The effect of Air Pressure fluctuations inside the Pipette tip

The dead air pressure prior to aspiration will always equal the Lab's environmental air pressure.

• When liquid is aspirated into the pipette tip, the humidity of the dead air space will increase, how much is determined by many factors including temperature, the liquid, the volume of air and the volume of liquid.
• As humidity increases, the number of molecules in the dead air space will increase, thus the pressure will increase.
• To try and equalise with the environmental pressure, the volume of dead air space will increase. The volume of liquid may also decrease slightly as the air displacement above causes dripping below.

These changes may not be noticeable in some applications, but if you attempt to transfer a small volume of a highly volatile liquid (e.g. ethanol) the difference between target volume and actual volume will be greater. It should be noted that this can result in workflows being successful in one lab, but not in another where the environment is extremely different.

How to Mitigate:

Firstly, Include a pre-wet step every time a new tip is used as this will increase the humidity within the tip prior to introducing the aspiration volume. This means that there is less drive for the liquid volume to evaporate into the dead air space as the difference is narrower. Secondly, using a travelling air gap will help to prevent any dripping after the aspiration.

Minimising the dead air space by increasing the aspiration volume ensuring that the pipette tip certainly contains enough liquid at the dispense to reach the target volume. A smaller dead air space above the liquid will reduce the effects of air displacement on pipetting accuracy. There is also the added bonus that the entire system is less "soft". This means that when the mechanical pressure of SQWERTY is exerted on the dead air space, there is less volume movement than if the same amount of pressure was applied to a liquid. The air molecules will simply increase in pressure before the liquid sample beneath is forced to move. The greater the dead air space, the more this effect is exaggerated.

If you require more information please contact [e-mail] for assistance.