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Filtration in HPLC

Introduction of Syringe filter

Principles of Filtration

What is Membrane Filtration


Principles of Filtration
Filtration is a science of growing information, distinctive terminology, and proprietary knowledge. As always, if you have questions about any of these concepts or how they apply to your specific applications, contact our Technical Service Department.

We will explain some of the fundamental aspects of filtration technology and how they relate to each other and to your application. Then, we will guide you through the logic of selecting the proper filter media and devices.

Filter media have many different properties that affect the performance of the filter in certain applications. When selecting the best filter media or device for your application, consider the important properties described below.

Depth vs. Membrane Filtration
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.

Chemical Compatibility
Chemical Compatibility is defined as the ability of a filterí»s materials of construction to resist chemicals so that the filterí»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 vs. Hydrophobic
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, í░hydrophilicí▒ is used somewhat differently than in some other fields, where it refers to a material to which water clings.

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. 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.

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 (Ž╠m). 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.

Binding is the tendency of certain substances to í░stickí▒ 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.

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.

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

Thermal Stability
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
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.

Measuring a Filter's Performance
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 filterí»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 í░United States Pharmacopoeia (USP) Biological Reactivity Test, In Vivo <88>í▒ 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.

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