Syringe Filters

Syringe filters are single use, self contained, filtration devices that are typically used to remove contaminating particulate from liquids or gasses.

 

When selecting the correct syringe filter for an application there are a number of factors to consider, these include:

 

• Filter and housing materials
• Pore size
• Effective filtration area 
• Hold-up volume

Acrodisc Syringe Filters

 

In 1974 Pall revolutionized sample preparation with the development of the Acrodisc syringe filter. Pall has continued to drive innovation within the syringe filter market, introducing novel and unique products:

 

• Acrodisc® MS Syringe Filters that are certified for low extractables in highly sensitive LCMS methods
• Acrodisc Syringe Filters with universal membranes that are suitable for all HPLC/UHPLC analytical sample preparation, featuring a universally solvent compatible membrane in a chemically compatible housing, users need only select pore size and device size
• Acrodisc Syringe Filters optimized for scale up, devices constructed of the same materials as larger-capacity capsules and cartridges
• Acrodisc White Blood Cell (WBC) syringe filter, designed specifically to capture and recover Leukocytes while separating red blood cells and platelets, thereby providing scientists with a simple and efficient method to isolate cells needed for use in a variety of research applications or prepare leukocyte depleted blood for analysis

 

Pall supplies both sterile and non-sterile syringe filters that can be used across a range of laboratory methods. The comprehensive family of Acrodisc syringe filter products that Pall supplies gives laboratory scientists a wide selection to choose from, helping customers obtain the best possible results for their demanding applications.

Syringe filters are available in a 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 and greater the throughput. Throughput is the amount of fluid able to pass through a filter prior to plugging.

 

The diameter of a syringe filter can be a good indicator of the EFA of the product, however due to advances in design and molding technologies the EFA of two syringe filters of the same diameter can be significantly different. For example, Pall supply both a traditional ring molded 25 mm syringe filter and the Premium Syringe Filter (PSF) 25 mm syringe filter. The PSF syringe filter has a much larger effective filtration area compared with the older traditional model, 3.9 cm² compared to 2.8 cm². Therefore, this gives the PSF syringe filter a much improved throughput and flow rate.

 

For difficult to filter samples that may contain a high particulate load Pall supplies Acrodisc syringe filters with an integrated prefilter configuration.

 

A dual layer filter combines different membranes of different pore sizes (larger layered over smaller), or combines depth media and a membrane filter, essentially combining a prefilter and a final filter to create self-contained serial filter units. This configuration leads to a much more efficient filtration, increasing both throughput and flow rate.

 

Syringe filters which contain a combination filter design can offer an economical alternative to using an individual prefilter and a final filter. They also offer greater ease of handling and reduced waste.

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.

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 under specific conditions. Pore size ratings are usually stated in Micrometers (μm).

 

Syringe filters with different pore size ratings are suitable for different applications. For example, syringe filters containing 0.45 μm are typically used for HPLC sample preparation and 0.2 μm is used for UHPLC. In life science applications 0.1 and 0.2 µm pore sizes provide sterilization of buffers, culture media, and additives. Larger 0.45, 0.8, 1.2 and 5 µm membrane pore sizes can be used for prefiltration and/or clarification.

Non-sterile Syringe Filter Applications

 

• HPLC certified for low UV absorbing extractables 
• IC certified for low levels of chloride, nitrate, phosphate, and sulfate 
• Mass spectrometry certified for low levels of extractable material 
• Automation-certified to be compatible with automated and semi-automated workstations
• Dissolution testing
• Gas filtration

Sterile Syringe Filter Applications

 

• Filtration of cell and tissue culture media
• Filtration of viral suspensions
• Clarification of biological, protein, enzyme, probe and hybridization buffers and other aqueous samples
• Filtration of aqueous/organic solutions
• Leukocyte depletion, capture and recover

Another aspect of choosing the right filter size is the hold-up volume. This is the volume of liquid remaining in the filter after use. A filter with a low hold-up volume is recommended for use with expensive fluids or those with limited availability.

 

Here are examples of the hold-up volumes of Pall’s non-sterile syringe filter range:

 

Volume to be filtered	Acrodisc syringe filter type	Typical hold-up volume
< 2 mL	4 mm Acrodisc Syringe Filter	< 10 μL
< 10 mL	13 mm Acrodisc Syringe Filter (Minispike)	< 14 μL
< 10 mL	13 mm Acrodisc Syringe Filter	< 30 μL
< 125 mL	25 mm Acrodisc PSF Syringe Filters	< 200 μL