There may be multiple reasons for filtering a sample, such as:

• Sample clarification before analysis
• Sample sterilization (using pore sizes of 0.2 µm or less)
• Particle sampling for quantitation (microscopy, gravimetry, microbiology)
• Bioburden reduction
• Observation

The filtration step may be a direct part of the analysis or process or may be used to facilitate the analysis or process by for example removing contaminants.

• Ash content
  Determined by ignition of the cellulose filter at 900°C in air. Essential concept in gravimetric applications and also a measure of general purity.
  
• Chemical compatibility
  GE Healthcare provides chemical compatibility charts to help you select filter media that will not be affected by exposure to the chemicals you use. View our chemical compatibility chart for membranes and housings.
  
• Hydrophilic
  Hydrophilic filters possess an affinity for water and can be wetted with virtually any liquid. They are typically used for aqueous solutions and compatible organic solvents.
  
• Hydrophobic
  Hydrophobic filters repel water and are thus best suited for filtering organic solvents, as well as for venting and gas filtration applications.
  
• Loading capacity
  The ability of a filter to load particulates into the fibrous matrix while maintaining an acceptablefiltration speed and workable pressure differential across the filter. In general, glass microfiber filters have higher loading capacity compared with cellulose filters, while membranes have inherently low loading capacity.
  
• Particle retention (air/gas)
  Efficiencies for air filtration are normally expressed as percent penetration or retention for a stated airborne particle size, commonly 0.3 µm dioctyl phthalate particles (DOP test).
  
• Particle retention (liquid)
  In a filtration process, the particle retention efficiency of a depth-type filter is often expressed as the particle size (in µm) which is retained at a set level. The level is usually quoted at 98% efficiency to allow for secondary filtration effects.
  
• Pore size (membranes)
  Pore sizes are usually stated in micrometers (µm) and are nominal for all membranes apart from Track-Etch and Anopore membranes for which the pore sizes are absolute.
  
• Porosity
  A measure of how porous a filter material is. For membranes, this is normally expressed as the percentage of the total volume of the filter that is composed of pores. For depth filters, the porosity is often expressed in terms of the flow characteristics in units of time (Gurley porosity).
  
• Prefilters
  Placing a prefilter upstream of a membrane filter considerably reduces the total particulate load, allowing the membrane to operate efficiently and extended its lifetime.

For a more extensive filtration glossary, please refer to our laboratory filtration brochure.

Our Whatman  brand features a large variety of filter media, such as filter papers, glass microfiber filters, and membrane filters. 

Cellulose filter papers

Our qualitative and quantitative cellulose filters are used for general filtration and exhibit particle retention levels down to 2.5 µm. We offer a wide choice of retention/flow rate combinations to meet the needs of numerous laboratory applications.

Glass microfiber filters

Borosilicate glass microfibers combine fast flow rates with high loading capacities and retention of very fine particles. Glass microfiber filters must be used flat and should not be folded. Binder-free glass microfiber filters withstand temperatures up to 500ºC and can therefore be used in gravimetric analysis where ignition is involved. Glass microfiber filters exhibit excellent compatibility with organic solvents and strong acids (apart from hydrofluoric acid) and bases. They can be used as prefilters or as final filters, and can retain particles as small as 0.7 µm.

Membrane filters

Unlike cellulose and glass microfiber depth filters, membrane filters are conventionally classified as surface filters because the filter matrix retains particles almost entirely on membrane the surface. The narrow effective pore size distribution is one of the major features of Whatman membrane filters, and the retention levels for these filters extend down to 0.02 µm.

Learn more about different membrane filter materials

Much of GE Healthcare’s filtration media are available in either flat format or in encapsulated formats, such as syringe filters, capsule filters, and filtration microplates.

• Flat filters: Circles or sheet format. Circles would normally be used in an appropriate filter holder.
  
• Prepleated filters (only for cellulose papers): Prefolded/pleated filter paper circles for use in conical filter funnels.
  
• Syringe filters: Small, encapsulated, disposable filter units intended for filtration of small sample volumes using a syringe.
  
• Syringeless filters/filter vials: Encapsulated, disposable filter units for small volume sample preparation not requiring the use of a syringe to drive the filtration step.
  
• Inline filters: Encapsulated, disposable filter units intended for connection to flexible tubing. Units are available for filtration of liquids and for air/gas applications.
  
• Capsule filters: Disposable filter units incorporating pleated filter media for larger volumes of liquid and air/gas applications.
  
• Filtration microplates: Microplates with incorporated filter media for simultaneous filtration of multiple samples.

Filter characteristics and typical properties

The selection of a laboratory filter depends on the conditions and objectives of your experiment or analytical procedure. The three most important characteristics of any laboratory filter are:

• Particle retention efficiency
• Fluid flow rate through the filter
• Loading capacity

According to your particular application, other important filter characteristics may also require examination, for example wet strength, chemical resistance, purity, and ash level. 

Typical properties of filter papers and glass fiber filters

Membrane materials

Chemical compatibility chart of membranes and housings

Filtration Methods for flat filters

Maximum practical volumes of cellulose filter paper circle sizes (quadrant folded)