Filtration – an ancient principle of separation
Filtration is a process in which substances, such as fluids, air and gasses are retained while passing through a permeable filter material. The use of hydrophobic filter materials also makes it possible to separate fluids from gasses.
- Surface filters
TRAKETCH® surface filters made by SABEU
not only feature exactly defined absolute removal rates, but also exactly defined pore sizes with narrow tolerances. When cells in cellbiology and particles in environmental analyses need to be analysed quantitatively at the membrane surface these track etched membranes are the best choice.
A PET or PC foil is being irradiated with heavy ions and the ion track is then etched off. This creates exact cylindrical pores. Depending on the process parameters the pore diameter can be defined between 0,1 and 12 µm. The membrane thickness is 8 – 36 µm.
- Surface filters
Membrane filters
are made of chemical polymers or cellulose fibers and have defined absolute removal rates. Each of these membranes consist of various pore sizes. During the microfiltration (0,02 - 10 µm) microorganisms and particles are being removed quantitatively through the internal structure of the ca. 150 µm thick membrane.
The lower removal rates are being defined by using secific microorganisms. For example, the size of Brevundimonas diminuta is approximately 0.3 µm. If this bacterium is unable to pass the filter membrane under precisely defined conditions, the removal rate of the membrane filter is 0.2 µm by definition. Substances that are greater than this removal rate will be retained.
Disadvantages may occur, if cells should remain quantitatively on the membrane surface for cell culture research, e.g. for cultivating tissue and observing the cells microscopically. The advantage is a high flow rate of fluids.
- Surface filters
Depth filters
generally consist of randomly arranged, interlaced fibers, e.g. cellulose, fiber glass or melt blown fibers. The advantages include relatively low manufacturing costs and a very high flow rate of fluids.
Possible disadvantages may arise because no precise removal rates and pore sizes are possible and fibers may potentially be extracted. Typical applications include clarification filtration (10 - 100 µm), where quantitative retention is not required and prefiltration will be needed in order to protect a final filter from being blocked by larger-sized particles.
