When selecting a syringe filter an important consideration is the type of material that both the filter and housing are made from. A syringe filter should be chemically compatible with the sample to be filtered.
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. The filter material should not shed particles or fibers, or add extractables to the sample being filtered .
Extractables are substances that may leach or otherwise come off a syringe filter and into 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. Careful manufacturing procedures can eliminate the need to flush filters.
Extractables can affect filtration in almost every application:
- In HPLC analysis, extractable chemicals can add extraneous peaks
- In cell culture, they can cause cytotoxicity (kill cells)
- In environmental analysis, they can appear as additional contaminants
Another consideration when selecting a filter is binding. This is the tendency of certain substances to “stick” to the filter medium (or other filter components) and be removed from the fluid. This is usually based on charge.
Binding can sometimes be a desirable characteristic, for example, when using a syringe filter with a chromatography membrane, that could either be a strong anion or cation exchanger. However, binding can also be an undesirable characteristic, as in the case of protein binding during filtration, which can lead to the loss of valuable products. Binding can also be an issue when performing analytical sample preparation, potentially reducing detectable levels of a compound of interest in a sample.
Membranes can be hydrophilic or hydrophobic in nature. Hydrophilic filters can be wetted with virtually any liquid and are the preferred filters for aqueous solutions, if chemically compatible. Hydrophobic filters will not wet in water but will wet in low surface tension liquids, for example, organic solvents such as alcohols. Hydrophobic filters are best suited for gas filtration, low surface tension solvents, and venting.