FREQUENTLY ASKED QUESTIONS

Top 3 Questions:

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.

Q. What is the difference between nominal and absolute pore size ratings?

Nominal pore size ratings are provided as a general indication of filter retention.  It is understood that some quantity of particles greater than, and equal to, the nominal pore size ratings will pass through the filters into the filtrate.  Some manufactures may associate nominal pore size ratings with percentage filtration efficiencies. Nominal pore size ratings vary from manufacturer to manufacturer and, consequently, are not necessarily equivalent. Filters from different manufactures with similar nominal pore size ratings may not actually exhibit similar retention characteristics.

Absolute pore size ratings are typically based on retention studies performed using challenge suspensions of standard microorganism cultures or particles of known size. Absolute pore size ratings represent the size of the smallest microorganisms or particles completely retained during these studies. Absolute pore size ratings are almost always correlated to bubble point specifications that are used for quality control during membrane manufacturing. For the most part, absolute pore size ratings, especially those based on microbial retention, are comparable from manufacturer to manufacturer. There is more uncertainty for absolute pore size ratings based on particle retention studies, especially for pore size ratings <0.2µm, since there are no standard methods for these studies.

Regardless of pore size ratings, it is important to understand that application conditions do influence particle retention. Even filters with absolute pore size ratings can be operated in conditions that will allow unexpectedly sized particles to pass.


Depth filters are constructed with relatively thick filtration media and typically have nominal pore size ratings >1µm. Due to their large void volume, they capture significant amounts of particulate within their pore structure.
Membrane filters are typically composed of polymers that have been chemically processed, resulting in highly porous thin films with microscopic pore structures. Membrane filters typically have absolute pore size ratings <1µm, with some exceptions. Because of their very fine pore structure, membrane filters tend to trap the majority of particles on the surface. However, smaller particles with diameters near or below the pore size rating can be captured within the membrane or pass through the membrane.

Top 3 Questions:

Alumina oxide is free of organic extractables and leachables and shows minimum adsorption.

Unfortunately,we are unable to supply the alumina oxide membrane filters with custom diameters.  Please contact us at sales@sterlitech.com to inquire about alternatives.

Yes, the Sterlitech alumina oxide membrane filters can be
used as alternatives for the Whatman Anodisc filters.  The alumina oxide filters do not have perimeter support rings, so somewhat greater care must be taken when handling the filters to avoid damage.

Cellulose Acetate

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Top 3 Questions:

Sterlitech Cellulose Acetate (CA) membranes are made from cellulose diacetate. These membrane filters also have an integral nonwoven polyester (polyethylene terephthalate) support layer. When evaluating application compatibility, both materials should be considered.

Not including the polyester support layer, the Cellulose
Acetate (CA) membrane filters are composed entirely of cellulose acetate
polymer.  There may, however, still be a small amount of residual lignin present. 


 Cellulose acetate (CA) membrane filters are one of the lowest protein binding filters available. They will generally have greater throughput with proteinaceous solutions when compared to other membrane filters. CA membrane filters are ideal for filtration of protein and enzyme solutions, tissue culture media and serums, biological fluids, and similar applications where maximum recovery of protein is critical.
CA membranes are manufactured with an integral nonwoven polyester support layer resulting in a dimensionally stable strong membrane that is easier to handle and resistant to curling. The filters have superior resistance to tearing and can withstand steam sterilization up to 135°C. They are suitable for use at elevated temperatures.
CA membranes are hydrophilic and readily wet in water and aqueous solutions. They have good chemical resistance and can be used with low molecular weight alcohols. 

Top 3 Questions:

Ceramic membranes are composed of a matrix of zirconium oxide and titanium dioxide. These rigid, inert inorganic filters have superior chemical and thermal resistance. They can be operated at temperatures that would destroy conventional polymer membranes, up to 350°C. These attributes are uniquely suited to applications where the filters are subjected to repeated regeneration with chemical and high temperature cleanings.

The ceramic membrane filters are only available with 47mm and 90mm diameters. You may want to consider other inorganic membranes, such as alumina oxide or silver, for applications requiring different diameters.

The ceramic membrane disk filters are considerably thicker
than conventional membrane disk filters and will not fit in conventional disk
filter holders.  The ceramic membrane disk
filters must be used with the specially designed holders offered at https://www.sterlitech.com/ceramic-membrane-disc-holders.html.

Top 3 Questions:

MCE membranes feature fast flow rates, a high protein binding capacity, and great thermal stability, making them a staple for many environmental and biological laboratories. Furthermore, they are available as presterilized, individually wrapped membranes, and can include a gridded pattern for quantifying microbial growth.

Unfortunately, in most instances, we are unable to supply the MCE membrane filters with custom diameters. Please inquire with your Sterlitech sales representative about alternatives.

Sterile mixed cellulose esters (MCE) membrane filters are used in vast quantities for microbiological studies across many industries and are manufactured in very high volumes to accommodate this demand.  Economies of scale and process automation allow the sterile MCE membrane filters to be offered at lower pack costs compared to non-sterile MCE membrane filters.  Non-sterile MCE filters are used much less frequently, necessitating less efficient, smaller volume manufacturing runs and packaging methods.  Consequently, the non-sterile MCE membrane filters have intrinsically greater manufacturing costs and must be offered at higher prices.

Top 3 Questions:

Nylon Membranes exhibit high protein binding, solvent resistance, and dimensional stability due to support by inert polyester.

The Sterlitech nylon membrane filters are constructed of nylon 66 polymer. Nylon 66 in inherently hydrophilic, nontoxic, and has good resistance to organic solvents. These membrane filters also have an integral nonwoven polyester (polyethylene terephthalate) support layer. When evaluating application compatibility, both materials should be considered.

Polycrylonitrile (PAN)

View all Polycrylonitrile (PAN) FAQ

Top 3 Questions:

PAN Membranes combine excellent selectivity, high flow rates and low pressure requirements for use. 

The polyacrylonitrile (PAN) membrane filters are absolute rated at 0.2µm and are bacterially retentive with typical 6 log reduction value (LRV). This level of retention can be expected to meet EPA standards for safe drinking water with respect to microorganisms. It is important to realize that the integrity of the combined filter holder and disk filter assembly must be considered in critical applications.

Top 3 Questions:

PEEK Membranes exhibit outstanding resistance to almost any known organic solvent, as they consist of pure PEEK (no sulfonation or similar). 

The polyether ether ketone (PEEK) membrane filters are hydrophobic. However, they tend to have lower water entry pressures than other common hydrophobic membrane filters such as PTFE, polypropylene, and polyvinylidene difluoride (PVDF).

Yes, the polyether ether ketone (PEEK) membrane filters may be purchased with diameters other than 25 and 47mm.  Please contact sales@sterlitech.com to inquire about availability and pricing.

PES (POLYETHERSULFONE)

View all PES (POLYETHERSULFONE) FAQ

Top 3 Questions:

PES Membranes are low protein binding, PES membrane filters are ideal for tissue culture media sterilization, life science and microbiology fluid applications. 

The polyethersulfone (PES) membrane filters have asymmetrical pore structure. The pore structure varies within the thickness of the membrane such that the largest openings occur on one side and the smallest openings occur on the opposite side. When viewing the membrane with reflected light at low incidence angles, each side has a somewhat different visual appearance. The side with the largest pores will appear more dull or matte than the side with the smallest pores. With a little bit of experience, most users can easily identify the sides. For optimal throughput, the PES membrane filters should be oriented so that side with the largest pores (the duller side) is facing upstream. For applications involving microscopic analyses of captured particles or microbes, the user may choose to orient the filter so that the side with the smallest pores (the shinier side) is facing upstream. This orientation may reduce throughput but it improves the likelihood of capturing particles of interest on the surface of the membrane instead of within the pore structure.

The polyethersulfone (PES) membranes used in the Sterlitech membrane filters have asymmetric pore structure.  The pore structure varies within the thickness of the membrane such that the largest openings occur on one side and the smallest openings occur on the opposite side.  When viewing the membrane with reflected light at low incidence angles, each side has a somewhat different visual appearance.  The side with the largest pores will appear more dull (or matte) than the side with the smallest pores
(which will appear shinier).  With a little bit of experience, most users can easily identify the sides.  The membranes can be used with either surface oriented upstream without affecting retention.  However, orienting the dull side upstream increases total throughput
while orienting the shiny side upstream allows for better analyses of the retained particles.

Top 3 Questions:

It is possible to estimate the pore diameter of polyester track-etch (PETE) membranes from SEM images. In fact, this is how the pore size is characterized during manufacturing for most of the track-etch membranes. However, it is important to understand that there are calibration and performance variations between different SEMs. There is a good likelihood that a user’s results will not correlate to the manufacturing results that were used to characterize the membrane.

The Sterlitech polyester track-etch (PETE) membranes are made of polyethylene terephthalate.

Hydrophobic membrane filters are necessary for applications where the membrane is used to retain liquid water while allowing gases to pass through.  The PETE membrane filters are hydrophilic and are not suitable for these applications.  The hydrophobic PCTE membranes typically have insufficient water entry pressures for these applications and will allow liquid water to pass at pressures lower than required.  Hydrophobic PTFE and polypropylene membrane filters have the highest water entry pressures for membrane filters and are commonly used for these applications.

Top 3 Questions:

PP Membranes exhibit good hydrophobicity and can be considered in applications that don't require the chemical compatability of PTFE. 

You can find the current specifications for the Polypropylene (PP) membrane filters at https://www.sterlitech.com/polypropylene-membrane-filters.html.  Click the "Application/Specification" tab near the middle of the page and scroll down as necessary.

Sterlitech polypropylene (PP) membrane filters maybe sterilized using compatible chemical agents such as ethylene oxide (EtO) or formalin.

Top 3 Questions:

PTFE Membranes are extremely hydrophobic and exhibit superior chemical compatability with agressive solutions. 

Top 3 Questions:

Polyvinylidene difluoride (PVDF) membrane filters are mechanically strong, exhibit superior chemical resistance, and high thermal stability. 

Top 3 Questions:

Most of the syringe filters offered by Sterlitech do not contain surfactants.  The exceptions are syringe filters with cellulose acetate (CA) and mixed cellulose esters (MCE) membranes.  CA and MCE membranes are treated with surfactants to ensure good wettability.    

From our standard offering, the largest pore size rating available is 5 microns.  Both cellulose acetate (https://www.sterlitech.com/cellulose-acetate-syringe-filters.html) and nylon (https://www.sterlitech.com/nylon-syringe-filters.html) membrane syringe filters are available with 5 micron pore size rating.

Sterlitech can provide custom syringe filters (https://www.sterlitech.com/custom-syringe-filters.html) with pore size ratings up to 250 microns.  Please contact sales@sterlitech.com to inquire about availability and pricing.

Yes, for water and some aqueous solutions, a hydrophobic membrane syringe filter installed in the tubing between the vacuum pump and the receiving flask can prevent accidental overflows from reaching the vacuum pump.  Sterlitech recommends PTFEVS4550BAR10 (https://www.sterlitech.com/ptfevs4565bar10.html) for this application.  Please note that this method will not work for low surface tension fluids, such as alcohols and solvents.

General Filtration

View all General Filtration FAQ

Top 3 Questions:

Q. What is the maximum temperature for the different filter membranes?
A. The maximum operating temperatures for Sterlitech filter membranes are listed below.

Q. How is the performance of a filter measured?

A. Design and material selection determines the performance of a filter. Three important measures of filter performance are flow rate, throughput and bubblepoint, defined as follows:

Flow Rate: Determines the volume of liquid or air that will flow through the filter at a fixed pressure and temperature. This is usually displayed as ml/minute/cm^2.

Throughput: Describes the dirt handling capacity of a filter. Namely, how long the liquid will continue to flow through the membrane before the membrane clogs. The lower the flow rate and throughput, the longer it takes the researcher to complete the analysis.

Bubble point: A test to determine the integrity and pore size of a filter. The differential pressure at which a steady stream of gas bubbles is emitted from a wetted filter under specific test conditions. The bubble point test measures the largest pore. Bubble point is generally determined using water or an alcohol (methanol or isopropynol) and is displayed as PSI.

Q. What variables affect the performance of a filter?

A. Viscosity: The viscosity of a liquid determines its resistance to flow; the higher the viscosity, the lower the flow rate and the higher the differential pressure required to achieve a given flow rate.

Porosity: The flow rate of a membrane is directly proportional to the porosity of a membrane, eg. the more pores, the higher the flow rate.

Filter Area: The larger the filter area, the faster the flow rate at a given pressure differential and the larger the expected filter throughput volume prior to "clogging for a given solution."

Top 3 Questions:

A. There is no predetermined shelf life for the silver membraneThe filters should be stored sealed in the original packaging until needed.  Over time, silver compounds may form on the surface of the membrane.  Any resulting surface discoloration is essentially cosmetic and does not affect filter performance.

Q. What if my membrane is slightly discolored?

A. Although the silver metal membrane is 99.97% pure silver, the formation of extraneous compounds is possible over time. For example, silver can become tarnished, especially when the environment contains certain emissions as described below. To minimize contamination of the membrane, leave it in sealed packs. Silver compounds may form on the surface which are primarily cosmetic imperfections and do not affect the pore structure or membrane filtration performance. Examples of colored compounds that can form on the surface of the silver metal membrane are:

• Ag2S (black)
• Agl (yellow)
• Ag3PO4 (yellow)
• Ag2CrO4 (dark red)
• AgCl (dark brown)
• Ag2O (dark brown)
• AgBr (light yellow)

The most common compounds that form on the silver metal membrane are Ag2S and AgCl. AgCl is a photosensitive salt that can be removed by flushing the membrane with an ammonia solution. Typically, just a brief soak or dip in the ammonia solution will dissolve AgCl. Ag2S is a very stable compound and is very difficult to remove from the membrane without altering the structure. A flush with methyl or ethyl alcohol can be used to remove some of the other compounds.

These compounds should not be confused with the natural grayish white appearance of the silver metal membrane surface. This appearance is due to the microporous structure of the media which reflects light in a manner different than polished silver. The slight difference in color between the two sides of the membrane is due to the manufacturing process and is most noticeable on 3 and 5 micron pores sizes.

Q. What NIOSH Standards are Silver Membranes specified for?

A. National Institute for Occupational Safety and Health (NIOSH) - used for industrial hygiene in foundries, glass plants, quarries, mines, ceramic manufacturing - Methods using 0.45 µm, 25 mm:

N6011 (Bromine & Chlorine) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/6011.pdf
N7500 (Silica, Crystalline) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/7500.pdf
N7501 (Silica, Amorphous) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/7501.pdf
N7504 (Vanadium Oxide) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/7504.pdf
N7505 (Lead Sulfide) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/7505.pdf
N7506 (Boron Carbide) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/7506.pdf
N9000 (Asbestos, Crysotile) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/9000.pdf

Track Etch Membrane

View all Track Etch Membrane FAQ

Top 3 Questions:

Q. What is a Polycarbonate or Polyester Track Etch filter membrane?

A. These types of filter membranes are precise, two-dimensional micro porous screens with straight through, cylindrical pores.

As in the case of other screen-type filters, particle capture takes place only on the surface, therefore there is more accurate separation cut-off. The precision cylindrical pores of Track Etch membranes have the most accurate size cut-off of any membrane. In depth filters, particles get caught throughout the torturous paths within the matrix as well as on the surface of the membrane.

Track Etch filters are also very thin (between 6 - 15 microns thick) but very durable (can withstand over 3,000 psi when properly supported).   They range in color from opaque to almost transparent and black.

Q. What are the benefits of using Sterlitech Polycarbonate or Polyester filter membranes?

A. Sterlitech Polycarbonate Track Etch (PCTE) and Polyester Track Etch (PETE) filters offer the lowest, non-specific binding of any filter membrane. The capture of samples occurs on a flat, glass-like smooth surface with an even distribution of particles captured on a single plane, simplifying microscopic and SEM examination of samples captured on the surface of the membrane.

  • Sterlitech Track Etch filter membranes are manufactured and produced under class 100 condition during critical manufacturing steps. Therefore, the membrane is free of contaminants and pyrogens.
  • Sterlitech PCTE and PETE membranes offer very low extractables. Both PCTE and PETE membranes are integral, plastic films, therefore, there is no sloughing or particle shedding.
  • Sterlitech PCTE and PETE membranes are biologically inert. 
  • They offer superior strength, with pressure tolerances in excess of 3,000 psi (when placed in an appropriate filter holder).
  • Both filter membranes offer excellent chemical resistance and thermal stability, with PETE offering a higher chemical resistance.

Q. Will Sterlitech Track Etch filter membranes keep liquid behind the filter and let gases pass through?

A. The need for a gas vapor barrier requires the use of a hydrophobic filter with a water contact angle greater than 120°. If the contact angle is less than 120°, the filter will "bleed" through. The best filter material to use for a vapor barrier is made of PTFE (Teflon™). Most hydrophilic membranes generally have a contact angle that is less than 50°, and an hydrophobic membrane generally has a water contact angle greater than 120°. Raw polycarbonate film has a contact angle of approximately 70°, which is neither hydrophilic nor hydrophobic. It is for this reason that on most Sterlitech PCTE filter membranes, we apply the wetting agent PVP. As a result they are considerably more hydrophilic than standard membranes. They also have a typical water contact angles that range from 70° to 90°.

Sterlitech PETE filter membranes are naturally hydrophilic with a contact angle of approximately 40°, therefore, no wetting agent is added in the manufacturing process.

Membrane/Process Development

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Top 3 Questions:

Cross flow velocity is calculated by dividing the volumetric flow rate through the cell by the cross section area of the cell. 

Cross flow velocity limits for commercially available spiral wound elements depend on the element construction limits, recommended maximum pressure drop in an element, and feed characteristics.  The recommended values could be obtained form the manufacturers. Please contact Sterlitech for more information.

Cross flow velocity affects the hydrodynamic conditions in the system and therefore affects the rate of fouling. If the objective of the experiment is to mimic the hydrodynamic conditions in commercially available spiral wound elements it is recommended to stay in the range recommended by the manufacturers. Please contact Sterlitech for more information. 

If the objective of the experiment is to shed light into the effect of cross flow velocity on the membrane performance/fouling, the optimum range of cross flow velocity should be identified experimentally.

HP4750 Stirred Cell

View all HP4750 Stirred Cell FAQ

Top 3 Questions:

Q. What size membrane does the HP4750 Stirred Cell use?

A. Any 47-50mm membrane disc can be used.

Q. What material is the HP4750 Stirred Cell made out of?

A. The cell itself is constructed of 316 stainless steel and utilizes a PTFE coated stirrer for chemical resistance.

Q. What type of stir plate do you recommend for use with the HP4750 Stirred Cell?

A. Any conventional magnetic stirrer will work.

Top 3 Questions:

Q. What is the difference between Sepa CF and Sterlitech HP4750?

A. The Sterlitech HP4750 is an enclosed batch system (limited to 300ml) with direct filtration under pressure. There is a stir bar mixing the solution and pressures up to 1000 psi may be applied.

The Sepa CF is a crossflow system that allows continuous sampling and testing under different pressure and flow rate parameters depending on the pump and fluid.

Q. Does the feed spacer penetrate the membrane?

A. The mesh spacer usually leaves an imprint on the membrane which is not a problem - unless too thick of a mesh is used - then it could damage the membrane.

Q. How do I calculate Reynolds number based on the feed cross-flow velocities for the various feed spacers?

A. The Sepa CF cell for use with high fouling spacer has a flow area width and height of 3.7 inches by 0.068 inches. Once a spacer material is placed in the channel the actual flow channel cross section is significantly reduced. We have not calculated that. We typically operate the cell and estimate the Reynolds number at the transition from laminar to turbulent flow by monitoring the increase in pressure drop as the crossflow is increased.

Another usage implementation which we have used is the placement of a rubber gasket in the flow channel. A specifically sized flow channel can be cut out of the gasket to define a flow channel of the desired cross section.

We actually have done more study of the CF cell for use with standard spacer (3.7 in. by 0.034 in. cell cross-section). We evaluated that cell for various cross flow velocities at various feed flow rates.

Flat Sheet Membranes

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Top 3 Questions:

Q. The Flat Sheet Membranes appear dry in their packaging. How do I pre-wet them? Do I need to do this?

A. Yes, you need to pre-wet the membranes. The best procedure is to place them in a dry holder and allow them to wet from the inlet side first. It may be best to perform this operation with water or a buffer, then dispose of the first rinse, and introduce the process fluid. This prevents any wetting agents or preservatives from mixing with the process solution.

Prior to use, the membranes should be stored in a climate controlled environment, away from sunlight and heat, while sealed in the original packaging. We recommend using the membranes as soon as practical after receiving them. However, most of the flat sheet membranes may be stored for up to one year without affecting performance. The exceptions are the GE cellulose acetate membranes (CE and CK) and the Aquaporin FO membrane which should be used within 6 months.

Prior to use, the Aquaporin Inside™ FO membranes should be soaked in DI water for a minimum of 30 min. Once wetted, the membranes should not be allowed to dry.

Glass Fiber Filter

View all Glass Fiber Filter FAQ

Top 3 Questions:

Glass Fiber Filters are exhibit high operating temperatures and are particularly economical for use as a pre-filter.

The acrylic (PMA) resin binder significantly improves the wet strength of the glass fiber filters. Resin bonded glass fiber filters are easier to handle and are resistant to fiber shedding. When evaluating application compatibility, it is important to consider the acrylic (PMA) resin.  

DOP is an abbreviation for dioctyl phthalate.  Aerosol particles made with DOP have a very uniform size of 0.3µm and are used to characterize air filter retention.  For example, DOP particles are used in ASTM D2986-95a, Standard Practice for
Evaluation of Air Assay Media by the Monodisperse DOP (Dioctyl Phthalate) Smoke
Test.