Choosing the Right Filtration Products

 Using the correct laboratory filter for processing liquids is an important decision, but it does not have to be difficult or complicated. Filter membranes are available in a variety of materials, pore sizes and other specifications, but you can narrow the selection by asking just a few questions.

Why are you filtering?

There are many reasons why a filtration step may be part of your laboratory protocol. Most often, filtration is performed to remove something from the liquid, a function that is dependent on the membrane pore size.

• To remove Mycoplasma from a solution, use a 0.1μm pore filter
• Routine laboratory sterilization of media, buffers, biological fluids and gases requires a 0.2 or 0.22μm pore filter membrane
• Clarification or pre-filtration of solutions and solvents can be accomplished with a 0.45μm pore size
• Glass fiber filters can be used for water testing or as pre-filters (to improve subsequent filter performance) 

• Glass fiber filters can be used for water testing or as pre-filters (to improve subsequent filter performance)

What are you filtering?

The chemical or biological characteristics of the liquid will also affect the choice of filters.

• Some membranes contain wetting agents or other non-toxic substances that can interfere with results
• The membrane material may bind proteins, nucleotides or other macromolecules that can clog the filter, remove important constituents or otherwise compromise the sample
• Not all filter membranes are compatible with all chemicals, so confirm that the filter that you choose will work with your solvent or solution 
• The temperature and length of exposure to the chemical may also affect the membrane’s integrity

How are you filtering?

The filter choice will also depend on the volume of liquid to be filtered and the method or device that is being used. Filter membranes are available in a variety of formats:

• Flat or disc filters
• Syringe filters
• Filter/storage devices
• Vacuum filters
• Bottle-top filters
• Centrifuge or microfuge tube filters

If the membrane is part of a filtration device, confirm that the housing material is also compatible with the liquid being filtered.

Membrane Material	Physical Characteristics	Chemical Compatibility	Biological Compatibility	Applications
Cellulose Nitrate	Strong and flexible Hydrophilic Autoclavable Limited pore size range	Low extractables Limited resistance to organic solvents	High protein binding	Protein or RNA transfer Sample preparation Microbiological and sterility studies Food, beverage and water testing
Nylon (Polyamide)	Hydrophilic Autoclavable	Very low extractables Moderately stable with esters and alcohols Resistant to organic solvents Suitable for high pH samples	Binds proteins	Media sterilization Solvent filtration and degassing Aqueous filtration Organic solvent filtration
PES	Hydrophilic Autoclavable	Low extractables; broad range solvent compatibility	Low protein binding	Media sterilization Solvent filtration and degassing General lab filtration Aqueous and compatible organic solvents
Polypropylene	Strong and flexible Hydrophobic Autoclavable	Low extractables; broad range solvent compatibility	-	Media sterilization Solvent filtration (HPLC) Vacuum degassing (acetonitrile) Biological solution filtration
PTFE (laminated to PP or PE)	Easy to handle Hydrophobic Wide range of pore sizes	Low extractables Resistant to a wide range of organic solvents	-	Solvent and gas filtration Sterile air venting Organic solvent filtration Non-aqueous sample filtration
PVDF	Hydrophilic	Resistant to a broad range of organic solvents	Low protein binding	-
Cellulose Acetate	Hydrophilic Autoclavable (when wet)	Limited solvent resistance Resistant to alcohol and oil Compatible (short-term) with MEK	Very low protein binding	Cold sterilization of proteinaceous solutions
Glass Microfiber	Hydrophilic	Compatible with organic solvents Compatible with strong acids and bases (not HF)	-	Air or water monitoring Filter or pre-filter