A filter with a pore size of 0.2 or 0.22 μm is typically considered sterile, as it is effective at removing bacteria from a solution. These filters are often referred to as sterilizing-grade membranes.
Why 0.2/0.22 μm?
The 0.2/0.22 μm pore size is significant because it's smaller than the average size of most bacteria. Therefore, when a liquid is passed through such a filter, the bacteria are physically retained on the filter, resulting in a sterile filtrate.
Important Considerations:
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Filter Material: The material of the filter membrane also matters. Common materials include:
- Polyethersulfone (PES): Known for its high flow rates and low protein binding.
- Polyvinylidene fluoride (PVDF): Offers good chemical resistance.
- Cellulose Acetate: A common, cost-effective option.
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Application: While 0.2/0.22 μm filters are generally considered sterile for removing bacteria, they may not remove viruses or other very small microorganisms. For applications requiring the removal of viruses, filters with smaller pore sizes (e.g., ultrafilters) are necessary.
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Pre-filtration: Using a pre-filter with a larger pore size (e.g., 0.45 μm) before the 0.2/0.22 μm filter can help to remove larger particles and prevent clogging, extending the life of the sterilizing filter.
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Integrity Testing: It's crucial to perform integrity testing on filters before and after use to ensure they have not been compromised and are functioning correctly. Bubble point testing is a common method.
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Sterilization of the Filter Itself: The filter unit itself must be sterile before use. This is typically achieved through autoclaving or gamma irradiation.
Summary
In summary, a 0.2 or 0.22 μm filter is generally considered sterile for removing bacteria from liquids. However, it is important to consider the filter material, the specific application, the use of pre-filters, integrity testing, and proper sterilization of the filter unit to ensure effective sterilization.
