Why there is air bubble when using syringe filter? How to avoid it?

The cause of the air bubble might be high pressure applied to the filter. If the pressure applied to the syringe is above the bubble point of the membrane and there is air in the syringe, air bubbles will be forced through the membrane in bulk flow compared to the normally low rate of diffusion. It might also occur at low pressure when using solvents with low surface tensions.

To avoid the air bubble issue for the syringe filter, filter a little slower and don’t push as hard on the plunger. You can also use a 0.45μm membrane if there is no need to use 0.2μm membrane. A larger diameter membrane will help to diffuse the sample over a larger surface area. This will help to prevent the air bubbles as well.