Key Parameters of the Filter Membranes

The importance of filtration in chromatographic pre-treatment

Due to the precision of the chromatography system and the requirement for accuracy of the result analysis, the filtration of the sample is especially important. Filtration can protect the chromatographic system and column, extend the life of the column, and improve the degree of data. Filtration eliminates pressure fluctuations due to the frictional generation of particles and baseline fluctuations due to irregular impurities. Therefore, microporous filter membranes are usually used to filter the mobile phase.

The main function of microporous membranes is to retain particles, bacteria, and other impurities from the gas or liquid phase for separation, purification, and refinement.

Key parameters of the filter membranes

Pore size: The pore size class refers to the pore size differentiated by the 100% interception of a particular size of challenge bacteria under very strict test conditions. The conditions that must be specified are test organism (or molecule) size and concentration, test pressure, and detection method. For example:
- Pore size 0.45μm filter membrane:
  Used for regular sample mobile phase filtration, capable of meeting general chromatographic requirements. For example, it is used for chromatographic systems with 3μm or larger particle size packing.
- Pore size 0.22μm filter membrane:
  Capable of removing very fine particles from samples and mobile phases as required. E.g., for chromatographic systems with <3μm packing.
- Pore size 1-5μm filter membrane:
  Commonly used for turbid solutions that are difficult to handle, which can be pre-filtered with 1-5μm filter membrane and then re-filtered with the corresponding filter membrane.
Bubble point: In the microporous membrane industry, the minimum pressure at which liquid must be expelled from the pores of a membrane when the membrane is wetted with a specific liquid and at a specific temperature.
Air flux: A method of measuring air flux filters. That is, at different pressures, different pore rates, and different filter areas, the flow of air through the flow.
Filter efficacy: The filter’s filtration efficacy is defined by the total amount of filtration and the size of particles obstructed under its specific pressure. Generally speaking, the lower the level of obstruction and pressure, the greater the effectiveness of the filter.
Filter material life: The maximum time a filter can be used under specific operating conditions. For the service life, there are many different factors affecting it, for instance, the specific material, the operation condition, the nature of the filtrate, etc.
Hydrophilic or hydrophobic: Hydrophilic membranes usually have a special chemical layer that allows the membrane to be saturated with water; hydrophilicity is a reference to the repulsion of water. Hydrophobic membranes rarely or do not absorb water. Small droplets of water can be visually observed resting on the surface of the membrane without being adsorbed by the surface and spreading into the water’s surface. The filter material’s pore size and characteristics decide the hydrophobicity.
Flow rate and flow volume: Flow rate is the total amount of filtrate passing through the membrane per unit of time at a given temperature and pressure. The flow rate is closely related to the surface properties of the membrane. Flow rate and flux are two important parameters for the performance of the filter media and design. This performance depends on several aspects.
- (1) Viscosity: It affects liquid flow. The higher the liquid’s viscosity (at a certain temperature and pressure conditions), the lower the flow rate.
- (2) Pressure difference: The pressure difference between the inlet and outlet infiltration, when the full load of the filter, the filter pressure difference increases.
- (3) Pore rate: It is the ratio of the volume of all the holes in the filter membrane to the volume of the entire membrane.