Membrane Filters FAQ

The pores of microporous membrane filters act as small capillaries.  When hydrophilic membranes come into contact with water, capillary action associated with surface tension forces causes the water to spontaneously enter and fill the pores.  In this manner, the membranes are easily wetted and allow the bulk flow of water through the pores.  Once wetted, hydrophilic membranes will not allow the bulk flow of air or other gasses, unless they are applied at pressures greater than the membrane’s bubble point. Hydrophilic membrane filters are typically used with water and aqueous solutions.  They can also be used with compatible non-aqueous fluids.  Hydrophilic membrane filters are typically not used for air, gas or vent filtration since the filters would block flow if inadvertently wetted, by condensation for example. When hydrophobic membranes come into contact with water, surface tension forces act to repel the water from the pores.  Water will not enter the pores and the membranes will act as a barrier to water flow, unless the water is applied at pressures greater than the membrane’s water entry pressure.  Low surface tension fluids, such as alcohols, can spontaneously enter and fill the pores of hydrophobic membranes.  Once all the air in the pores is displaced, there are no longer any surface tension forces and water can easily enter the pores, displace the low surface tension fluid, and pass through the membrane.  The membrane will then allow bulk flow of water for as long as the pore remain water filled.  If the membrane is allowed to dry (i.e. air enters the pores), then it must be pre-wet with a low surface tension fluid again prior to use with water. Hydrophobic membrane filters are typically used with compatible non-aqueous fluids.  They are also commonly used as air, gas, or vent filters.  Hydrophobic membrane filters are sometimes used with water or aqueous solutions; and, in these applications, they must first be prewet with a low surface tension, water miscible fluid prior to use.

Membrane Wettability Demo by Sterlitech (opens a new tab)

The maximum operating temperatures for Sterlitech filter membranes are listed below.

• Sterlitech Silver Metal - 427°C
• Sterlitech Ceramic - 350°C
• Sterlitech Polycarbonate Track Etch - 140°C
• Sterlitech Polyester - 140°C
• Sterlitech Nitrocellulose (MCE) - 130°C
• Sterlitech Nylon - 180°C
• Sterlitech Polyethersulfone (PES) - *130°C
• Sterlitech Polypropylene - 82°C
• Sterlitech Cellulose Acetate - 135°C
• Sterlitech PTFE (Laminated) - 130°C
• Sterlitech PTFE (Unlaminated) - 260°C

*5.0um and 8.0um - max temp is 180°C

Nominal pore size ratings provide a general indication of filter retention efficiency, meaning some particles equal to or larger than the stated pore size may pass through the filter. Nominal ratings can vary by manufacturer, so filters with the same nominal pore size may not offer equivalent filtration performance. Absolute pore size ratings are determined through controlled particle or microbial retention testing and represent the smallest particles that are consistently retained by the membrane. These ratings are often correlated with bubble point specifications and are generally more comparable across manufacturers. Important: Actual filtration performance depends on application conditions, even when using filters with absolute pore size ratings.

A. Pore size and porosity describe different characteristics of a membrane filter. Pore size is the diameter of an individual pore, typically measured in micrometers (µm). Most filters contain a range of pore sizes. A nominal pore size refers to the predominant pore size, while an absolute pore size indicates the largest pore, meaning all pores are expected to be equal to or smaller than the rated size. Porosity describes how much of the membrane is made up of pores. For Polycarbonate Track-Etched (PCTE) and Polyester Track-Etched (PETE) membrane filters, porosity is the percentage of the membrane surface area occupied by pores, typically ranging from less than 1% to 16%. For most other microporous membrane filters, porosity refers to the percentage of the membrane volume occupied by pores, typically ranging from 40% to 80%. In general, pore size determines the size of particles retained, while porosity influences flow rate, permeability, and filtration capacity.

We have several membranes to recommend for gravimetric analysis.

• Mixed Cellulose Esters (MCE) Membrane Filters, Plain: In gravimetric analysis using ashing techniques, (MCE) Nitrocellulose filters yield a residue of less than 0.045% of their initial weight.  They are hydrophilic with a non-cytotoxic wetting agent extractable level of less than 4% of their weight.
• Polycarbonate Track-Etch Membranes (PCTE) - 25mm:  Polycarbonate Track-Etch or our Polyester Track-Etch (PETE) membranes are two membranes that offer exceptionally low tare weights, are non-hygroscopic, and exhibit extremely low absorption and adsorption losses. Since these membranes are non-hygroscopic, they are particularly well suited for gravimetric analysis.  They do not require drying when used directly out of the package.  If they are wet, they can be dried rapidly and will not pick up moisture from the air during weighing.
• Glass Fiber Filters & Prefilters:  Glass fiber filters without binders are recommended for analytical and gravimetric determinations.

You can find the Sterlitech compatibility guide.  It is important to realize that application conditions, such as operating temperature, affect compatibility.  Please contact us at [email protected] if you need assistance.

The bubble point is the minimum pressure required to force air through the largest pore of a wetted membrane filter. It is inversely related to pore size—smaller pores have higher bubble point values, while larger pores have lower bubble points. Because membrane filter retention is directly related to pore size, bubble point testing provides a reliable, non-destructive membrane integrity test. During manufacturing, destructive challenge tests using microorganisms or standard particles are used to establish the relationship between pore size and bubble point pressure. This correlation is then used to define the minimum bubble point specification for each membrane pore size. Users can perform a bubble point test before or after filtration to verify the integrity of the membrane and confirm that it has not been damaged or compromised during use.

Depth filters and membrane filters differ in how they capture particles and their typical filtration applications. Depth filters are made from relatively thick filtration media with nominal pore size ratings, typically greater than 1 µm. Their high void volume allows particles to be captured throughout the filter matrix, making them well suited for prefiltration and applications with high particulate loads. Membrane filters are thin, highly porous polymer films with precisely controlled pore structures and are commonly available with absolute pore size ratings of 1 µm or smaller (with some exceptions). They retain most particles on the membrane surface, providing highly accurate and reproducible filtration for particle analysis, microbiological testing, sterile filtration, and laboratory applications. Smaller particles near or below the membrane's pore size may pass through or become trapped within the pore structure.

Membrane filter sample packs let you evaluate different membrane materials, diameters, and pore sizes at a low cost before purchasing standard pack quantities. They are ideal for application testing, helping you identify the best membrane filter for your filtration process while reducing cost and minimizing product selection risk.  

Yes, in many cases. Sterlitech may be able to provide membrane filter samples for materials not included in the standard sample packs. Please contact [email protected] with your application requirements to inquire about sample availability, pricing, and custom membrane options.

Yes, in many cases. Membrane filter samples may be available in custom diameters beyond the standard 13 mm, 25 mm, and 47 mm sample packs. Please contact Sterlitech at [email protected] to discuss your application and inquire about availability, pricing, and custom sample options.

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Membrane filters are packaged with separator papers between each filter to protect them during storage and handling. The easiest way to distinguish them is by color and appearance. Most membrane filters are white, while Polycarbonate Track-Etched (PCTE) membrane filters are colorless and translucent. Some specialty membrane filters, such as black polycarbonate membranes, appear dark gray or black. In contrast, the separator paper is always a different color than the membrane and is typically blue, tan, or printed—it is not intended for filtration.

The best membrane filter depends on the IR spectroscopy method:

Silver membrane filters: Best for reflectance IR spectroscopy; not suitable for transmission IR.

Aluminum Oxide (AAO) membrane filters: Suitable for transmission IR spectroscopy (depending on the spectral range), but not recommended for reflectance IR. These membranes are brittle and should be handled with membrane tweezers.

Glass microfiber filters: Suitable for both reflectance and transmission IR spectroscopy (depending on the spectral range). Thin, binderless grades such as Grade C or GC-50 provide better light transmission.

Gold-coated Polyester Track-Etched (PETE) membrane filters: Ideal for reflectance IR spectroscopy, especially with aqueous samples, due to their inherent hydrophilicity. They are not suitable for transmission IR.

To allow water flow through hydrophobic membrane disc filters, you should first pre-wet the filter by briefly submerging it in alcohol (>90% concentration of ethanol, methanol, or IPA are fine) immediately before water filtration. A small weighing dish or petri dish is a good container for this step. The low surface tension of the alcohol will allow spontaneous filling of the dry membrane pores by capillary action. Once the pores are filled with liquid, they will no longer repel water. If the presence of alcohol is problematic during filtration, then the alcohol wet filter can be submerged in a large beaker (≥1L) of purified water and allowed to sit for several minutes, perhaps with occasional gentle stirring. This will effectively dilute the alcohol in the pores. If necessary, the water can be replaced one more time during the soak, and the soak time extended, for a more thorough dilution. The filter should remain submerged until ready for use. When loading the wet filter in the holder, it is important to have a sense of urgency. The water filtration must be initiated before the filter dries. If the filter is allowed to dry, then the pre-wetting process must be repeated. 

It depends on the membrane type. Polycarbonate Track-Etched (PCTE) and Polyester Track-Etched (PETE) membrane filters are symmetric and do not have a preferred orientation. Either side can face the feed solution without affecting filtration performance or particle retention.

Polyethersulfone (PES) membrane filters are asymmetric and do have a preferred orientation. For maximum flow rate and sample throughput, install the membrane with the coarse (larger-pore), dull/matte side facing the feed (upstream). This depth filtration structure reduces surface clogging and extends filter life. For applications where the retained particles are the focus—such as microscopy or particle analysis—the fine (shiny) side may be oriented toward the feed to keep particles closer to the membrane surface for easier visualization. Regardless of orientation, the rated pore size and particle retention remain unchanged.

No. Sterlitech does not currently offer a commercial bubble point test instrument or bubble point testing holder. The apparatus shown in our bubble point testing video was designed exclusively for internal use and has not been validated or manufactured for commercial sale. If you plan to build a custom bubble point testing apparatus, ASTM F316 provides a conceptual design for a bubble point test holder that can serve as a useful starting point. For best compatibility with commercially available membrane filters, Sterlitech recommends designing the holder for 47 mm membrane disc filters rather than 2-inch filters, as 47 mm is the industry standard.

Open area, also called membrane porosity, is the percentage of the membrane occupied by pores or void spaces. In general, higher porosity provides higher flow rates because more open pathways are available for fluid to pass through.

Membrane porosity is measured empirically, typically using a liquid displacement method, rather than calculated mathematically. Because many membrane filters have a complex, three-dimensional pore structure, porosity values are typical rather than lot-specific specifications and may vary between production lots.

It depends on the membrane type. For fibrous membrane filters, the pores are not cylindrical tubes. Instead, they form a three-dimensional network of randomly entangled fibers, rather than a woven mesh. This structure allows particles to be captured both within the membrane depth and on the membrane surface, depending on particle size and filtration conditions.

In most applications, no. While hydrophobic PTFE membrane filters can initially allow oil to pass and repel water when the membrane is dry, this effect is temporary. Once the membrane pores become filled with oil, the surface tension that repels water is lost, allowing water to pass through under relatively low differential pressures. Therefore, hydrophobic membrane filters are not a reliable solution for continuous oil-pass/water-reject separation. For oil/water separation, ultrafiltration (UF) membranes are typically recommended. Membranes such as the GE MW ultrafiltration membrane are designed to allow water to permeate while rejecting oil, making them more suitable for wastewater treatment and oil removal applications. For research or proof-of-concept studies, Sterlitech also offers PTFE membrane sample packs if you wish to evaluate hydrophobic membrane performance under your specific operating conditions.