Dear Dr. Luigi Marongiu

0.2μm filters are specifically designed to remove bacteria. Your failure to get a clear filtrate could indicate that there may be an issue with the filtration technique. (See below).

You may use the protocol provided below.

What type of defects can be expected?

A. The filter itself could have defects, such as pinhole leaks, that allow bacteria to pass through.

B. Excessive pressure during vacuum filtration can compromise the filter's structure and lead to cracks or ruptures.

C. The filter membrane may not be properly or completely wet, which can interfere with its ability to retain particles.

D. An incomplete seal in the filtration assembly can create a bypass, allowing unfiltered suspension to mix with the filtrate.

E. O-rings, tubing, or flask connections may be faulty, leading to leaks.

In case you encounter such problems, a) perform an integrity test on your filter, such as a bubble point test, to verify it is not defective; b) ensure that pressure and temperature conditions are within the manufacturer's specified limits; c) visually inspect all components of your setup for signs of wear or damage; d) test for leaks by applying soapy water or using an ultrasonic leak detector, depending on the scale and complexity of your equipment.

Regards,

Malcolm Nobre

It is always best to remove as much of the cell debris (and intact cells) as possible by centrifugation before applying your sample to the filter. It is fine if you only have a few cells but if there is visible turbidity in your sample then you will undoubtedly clog the filter very quickly. For small volumes I usually spin for one minute at top speed in a normal microcentrifuge.

Alternatively you can pre-filter through a much courser grade filter to remove most of the debris before using the 0.22 micron filter. It depends on the volumes you are trying to deal with and if you have large volumes this might be easier.