What Is a Microporous Filter Membrane?
Introduction of Membrane Filters
The microporous filter membrane is a kind of macromolecule membrane material, which has many uniform micropores, and its pore size varies from 0.025 to 14 microns. Its filtration mechanism is mainly physical screening.
Nowadays, membrane filtration is a rapidly developing science, which could be divided into microfiltration, ultrafiltration, nanofiltration, and reverse osmosis.
Filtration of the sample is particularly important due to the precision of the chromatographic system and the accuracy requirements for the analysis of the results. Filtration protects the chromatographic system and column, extending column life and improving data accuracy. Filtration eliminates pressure fluctuations caused by friction-generating particles and baseline fluctuations caused by irregular impurities. It is possible to eliminate interference with the detection system due to the presence of air bubbles.
Therefore, a microporous membrane is usually used to filter the mobile phase, and a syringe filter is used to filter the sample. There are many kinds of microporous membranes, such as cellulose acetate membranes filter, polypropylene membranes filter, and polytetrafluoroethylene membranes filter.
The advantages of microporous filter membranes are high porosity, fast filtration speed, small adsorption, no retention of drug solution, no influence on drug content, simple equipment, easy removal, etc. The disadvantages are poor acid and alkali resistance, poor adaptability to some organic solvents such as propylene glycol, and the retained particles easily blocking the filter membranes and affecting the filtration rate. The membrane can be used only after other filters have been preliminarily filtered.
Microporous filtration membranes are divided into organic membranes and water membranes. Organic membranes can pass through aqueous solutions, while water membranes can not pass through organic solutions.
There are different aperture specifications. One has a smooth surface, and some have no positive or negative sides. In general, 2,3 ml of primary filtrate is discarded before filtration, so as not to interfere with the detection of filter membrane impurities. After the steps of filtration, the adsorption of drugs on the lower filter membrane was investigated. The membrane filters are comparatively thin and are made of synthetic polymer. It is also called a screen filter which plays the separating role by reserving the particles which are larger than the aperture of the membrane.
On the other hand, the particles which are smaller than the standard aperture could go through the membrane and be captured by other mechanisms in the membrane structure. So it well suits the key application which needs the maximum particle recovery rate. There are many advantages of membrane filters. For example, the absolute submicron aperture rating is probable. Bacteria and particles could be reserved by the pore size. In addition, the extractability is lower than usual and NDT (Non-destructive Testing) keeps at the average level.
Moreover, large sample testing is permitted. Besides, the result could be got much quicker than that of the traditional MPN standard method. However, there exist disadvantages as well. The main drawbacks are shown as the lower flow rate and higher cost than depth filtration.
Membrane Separation Technology
Membrane separation technology is considered to be “one of the most promising and promising major high-tech developments in the 21st century, and it plays a strategic role in industrial technological transformation.”
Membrane separation technology refers to the separation, classification, purification, or concentration of liquid or gas mixed with two components or even multiple components by the osmosis of a specific membrane driven by external energy or chemical level difference.
Polyvinylidene fluoride (PVDF) is a common membrane material. It has excellent anti-oxidation activity, strong thermal stability, high mechanical strength, and good film-forming properties. It is widely used in people’s production and life.
However, the strong hydrophobicity of PVDF itself makes it easy to block membrane pores in the membrane and retentate in the water treatment system, which results in a decrease in water flux and anti-pollution performance during membrane filtration. Multiple cleanings reduce the service life of the membrane. These shortcomings of the membrane constrain the development and application of membrane separation technology.
Therefore, in order to improve the separation performance of the membrane, increase the service life of the membrane, improve the operational stability, enhance the anti-pollution ability, and improve the hydrophilic properties of the membrane is particularly important. To this end, researchers often improve the membrane surface and the properties inside the membrane by a hydrophilic modification to improve the filtration capacity and anti-pollution performance of the PVDF membrane.
Filter Parameters and Common Sense
1. Filter life: The maximum operating time of the syringe filter under certain operating conditions. It depends on many factors, such as the nature of the filtrate, the operating temperature, and the choice of filter material.
2. Hydrophilic: The hydrophilic membrane usually has a special chemical layer so that the membrane can be wetted by water; hydrophilicity is a reference to the repulsive force of water. Hydrophobic filters rarely absorb water or do not absorb water at all.
3. Hydrophobicity: A property that describes a membrane or membrane material that is not readily soluble in water. Hydrophobic filter membranes rarely absorb water or do not absorb water at all.
Precautions for Using a Filter Membrane
- Before use, the pore size of the membrane filter must be selected accurately according to the need, and the material to be filtered must be larger than the pore size of the membrane.
- Before use, they should be cleaned, i.e. soaked in distilled water at 70-80 degrees for 4 hours or in distilled water at room temperature for 12 hours.
- When there are more particles and bacteria in the filtrate, they should be pre-filtered, i.e. filtered with a pre-filter medium or large pore size filter membrane filter first, so as to avoid too fast blockage.
- Disinfection: Usually the membrane filter is not disinfected before leaving the factory, so it needs to be sterilized before use. Hot-pressure sterilization, i.e. steam sterilization at 121 °C for 30 minutes, is usually used.
When sterilizing by hot-pressing, it is necessary to avoid the direct one-way rush of steam on the film and to prevent the back of the film from being damaged by negative pressure caused by steam condensation. The air bubble method is used to test whether the seal is tight and whether the filter membrane is damaged due to improper disinfection.
