Frequently Asked Questions

We have a Chemical Compatibility Chart that you can use for reference.

During high-pressure filtration processes, the membrane sheet inside the cell may deflect outwards into the feed channel.  This occurs due to variations in pressure during operation. This movement can cause rubbing against O-rings which can result in abrasion, possibility rupture, in the membrane sheet. Therefore, a piece of material (shim) can also be placed between the membrane layer and the test cell to take up the free space and help to prevent movement.   

For more information on shims incorporated into our test cells, please visit here. 

The following are some general guidelines for welding microporous membrane filters:  

• Filter welding is typically achieved by applying a heated die, with compressive force, directly to the membrane filter around its perimeter to bond it to a plastic substrate. The plastic substrate should have a melting temperature similar or lower than the membrane filter. Note that plastics with different surface energies may not bond well. In ideal cases, the plastic substrate is the same material as the membrane. For laminated PTFE membrane, the ideal case would be the plastic substrate is the same material as the support layer.  
• Keep the geometry of the membrane filter simple. This minimizes tooling complexity and lurking variables. Typically, circular membrane filters provide the most consistent results. 
• The contact surface of the sealing die should be polished and coated with a high-temperature nonstick coating to prevent adhesion and plastic buildup. To minimize plastic buildup and maximize coating life, sealing temperatures should be minimized. It may be necessary to periodically clean the contact surface and, at longer intervals, recoat the sealing die. 
• A minimum seal width of 2mm is recommended. For confined part geometries, narrower widths may be attempted, but sealing optimization becomes much more difficult and critical. A good seal typically appears transparent. In critical applications, it may be prudent to devise tests to verify integrity. 
• Optimal settings for sealing die temperature, contact pressure, and dwell time are interrelated and determined through experimentation.  

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. 

• Silver Membrane Filters:  These are the best choice for reflectance IR spectroscopy, but cannot be used for transmission IR spectroscopy.  
• Aluminum Oxide Membrane Filters: Depending on the spectral range, these are a good choice for transmission IR spectroscopy, but they are a poor choice for reflectance IR spectroscopy. These filters are extremely brittle and will fracture if subjected to bending. They cannot be handled with fingers; membrane tweezers must be used. 
• Glass Microfiber Filters: Depending on the spectral range, these filters can be used for both reflectance and transmission IR spectroscopy. The paper does not specify any particular pore sizes; however, most of our offerings should have good efficiency with ≥5-micron sized particles. For good light transmission, I would guess that a thin, binder- less filter media would be best, such as Grade C or GC-50.  Gold-Coated Polyester Track-Etch Filters: Gold coated polyester track-etch filters have essentially the same spectral properties as gold-coated polycarbonate track-etch filters; but they are inherently hydrophilic and, consequently, much more convenient for use with water samples. These are a good choice for reflectance IR spectroscopy, but cannot be used for transmission IR spectroscopy. 

If you are looking for polypropylene membrane filters, you can consider PTFE membranes instead.

We may be able to offer some additional membrane options that are not listed.  Please contact us at [email protected] to inquiry about availability and pricing.

In most cases, membrane filter samples can be purchased in sizes that are not listed in the standard sample packs.  Please contact us at [email protected] to inquire about availability and pricing.

Sample packs allow the customer to purchase small quantities of membrane filters at nominal cost, with various diameters and pore sizes as selected.  This allows the customer to preform trials as needed to determine the optimal filter for their application before committing to purchasing standard pack quantities.

 

The bubble point is the minimum amount of pressure required to push air bubbles through the largest pore of a wet membrane.  The bubble point is inversely proportional to the pore diameter, as the pore diameter decreases the bubble point increases and vice versa.

Retention efficiency of membrane filters can be directly measured by challenging the filters with suspensions of standard microorganism cultures or particles of known size.  Unfortunately, such efficiency testing is necessarily destructive.  However, since retention characteristics are dependent on pore size, it is possible to correlate destructive challenge testing results to non-destructive membrane bubble point tests.  In this manner, the relationship between membrane pore size and membrane bubble point is empirically determined.  Typically, a minimum bubble point can be determined and specified for a particular pore size rating.  The bubble point specification is then used for quality control during membrane manufacture.  The bubble point can also be used by the consumer as a nondestructive test to verify membrane integrity before and/or after use.    

To ensure ease of use, the membrane filters as stacked in their packaging are interleafed with layers of separator paper.  In most cases, the membrane filters will be white in color except for the track-etch membranes which are colorless and translucent.  In some special cases, the membranes will be dyed dark grey to black in appearance.  In all cases, the separator paper will be a different color than the membrane and is usually not white.  Please contact us at [email protected] if you need assistance.

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 pore size refers to the diameter of the individual pores in a membrane filter.   Pore size is typically specified in micrometers (µm).   Most membranes and filter media actually contain a distribution of pore sizes.  Nominal pore size ratings typically refer to the predominant pore size of a filtration media; pores larger and smaller than the nominal rating may be present.  Absolute pore size ratings typically refer to the largest pore size of a membrane and it is expected that all pores will be equal to or smaller than the absolute rating.

For the polycarbonate track-etch (PCTE) and polyester track-etch (PETE) membrane filters, porosity is the percent of the total surface area occupied by the pores; it typically ranges from <1% to 16%.  For the other membrane filters, porosity is the percent of the total volume occupied by the pores; it typically ranges from 40 to 80%.

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.

No. It is a common misconception that sterile air/gas filtration requires the use of a filter containing small pores. A 0.2 µm filter is required for sterile liquid filtration; however, this pore size is not necessary when sterilizing air or gas since the filtration mechanisms in air differ significantly from those in liquid. For example, a filter that retains 0.3 µm aerosolized particles at 99.97% filtration efficiency can provide a sterile air/gas stream due to the mechanics of particle capture in the gas stream environment.

Typically, the criteria set by the ASTM (American Society for Testing of Materials) can be met using filtration media with pore sizes up to 1.0 µm; there are also published reports of filters that are an order of magnitude higher than captured particulate. The efficiency of a filter for the production of sterile (ultrapure) air is based upon the concentration of 0.3 µm Dioctylephthalate aerosol particles (DOP) that penetrate a filter at a predetermined flow and expressed as a percentage. A 99.97% DOP retention indicates a high efficiency particulate air (HEPA) grade rating (ASTM: D2986-71).

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 shelf life of your membranes?

A. Cellulose acetate (CA) and nitrocellulose (MCE) membranes will last 2 1/2 to 3 years before reverting to their natural hydrophobic states.  Nylon, on the other hand, is naturally hydrophilic so it does not have a shelf life.

An easy test for older membranes is to perform a "wetting out" test.  Dipping a small portion of the membrane into water (hydrophilic membranes) or an alcohol (hydrophobic membranes) works well.  If the membrane absorbs the material, or "wets out", it has not gone past the shelf life.

Q. What is the maximum temperature for the different filter membranes?
A. 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
• Sterlitech PVC Data not available

*5.0um and 8.0um - max temp is 180°C

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.

Q. Which membranes are recommended for gravimetric analysis?

A. 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.

• Analytical Filter Funnels:  Funnels are available complete with low hold-up polypropylene support pads, gravimetric analysis is one of the ideal applications for this product.  Each package of 12 units comes with an extender that adapts the unit for side-arm flask filtrations.  Optional No. 8 rubber stoppers may be ordered for filtering with flask.

• Glass Fiber Filters & Prefilters:  Glass fiber filters without binders are recommended for analytical and gravimetric determinations.