Cellulose acetate (CA) membranes are made of entirely cellulose acetate polymer.  These membrane filters have the lowest protein binding of the filer types in question and typically have the greatest throughput when used with proteinaceous solutions.  They are also used in applications where maximum protein recovery is critical. The Sterlitech CA membranes have an integral nonwoven polyester support layer for improved strength and handling ease.

Ideally, nitrocellulose (NC) membranes would be made entirely of nitrocellulose polymer. However, almost all commercially available filtration grade membrane filters labeled as “nitrocellulose” are actually composed of membranes made with a mixture of nitrocellulose and cellulose acetate polymers.   This is because pure nitrocellulose membrane filters are very difficult to manufacture with accpetable characteristics. In an effort to provide clarity to consumers, some manufactures refer to membranes containing a mixture of nitrocellulose and cellulose acetate polymers as mixed cellulose esters (MCE) membranes.  In almost all applications, commercial nitrocellulose (NC) membrane filters and mixed cellulose esters (MCE) membrane filters are equivalent.

Mixed cellulose esters (MCE) membrane filters do not have a discrete support layer and are somewhat fragile and more difficult to handle
when compared to CA membrane filters.  MCE membranes exhibit greater protein binding than CA membranes.  However, MCE membranes tend to have more uniform pore structure and intrinsically greater clean water flow.  MCE membrane filters have very good biocompatibility and are commonly used for microbiological studies.  MCE filters are also traditionally used for silt density index (SDI) measurments.

Regenerated cellulose (RC) membranes are made by chemically treating nitrocellulose membranes so that the material reverts back to natural
cellulose.  RC filters have the very good solvent resistance of traditional filter papers but with the much finer retention characteristics of polymer membrane filters.   Additionally, regenerated cellulose is inherently hydrophilic and RC filters do not contain the wetting agents required by MCE and CA membrane filters.

Yes, the cellulose acetate (CA) membranes contain a proprietary surfactant.  The surfactant is used to ensure the CA membrane filters have good wettability.

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%.

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.    

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.

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.

 

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.

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.