Ask the Expert - Answer by Dr. Nadine Mellies, Application Specialist Manual Liquid Handling at Eppendorf
Pipetting different types of liquids is a challenge in cell culture: cell culture medium tends to foam due to the high protein content, DMSO used as cryoprotectant and solvents for drugs or inhibitors remains somehow in the tip due to its viscosity, and ethanol used as a solvent for any lyophilized components dips out of the pipette tip after aspirating due to its high vapor pressure. All these physical properties of the used liquids have their impact on the outcome of cell culture experiments: foaming is a potential source of contamination or worse cell attachment and inaccurate pipetting of solvents results in wrong concentrations of reagents. Here, positive displacement pipettes help to overcome the challenges derived from the physical properties of the used liquids. As the liquid is soaked into a tip with an integrated piston, the liquid and the piston are in direct contact without any air cushion in between, thus avoiding the formation of air bubbles during cell seeding. Foaming liquids get not in contact with the interior of the pipette reducing the risk of contamination derived from aerosol formation. This system also allows the accurate transfer of most liquids, no matter if they are just aqueous, have a high density or high vapor pressure. So, positive displacement pipettes are less sensible than the often-used air-cushion pipettes and allow a more precise, accurate and convenient pipetting of problematic liquids.