What is Vapour Pressure?

Answer

Vapour pressure is the pressure that the gaseous form of a molecule exerts on its liquid. Normally, water molecules at room temperature are constantly transitioning back and forth between liquid and gas form. This isn't usually noticeable, but when in a contained environment, if more molecules are evaporated than condensed, the vapour pressure increases which puts pressure on the liquid below. If the vapour pressure is great, this could cause dispensing inaccuracy and inconsistency.

Low humidity and boiling point increase the number of molecules evaporating, increasing the vapour pressure. High temperature and elevation (lower atmospheric pressure) can also cause these same effects.

How to mitigate

High vapour pressure can be mitigated against by:

• Allowing the liquid to equalise with the ambient conditions. Waiting for the liquid temperature to come to the room temperature (or bringing the room temperature closer to the liquid temperature). 
• pre-wetting the tip with the liquid. This allows the atmospheric pressure within the tip to move further towards an equilibrium with the vapour pressure, which reduces the rate of evaporation within the tip.
• Increasing the aspiration volume can have the same effect as using a pre-wet.
• The blowout should also be increased to allow a greater air space above the liquid level within the tip. This means that the liquid has more room to evaporate into and thus less pressure is exerted onto the contained liquid.

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