Precise and Repeatable Pipetting of Problem 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 only have an air cushion pipette at hand.

Air Cushion Principle vs. Positive Displacement Principle

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 system working along the positive displacement principle the piston of the tip is in direct contact with the liquid without an air cushion. Contamination of the pipette is prohibited. Eppendorf offers this principle in the Multipette® (Northamerica: Repeater®), Biomaster®, Varipette® (North America: Maxipettor®).

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Viscosity

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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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Density

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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

Video

Vapour Pressure

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  • 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.

Video

Hazardous

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  • 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

Video

Detergent Containing

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  • 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

Video

Foaming

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  • 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.