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Process for Selecting a Filter
1. Determine the particle size to be retained or filtered. Membranes will retain all particles equal to, and larger than their designated pore size.
2. Assess the chemical compatibility of the membrane filter with the liquid or gas to be filtered. Consider the chemical resistance properties of all the parts that will contact the filtrate.
3. Depending on the procedure performed, the membrane color or surface pattern may be important.
4. Also consider hydrophilic or hydrophobic membranes, temperature, flow rate, throughput, and sterilization needs
Types of Membrane Materials
Cellulose Nitrate and Cellulose Acetate
Cellulose Nitrate (CN) membrane is the most popular membrane used in analytical and laboratory filtration. CN membrane has excellent wetting properties and gives fastest flow rates with aqueous solutions.
Cellulose Acetate membrane is a mixture of cellulose triacetate and diacetate that creates a strong membrane in both lateral and longitudinal directions. In addition, the membrane has a low static charge, a very low aqueous extractability, and good solvent resistance to low molecular weight alcohols.
Mixed Cellulose Ester
Mixed Cellulose Ester membrane provides a more uniform and smoother surface compared to pure nitrocellulose membrane. This membrane is typically used to count or analyze particles contained in liquids or captured from aerosols.
Nylon
Nylon membrane is strong, inherently hydrophilic, and compatible with a broad range of aqueous solutions including alcohols and solvents used in HPLC work.
Polyethersulfone (PES) and PTFE
Polyethersulfone (PES) membrane is hydrophilic and low protein binding. No external wetting agents are required, resulting in low extractables. PES membrane generally offers fast flow rate and better chemical resistance than cellulose acetate membranes.
PTFE membrane is strong, highly porous, and inert to most chemically aggressive solvents, strong acids, and bases. Chemical and thermal limitations are imposed by the backing material.