To help narrow your membrane filter options, Sterlitech has assembled the following guide highlighting important aspects of membrane filtration: Objective: What's the goal of your application?

1. If examining particles retained on the surface of the membrane, consider using silver, polycarbonate, polyester, or nitrocellulose mixed esters (MCE) filter membranes.
   


2. If purifying a liquid or examining the filtrate, use a hydrophilic filter membrane and consider other variables such as chemical compatibility and pore size.
   


3. If filtering a gas or doing a venting application, consider using a hydrophobic membrane, PTFE(Teflon) or polypropylene. Silver is also used in gas filtration due to its bacteriostatic nature and good chemical and thermal capabilities.

Chemical Compatibility: Chemical compatibility is defined as the ability of a filter material to resist chemicals. The most chemically resistant filters Sterlitech offers are silver metal and PTFE membranes. The relative chemical compatibility of most widely used reagents can be found in our chemical compatibility chart. Pore size: The pore size of a filter, normally stated in micrometers (µm), is determined by the diameter of a particle that is retained by the filter and theoretically by the bubble point test. The rating is stated in nominal or absolute terms:

1. Nominal pore size rating describes the ability of the filter to retain the majority of the particles at the rated pore size and larger (60-90%). Glass or polypropylene fiber and screen filters are good examples of nominally rated filtration. Nominal filters are generally used as pre-filters or in applications where it is not critical to filter "every" particle.



2. Absolute size rating describes the pore size at which a challenge organism or particle of a specific size will be retained with 99.9% efficiency under strictly defined test conditions. Most membrane filters are rated in absolute terms.

Temperature: The thermal stability of a filter is an important consideration when using elevated temperatures. Silver metal (427°C) and ceramic (350°C) filter membranes can sustain some of the highest temperature applications. The maximum operating temperatures for our filter membranes can be found on Sterlitech's website.

Binding/Protein Concentration: Applications involving proteins, nucleic acids, and enzymes are very common. However, most applications will require that either the material of interest flows through the membrane or be retained by the filter membrane. Two types of membranes exist to deal with this dilemma:

1. High protein binding membranes, such as nylon, are used when proteins are retained by the filter membrane.



2. Low protein binding membranes, including polyethersulfone(PES), cellulose acetate, and polypropylene, should be used when high recovery of the proteins in the solution is required.

Volume of solution being filtered: The volume of the solution is an important consideration in determining the correct diameter of membrane. Sterlitech's filtration products generally work with volumes of solution up to 2 liters (L). The chart below gives a good idea as to which volumes are suitable to a given membrane diameter: