A Depth Filter is a filter consisting of either multiple layers or a single layer of a medium having depth, which captures contaminants within its structure, as opposed to on the surface. (Example: Pall Life Sciences Glass Fiber media.)

Advantages:

• Lower cost
• High throughputs
• High dirt-holding capacity
• Protects final filters
• Removes variety of particle sizes

Potential Disadvantages:

• Media migration (shedding)
• Nominal pore size
• Particulate unloading at increased differential pressure

A Membrane Filter typically traps contaminants larger than the pore size on the addressed surface of the membrane. Contaminants smaller than the rated pore size may pass through the membrane or may be captured within the membrane by other mechanisms. Membrane filters are typically used for critical applications such as sterilizing and final filtration. (Examples: Pall Life Sciences Supor® and GN-6 Metricel® membranes.)

Advantages:

• Absolute sub-micron pore size ratings possible
• Can be bacteria and particle retentive (pore size dependent)
• Generally lower extractables
• Generally integrity testable

Potential Disadvantages:

• Lower flow rates than depth media
• More costly than depth media

A Combination Filter combines different membrane pore sizes, or combines depth media and a membrane filter to create self-contained serial filter units. They can offer an economical alternative to using individual prefilters and final filters. (Examples: Pall Life Sciences Acrodisc® syringe filters with GxF glass fiber/0.45 µm GHP membrane, Acrodisc PF syringe filters, VacuCap® PF devices, and AcroPak™ 500 capsules.)

Chemical Compatibility is defined as the ability of a filter’s materials of construction to resist chemicals so that the filter’s function is not adversely affected, and the filter material does not shed particles or fibers, or add extractables. It is important to remember that compatibility is specific for a particular chemical or combination of chemicals, at a particular temperature. To select the proper filter or device, you must determine the compatibility of the filter components with the fluid. Temperature, concentration, applied pressure, and length of exposure time affect compatibility. The materials used in the manufacture of filtration products are carefully chosen for their resistance to a wide range of chemical solutions. Still, understanding the compatibility between the fluid to be filtered and the filter elements under actual conditions of use is essential.

Hydrophilic filters are easily wet with water. Hydrophilic filters can be wetted with virtually any liquid, and are the preferred filters for aqueous solutions, as appropriate by compatibility. Note that in the filtration industry, “hydrophilic” is used somewhat differently than in some other fields, where it refers to a material to which water clings. (Example: Pall Life Sciences Supor membrane.)

Once wetted, hydrophilic filters do not allow the free passage of gases until the applied pressure exceeds the bubble point and the liquid is expelled from the pores of the membrane. (See the section on "Measuring  a Filter's Performance".)

Wetted membrane prior to bubble point.

Hydrophobic filters will not wet in water but will wet in low surface tension liquids, for example, organic solvents such as alcohols. Once a hydrophobic filter has been wetted, aqueous solutions also will pass through.

Hydrophobic filters are best suited for gas filtration, low surface tension solvents, and venting. In certain applications, hydrophobic filters are used to filter aqueous solutions because of compatibility requirements. (Example: Pall Life Sciences TF (PTFE) membrane.)

Water or aqueous solutions can also pass through a hydrophobic filter once the water breakthrough pressure is reached. (See the section on "Measuring  a Filter's Performance".)

Pore Size Rating is the pore size of the filter determined by the diameter of the particle that it can be expected to retain with a defined, high degree of efficiency. Pore size ratings are usually stated in Micrometers (µm). Ratings can be stated as either nominal or absolute.

Nominal filter ratings are an arbitrary value, indicating a particulate size range at which the filter manufacturer claims the filter removes some percentage. Nominal ratings vary from manufacturer to manufacturer and cannot be used to compare filters among manufacturers. Processing conditions such as operating pressure and concentration of contaminant have a significant effect on the retention efficiency of the nominally-rated filters. (Example: Depth media, such as Pall Life Sciences Glass Fiber media.)

Absolute filter ratings are a value associated with a filter that represents the size of the smallest particle completely retained. Complete retention is within the experimental uncertainty of a standard test method consistent with the intended filter usage. Among the test conditions that must be specified are test organism (or particle size), challenge pressure, concentration, and detection method used to identify the contaminant. (Example: Most membrane filters, such as Pall Life Sciences Supor membrane products.)

Below are typical challenge organisms for specific membrane pore sizes:

Absolute-rated Filter Media    (Pore Size)	Challenge Organism
0.1 µm	Acholeplasma Iaidlawii
0.2 µm	Brevundimonas diminuta
0.45 µm	Serratia marcescens
0.8 µm	Lactobacillus species
1 µm	Candida albicans

Binding is the tendency of certain substances to “stick” to the filter medium (or other filter components) and be removed from the fluid. Binding can be a desirable characteristic, as in the case of nucleic acid or protein binding on transfer membranes, which allows them to be separated and identified; or an undesirable characteristic, as in the case of protein binding during filtration, which can lead to a loss of valuable products. (Examples: Pall Life Sciences HT Tuffryn®, Supor, Omega™, Fluorodyne® II, and GH Polypro (GHP) membranes are extremely low protein binding.)

Extractables are substances that may leach or otherwise come off the filtration system and may be added to the fluid being filtered. These contaminants may include wetting agents in the filter media, manufacturing debris, chemical residue from sterilizing the filter, adhesives, or components of the filter materials of construction. The type and amount of extractables will vary with the type of liquid being filtered.

Extractables can be minimized by flushing the filter with either water or a process-compatible solvent before using it. Some filters are sold pre-flushed. Careful manufacturing procedures can eliminate the need to flush filters. (Example: Pall Life Sciences filter devices sterilized with gamma irradiation do not exhibit toxic extractables associated with ethylene oxide sterilization.)

Extractables can affect filtration in almost every application:

• in HPLC Analysis, they can add extraneous peaks,
• in Cell Culture, they can cause cytotoxicity (kill cells),
• in Microbiological Analysis, they can inhibit growth and affect recovery of microorganisms,
• in Environmental Analysis, they can appear as additional contaminants.

Thermal Stability is the ability of the filter media and device components to maintain integrity and functionality at elevated temperatures. Thermal stability is important when considering filter sterilization, such as autoclaving. Certain filters cannot be autoclaved because of insufficient thermal stability. Keep in mind that there is a relationship between chemical compatibility and thermal stability; many types of filter media may be compatible with a chemical at room temperature, but not at high temperature. Thermal stability can be characterized by determining the maximum operating temperature under specified conditions.

Flow Rate and Throughput are two important related measures of filter media and device performance described in this section. This performance is affected by many different variables. The most important variables are detailed in the subsequent text.

Water Flow Rate is a measure of the amount of water that flows through a filter. It is related to the degree of contamination, differential pressure, total porosity, and filter area. Expressed in the membrane industry in units of milliliters/minute/square centimeter (mL/min/cm²).

Air Flow Rate is a measure of the amount of air that flows through a filter. It is related to the degree of contamination, differential pressure, total porosity, and filter area. Commonly expressed in the membrane industry in liters/minute/square centimeter (L/min/cm²) at a given pressure.

Throughput is the amount of fluid able to pass through a filter prior to plugging. See Filter Life in the "Measuring a Filter's Performance" section.

Differential Pressure is the difference between the pressure in the system before the fluid reaches the filter (upstream pressure) and the system pressure after the fluid flows through the filter (downstream pressure). In a constant flow application, the differential pressure increases as the filter begins to clog.

Viscosity is a measurement of a fluid’s resistance to flow. For instance, a slow-flowing liquid like honey has a higher viscosity than a “thin” liquid like water. The higher the viscosity (at a constant temperature and pressure), the lower the flow rate through a filter (assuming that the fluid is Newtonian, that is, that the viscosity does not change as the conditions change).

Porosity (also called “open area” or “void volume”) is a measurement of all of the open spaces (pores) in the membrane. Generally, membranes are 50 to 90% open space. Flow rate is directly proportional to the porosity of the membrane (more pores = higher flow rate, for a given pore size and thickness of filter medium).

Filter Area. Filter media and devices are available in a wide range of sizes with different Effective Filtration Areas (EFA). EFA is the filter area that is available for filtration; for a given membrane, the larger the filter area, the higher the flow rate at a given initial differential pressure.

Filter Media and Device Configurations. Filter media and devices are available in a wide variety of sizes and configurations from disc membranes to small syringe filters to large capsule filters.

Disposable Filtration Devices such as syringe filters and capsule filters are the most convenient means for filtering any sample volume. These devices usually consist of a membrane integrally sealed into a polymeric housing with fittings that attach easily to a syringe, tubing, or piping on the inlet and/or outlet of the device. These devices are typically pre-sterilized, ready for use, and intended primarily for one-time use. A second common configuration for filtration products is the Disc Filter.

The Disc Filter is installed by the end-user into a reusable piece of hardware made of stainless steel, glass, or a polymeric housing material. While from strictly a material cost standpoint, the membrane disc is less expensive, this method requires the end-user to install the filter integrally (i.e., without bypass) into the filter holder, and often, to sterilize the filtration system prior to use.

Filter Area, Flow Rate, and Throughput Relationship

0.2 µm Supor Membrane Devices	Filter Area (cm2)	Typical Water Flow Rate Lpm at 0.7 bar (10 psid)	Throughput* (L)
25 mm Acrodisc Syringe Filter	2.8	0.039	0.1
AcroCap Device	15	0.20	2
AcroPak 200 Capsule	200	2.35	12
AcroPak 500 Capsule	500	9.0	25
AcroPak 1000 Capsule	1000	17.4	50

*Estimated throughput when filtering RPMI media with 10% bovine calf serum.

To help determine whether a filter will be suitable for your applications, manufacturers use various tests to rate the performance of the filter under certain conditions.

Biological Safety Test is a general term used to categorize tests performed to determine whether the filter’s materials of construction are capable of inducing measurable degrees of systemic toxicity, localized skin irritation, sensitization reaction, or other biological responses. Either in vivo or in vitro test methods may be employed. Tests like the “United States Pharmacopoeia (USP) Biological Reactivity Test, In Vivo <88>” ensure that the filters can be exposed to the test solutions without causing an adverse reaction.

Pyrogenicity is the tendency of a substance to raise body temperature when injected into the body. Filtration materials that come in contact with injectable liquids must meet pyrogenicity standards and be classified as non-pyrogenic. Pyrogenicity can be determined by such standard tests as the Limulus Amoebocyte Lysate (LAL) test.

Bubble Point is a measure of the air pressure required to force liquid from the largest wetted pore of a membrane. It serves as an indication of pore size and rates the filter’s ability to serve as a particle barrier. The bubble point is dependent on the liquid used to wet the membrane. For a given pore size, the bubble point will be higher in a liquid with a higher surface tension (such as water) than in a liquid with a lower surface tension (such as isopropyl alcohol). The bubble point rating is determined when the largest pore yields a bubble; the larger the pore, the less pressure required to form the bubble. Expressed in units of pounds per square inch (psi) or bar for membranes (ASTM:F316-03, Standard Test Methods for Pore Size Characteristics).

Water Breakthrough is a measure of the amount of pressure required to transmit water through the largest pore of a dry hydrophobic filter. It serves as an indication of pore size for a hydrophobic membrane, and rates a filter’s ability to serve as an aqueous barrier. The larger the pore size, the less pressure is required to push water through the pore. Expressed in the filtration industry in units of pounds per square inch (psi) or bar.

DOP Test is a measure of the efficiency of a filter for the removal of particulate from air, based on the retention of 0.3 µm Dioctyl Phthalate (DOP) aerosol droplets, usually expressed as a percentage. A High Efficiency Particulate Air (HEPA) filter must retain at least 99.97% of 0.3 µm DOP droplets (ASTM:D2986-95A). The 0.3 µm size was chosen because particles of this size are the most difficult to retain in many air filters.

Filter Efficiency measures the percentage of particles which are removed from the fluid by the filter. In filtration of liquids, filter efficiency is given on the basis of particles at or above a certain diameter in size. In gas filtration, efficiency is stated as including all particles, including those at the most penetrating particle size. See the DOP Test for a test of efficiency in air filtration. Some filter manufacturers will report efficiency in terms of the percentage removal of the particles by weight, which doesn't reveal the number of particles that may pass through the filter. This is a type of nominal filter rating. For high-efficiency filters, this is often replaced by a beta rating. Efficiency can be calculated from a beta value as follows:

Filters rated as one micron or finer are often rated using titer reduction values or log reduction values.

Filter Life is a measure of how long a filter will last before requiring replacement or cleaning. It can be stated either in terms of time (e.g., 30 days between changes) or volume of fluid filtered (e.g.,10,000 liters processed between filter changes). A filter's actual life will depend on what particulates and conditions it is exposed to in actual usage, so filter life ratings from lab testing with standard contaminants can be used for comparison, but do not necessarily predict actual service life. To predict actual life, testing with the actual application fluids under actual operating conditions is required.

Typically, the useful life of a filter can be determined by a two-to-four fold increase of differential pressure in a constant flow system or a drop in pressure of 50 to 80% in a constant pressure system. See Throughput in "Flow Rate and Throughput" section.

As you use our web site, you will find more helpful hints for selecting the proper filter media, devices, and hardware for your applications. If you have additional questions or just want to talk more about your application, please contact our Technical Service Department — we like to hear from you!