Millipore^® Filter Membranes

Membrane Filter Characteristics

• Chemical Compatibility: The filter material must be compatible with the chemical nature of the substance being filtered to avoid structural failure. It is important to consider both the compatibility of liquid samples and dissolved solutes that can interact with the membrane.
• Wettability: For liquid filtration, the membrane must be wettable with the fluid being filtered. Hydrophilic membranes are easily wet with water and are preferred for filtering aqueous solutions. Hydrophobic membranes are recommended for gas filtration, low surface tension solvents, and venting, and can be wet in organic solvents such as methanol, allowing filtration of both aqueous liquids and organic solvents.
• Pore Size: Pore size provides an indication of the largest pore diameter and can be related to the membrane’s ability to filter out particles of a certain size. Bubble point and bacterial retention testing are two commonly used methods for measuring pore size.
• Diameter: The diameter, size, and shape of the filter are selected based on the apparatus used for filtration or sample collection.
• Flow Rate: Defined as the time required for the flow stream to pass through the filter, flow rate may be measured for air or liquid. Flow rate generally decreases with smaller pore size, but altering membrane material, thickness, porosity, and pore architecture can lead to differences in flow rate.
• Analyte Binding: Analyte binding refers to the loss of analytes during filtration, resulting in a filtrate with a different molecular composition than expected. Membranes with limited functionality (e.g. PVDF, PTFE) show very low analyte binding, whereas membranes with higher functionality (e.g. nylon, MCE) show a high level of analyte binding)
• Optical Properties: When visually analyzing retentates, membrane optical properties must be compatible with the imaging method, such that the membrane provides a consistent background over the entire sample surface and does not impart additional noise during testing.
• Extractables: Extractables are contaminants present in the final filtrate that originate in the filter or device. Filter extractables occur as three different types: shedding of filter materials or particulate extractables, residual chemicals from the manufacturing process, and surface modification chemistries washing off the filter. The presence of extractables can also be related to the chemical compatibility of the membrane with the solution being filtered. Generally, if a membrane is not chemically compatible with the solution, a higher level of extractables is observed in the filtrate.
• Retentiveness: Retentiveness is the ability of a membrane to retain the particle or molecule of interest. 

Prefiltration and Depth Filters

• Prefiltration: Prefiltration utilizes large pore membrane filters to remove large particulates, such as dirt or sediment, from samples prior to filtration with a smaller pore membrane filter. Using prefiltration in sample preparation can prevent premature filter clogging or fouling.
• Depth filters: Depth filters retain particles internally rather than on the filter surface. Due to their high particle retention capacity, depth filters are frequently used for prefiltration.
• Binders: Commonly used in non-woven, fiber-based materials, binders provide shape and strength to the final product. While binders are routinely used in glass fiber filters, these additives reduce thermal stability and can result in sample contamination by extractables.
• Net Filters: With large and uniform pores, the net-like structure of net filters is used to remove large particulates (e.g. cells, proteins, dirt) for solution clarification of particulate analysis.

How to Choose a Millipore^® Filter by Membrane Type

How to Choose a Millipore^® Filter by Volume

The diameter of the filter is selected based on sample volume and should fit the apparatus or filter holder used for filtration.

How to Choose a Millipore^® Filter by Pore Size

Pore size provides an indication of largest pore diameter and can be related to the membrane’s ability to filter out particles of a certain size.