Precise and Repeatable Pipetting of Challenging Liquids

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 should only have an air-cushion pipette at hand.

Air-Cushion Principle vs. Positive Displacement Principle

Air-Cushion Principle

Air-cushion pipettes are most commonly used in labs around the world and ideal for liquids with physical properties similar to water. In this instrument type, the piston is separated from the liquid sample by a small air cushion. The air cushion will expand or shrink when humidity or temperature changes. This influences the pipetted liquid volume and may affect the performance of air-cushion instruments. To reduce these risks however, Eppendorf air-cushion pipettes (Research plus®, Reference®2, Xplorer® and epMotion®) work with extra small air cushions. Furthermore, they may easily be readjusted to special liquids with a temporary adjustment. Changing back to the manufacturer adjustment is always possible in less than a minute.

Positive Displacement Principle

In positive displacement systems, the piston is part of the tip and in direct contact with the liquid. There is no air cushion that may be affected by liquid sample properties. These tools are therefore ideal for liquids with varying viscosity, volatility, surface tension or density as well as hot or cold liquids. The disposable tips with integrated pistons (e.g. Combitips advanced) also prevent contamination and help to keep user and instrument safe when working with hazardous liquids. Eppendorf offers this principle in the Multipette® (Repeater®), Biomaster®, Varipette® (North America: Maxipettor®).

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Viscous Liquids

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  • Problem: Viscous liquids have a high resistance to flow (e.g. glycerol).
  • Observation: Unknown time until liquid has fully risen in tip, liquid residues stay attached to tip wall.
  • Prevention air-cushion pipette: Work slowly, reverse pipetting
  • Recommendation positive displacement: Sealing lip of piston wipes tip clean
     

Application Note 386

Application Note 211

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Dense Liquids

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  • Problem: The liquid’s density influences the size of air cushion (e.g. ethanol).
  • Observation: Too low / too high volume is pipetted.
  • Prevention air-cushion pipette: Adjust pipette to liquid (becomes fixed-volume pipette for adjusted volume).
  • Recommendation positive displacement: No air cushion therefore no problem.

Application Note 242

User Adjustement: Reference 2

User Adjustement: Research Plus

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Volatile Liquids

  • Problem: Liquids with high vapour pressure force air cushion to expand (e.g. acetone).
  • Observation: The pipette drips.
  • Prevention air-cushion pipette: Prewet at least 5 times, reverse pipetting (better accuracy, but dripping still occurs).
  • Recommendation positive displacement: No air cushion therefore no problem.

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Hazardous Liquids

  • Problem: Aerosols may contaminate the pipette.
  • Prevention air-cushion pipette: Use filter tips with high efficiency (e.g. ep  Dualfilter T.I.P.S.®).
  • Recommendation positive displacement: Liquids are safely enclosed in  dispenser tip due to sealing lip of the piston.

Application Note 327

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Detergent Containing Liquids

  • Problem: Detergents lower the surface tension of water. Liquid residues stay in tip.
  • Prevention air-cushion pipette: Use tips with low retention effect (e.g. epT.I.P.S.® LoRetention).
  • Recommendation positive displacement: Sealing lip of the piston wipes tip clean.

Applicatipon Note 218

Application Note 192

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Foaming Liquids

  • Problem: Liquid foams when moved. It is difficult to pipette sample accurately.
  • Prevention air-cushion pipette: Reverse pipetting.
  • Recommendation positive displacement: Reverse and residual stroke leave room for foaming, Sealing lip of the piston moves tip clean.

 

Summary & Further Reading