Pore Size, Materials, and Use Notes of Filter Membranes

Microporous filter membranes are made by using polymeric chemical materials and porogenic additives that are specially treated and coated on the support layer. In the application of membrane separation technology, microporous filter membrane is a membrane species with a wide range of applications, which is easy to use, fast, and has a wide range of applications. It is hydrophilic, non-toxic, and hygienic, and is a porous membrane filter material with a relatively uniform pore size distribution of penetrating micropores. Today, HAWACH will mainly introduce the frequently used filter membranes and their use notes.

Scanning-Electron-Microscope-Picture-of-Filter-Membrane

1. Frequently used filter membranes( based on pore size)

There are four kinds of filter membranes used frequently, with pore sizes of 0.22μm, 0.45μm, 0.8μm, and 1.0μm.

1) 0.22μm filter membrane

It can remove extremely fine particles from samples and mobile phases and meet GMP or Pharmacopoeia requirements for 99.99% sterility. Suitable for the processing of solvents and samples with high and higher requirements, such as ion-pair reagents for chromatography, and ultrapure water. Water, mass spectrometry analysis, solvent samples, etc., filtration. For less than 3μm, 3μm or larger packed color chromatography system.

2) 0.45 μm filter membrane

It can filter out most bacteria and microorganisms, is suitable for routine samples, mobile phase filtration, and can meet general chromatography requirements

3) 0.8-1.0 μm filter membrane

Removal of most insoluble particles, suitable for ultraviolet (UV) and other relatively low-demand detection, higher efficiency than 0.45μm membrane, especially for insoluble excipients, suspensions, and binders, and the use of UV detection for oral solid formulations dissolution detection.

4) 1-5μm filter membrane

Filtration of larger impurities or pretreatment of turbid solutions that are difficult to handle can be done with 1-5μm membranes before filtration with the corresponding membrane.

2. Frequently used filter membranes( based on materials)

Microporous filter membranes mainly include cellulose acetate filter membranes, mixed cellulose ester filter membranes, nylon filter membranes, polytetrafluoroethylene filter membranes, polyvinylidene fluoride membranes (PVDF), polyethersulfone filter membranes, polypropylene filter membranes (PP filter membrane). It can be divided into a hydrophilic system, an organic system (filtered water system requires pre-wetting (ethanol)), and a hydrophilic organic system (organic resistance, filtered water system does not require pre-wetting).

2.1 Membrane Materials

Different membrane materials provide different chemical compatibility, flow rates, and adsorption characteristics.

(1) Hydrophilic Membranes (Water-Compatible)

  • Polyethersulfone (PES)
    • High flow rate
    • Low protein binding
    • Ideal for biological samples
  • Cellulose Acetate (CA)
    • Low protein adsorption
    • Suitable for aqueous solutions
  • Nylon (Nylon)
    • Strong and durable
    • Broad chemical compatibility

(2) Hydrophobic Membranes (Organic-Compatible)

  • Polytetrafluoroethylene (PTFE)
    • Excellent chemical resistance
    • Ideal for organic solvents and gases
  • Polyvinylidene Fluoride (PVDF)
    • Low protein binding
    • Works for both aqueous and organic systems

(3) Specialized Membranes

  • Regenerated Cellulose (RC)
    • Broad solvent compatibility
    • Low extractables
  • Glass Fiber (GF)
    • High loading capacity
    • Used for prefiltration

2.2 Common Membrane Materials and Their Properties

Different membrane materials suit different chemical compatibilities, flow rates, and applications:

  • Cellulose Nitrate (CN):
    • Features: High protein binding, good flow rate.
    • Applications: Microbial analysis, especially membrane filtration of water samples for coliform testing.
    • Notes: Not ideal for filtering protein-sensitive solutions.
  • Cellulose Acetate (CA):
    • Features: Low protein binding, hydrophilic, high throughput.
    • Applications: Biological fluids, enzymes, and culture media.
    • Notes: Good choice for sterilizing protein-rich solutions.
  • Polyethersulfone (PES):
    • Features: High flow rate, low protein binding, broad chemical compatibility.
    • Applications: Sterile filtration of media, buffers, and biological samples.
    • Notes: Often used in the pharmaceutical and biotechnology industries.
  • Polytetrafluoroethylene (PTFE):
    • Features: Chemically resistant, hydrophobic by default.
    • Applications: Filtration of aggressive solvents, air/gas sterilization.
    • Notes: Can be made hydrophilic by treatment; ideal for organic solvents.
  • Nylon:
    • Features: Hydrophilic, mechanically strong.
    • Applications: General aqueous and organic solvent filtration.
    • Notes: Binds proteins moderately—avoid for sensitive samples.
  • Polyvinylidene Fluoride (PVDF):
    • Features: Low extractables, high chemical resistance.
    • Applications: HPLC sample filtration, sterile filtration.
    • Notes: Available in hydrophilic and hydrophobic forms.

Use notes on the filter membrane

1. Before using the microporous filter membrane, it should be cleaned and treated, soaked and washed with distilled water, and then installed in the filter, while the filtration of organic solvent is washed with ethanol immersion. For the filtration of spectrum samples, or if it is aqueous, distilled water is washed and soaked, and then distilled water is pre-filtered. If it is organic filtration, the product is first soaked in pure toluene and above 95% ethanol, respectively, overnight, then pure toluene and pure ethanol are pre-filtered, and the membrane is washed and used.

2. The filter membrane before use must be accurately selected according to the needs of the membrane pore size; the substances to be filtered out must be greater than the pore size of the membrane.

3. If using it on demanding occasions, such as the filtration of chromatography samples or reagents, wash with distilled water and then pre-filter with distilled water for the water system. Soak the product in pure toluene and 95% or more ethanol overnight, then pre-filter with pure toluene and pure ethanol, wash the membrane, and use it for the organic system filtration. The product becomes brittle significantly after entering water, and the reduction of strength in organic solvents is less significant.

4. If the filtrate contains more particles or bacteria, it should be pre-filtered, that is, pre-filtration media or a large pore size membrane filter first, to avoid too fast clogging.

5. Disinfection: The membrane is not disinfected before leaving the factory, so it needs to be sterilized before use. Disinfection in many ways, generally using hot-pressure sterilization, that is, 121 °C steam sterilization for 30 minutes. In hot-pressure sterilization, steam should be avoided directly one-way rush on the membrane to prevent negative pressure due to steam condensatio,n and the back of the membrane under pressure and breakage boiling sterilization, chemical disinfection, etc. can also be used for sterilization and filtration with bubble method to test whether the seal is tight, whether the membrane is broken due to improper sterilization. If you have any questions about the disinfected ones, pls contact HAWACH.

6. When filtering liquid, the membrane must be wetted. Apply sterile water to wet it if it becomes dry due to disinfection. If the wetting is not good to affect the flow rate, before sterilization and filtration, the membrane must be sterilized for all appliances, and then in the sterile room for sterilization and filtration, it should be strictly in accordance with the sterilization operating procedures.

7. The microporous filter membrane is flammable and not acid and alkali-resistant, not solvent-resistant, only for aqueous solution (PH5-10), oil, air, juice, wine, etc.

8. When using pressure, vacuum, and potential difference, we must pay attention to prevent the outside air in the vacuum from bacteria, and particles from inhaling contaminated filtrate, in pressurized filtration, the faster filtration speed, the higher the pressure, the faster the filtration speed, high pressure should consider whether the filter support body is intact.

9. In pharmaceutical analytical dissolution experiments, membrane adsorption is the most common drawback of using microporous filter membranes. There are many factors affecting membrane adsorption, including the diameter of the membrane, pore size, material, type of filter medium, ionic strength, pH, and the nature of the drug itself. The interaction forces, such as hydrogen bonding, van der Waals forces, and charge adsorption, between different materials of filter membranes and different compounds being filtered vary, and the degree of adsorption varies. It has also been reported in the literature that different pH buffer salts and different diluents (e.g, methanol) can reduce or eliminate adsorption.

10. In drug analysis of substances of interest experiments, attention needs to be paid to whether new impurities are generated after membrane filtration.