sticky samples will be pushed out of the tip completely by the integrated piston. The pipette itself is protected against hazardous samples. Eppendorf offers this principle in the Multipette^® (Northamerica: Repeater^®), Biomaster^®, Varipette^® (North America: Maxipettor^®).

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