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How to Stop Dripping When Pipetting Volatile Liquids

10.05.2016

Who isn’t aware of acetone, methanol & co. starting to drip out of the pipette tip directly after aspiration? I guess every one of us has experienced this. I used to try to work as fast as possible. I placed the tubes very close to each other to avoid chemical loss and spillage, but even so drops of chemicals were faster. So, I had to admit that my pipetting was not accurate and I had to change my technique.

Classic pipettes start dripping when pipetting volatile liquids due to the air inside the pipette. This so-called air cushion exists between the sample liquid and the piston inside the pipette. As commonly known, air is flexible and adapts to external influences such as temperature and air pressure by expanding or compressing. Liquids are also subject to external influences and naturally evaporate as the humidity of air is lower. A volatile liquid evaporates more than water. During pipetting, it evaporates into the air cushion forcing the latter to expand and liquid is pressed out of the pipette tip … the pipette drips.
One technique to reduce or even stop the dripping is to achieve a higher percentage of humidity in the air cushion. This is done by pre-wetting the pipette tip and thereby saturating the air cushion. When using low volatile liquids such as 70 % Ethanol or 1 % acetone, aspirate and dispense the sample liquid a minimum of 3 times into the source vessel, before aspirating the sample volume you want to transfer. If the concentration of volatile liquid is higher, repeat these pre-wetting cycles 5-8 times. However, with very high concentrations such as 100 % methanol or chloroform, this won’t be enough. It’s best to use another type of pipette: a positive displacement pipette. These pipettes use tips with an integrated piston without an air cushion. The sample is in direct contact with the piston and there is no risk of dripping.
You can easily improve your accuracy when pipetting volatile liquids by choosing the correct technique or changing the tool you are using. Additionally, you will increase safety by avoiding spillage and simplify your workflow. If you want to learn more on correct pipetting, watch our Youtube video tutorial “How to pipette correctly”.