Relevant Parameters and Common Sense of Filter Membrane

The importance of filtration in chromatographic pretreatment

Due to the precision of the chromatographic system and the requirements for the accuracy of the result analysis, the filtration of samples is particularly important. Filtration can protect the chromatographic system and the chromatographic column, extend the service life of the column, and improve the degree of data.

Filtering can eliminate pressure fluctuations caused by particles generated by friction and baseline fluctuations caused by irregular impurities and can eliminate the interference of the detection system due to the presence of bubbles. Therefore, a filter membrane disc is usually used to filter the mobile phase.

Select filter membranes of different types and pore sizes to filter the mobile phase and samples to clarify and eliminate bacteria and remove impurity particles. It is of great significance to protect the chromatographic system and the chromatographic column. So we should know the related parameters of the filter membrane.

Related parameters of the filter membrane

1. Absolute aperture
The absolute pore size grade refers to distinguishing the pore size by 100% interception of challenge bacteria of a certain size under very strict test conditions. Among the conditions that must be specified are the size and concentration of the test organism (or molecule), the test pressure, and the test method.

2. Air flux
It is a method for measuring air passing through a filter. That is, under different pressures, different porosities, and different filter areas, the flow rate of air.

3. Bubble point
In the microporous filter membrane industry, a specific liquid is used to wet the filter membrane, and at a specific temperature, the minimum pressure of the liquid that has to be squeezed out of the filter membrane pores is called.

4. Filter efficiency
Under its specific pressure, the filter defines its filtration efficiency in terms of the total amount of filtration and the size of hindered particles. In general, the lower the degree of obstruction and pressure, the greater the efficiency of the filter.
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5. Filter life
Under certain operating conditions, the longest use time of the filter. It depends on many factors, such as the nature of the filtered fluid, the operating temperature, the choice of filter material, etc.

6. Affinity/Hydrophobicity
Hydrophilicity is defined as hydrophilic. Hydrophilic membranes usually have a special chemical layer that allows the membrane to be wetted by water; Hydrophobicity is a reference to the repulsion of water. Hydrophobic membranes rarely absorb water completely. In observation, it can be visually observed that small water droplets stay on the surface of the filter membrane without being absorbed by the surface and diffusing into the water surface. The degree of hydrophobicity depends on the pore size of the filter material and the characteristics of the filter material.

7. Filter material life: the longest use time of the filter under certain operating conditions. Many factors will affect it, for instance, the filter material, nature, and its operating temperature.

8. Flow rate and flow
Flow rate is the total amount of filtrate passing through the filter membrane per unit of time at a specific temperature and pressure. The flow rate is closely related to the surface properties of the filter membrane. Flow rate and flow rate are two important parameters of filter media and design performance. This performance depends on the following aspects:
(1) Viscosity: Viscosity determines the difficulty of liquid flow. The higher the viscosity of the liquid (under certain temperature and pressure conditions), the lower the flow rate. The higher the pressure required to achieve the same flow rate.
(2) Pressure difference: The pressure difference between the inlet and the outlet during filtration. When the filter is at full load, the difference in filtration pressure increases.
(3) Porosity: refers to the proportion of the volume of all pores in the filter membrane to the volume of all filter membranes. Usually, the membrane has a 50-90% pore area, and the flow rate is directly related to the membrane’s porosity.

Therefore, the following factors should be considered when choosing a filter

1. The material of the membrane (chemical compatibility)
When choosing a filter, chemical compatibility must first be considered. Whether the syringe filter is resistant to acids, alkalis, organic solvents, etc.

2. The pore size of the filter membrane
For chromatographic column systems using 3um or larger particles, 0.45um syringe filters or membranes can be used; for chromatographic systems that use less than 3um packing, or chromatographic systems involving microbial growth, 0.2um filters are recommended. For turbid solutions that are difficult to handle, you can use a 1-5um filter membrane for pre-filtration, and then use the corresponding filter membrane for continuous filtration.

3. Characteristics of the sample
(1) Hydrophilic sample: Use the hydrophilic membrane. It has an affinity for water and is suitable for filtering water-based solutions. The available membranes are mixed cellulose membrane, polyethersulfone (PES), Nylon, etc.
(2) Strongly corrosive organic solvents: generally hydrophobic membranes are used. Filter membranes such as PTFE, polypropylene (PP), Microporous Glass Fiber Membrane Filters, etc.
(3) Protein solution: choose a filter with low protein adsorption, such as a PVDF filter.
(4) Ion chromatography: polyethersulfone (PES) is generally considered to be more suitable for the filtration of solutions with low inorganic ions.

4. When selecting a syringe filter
In addition to considering the above factors, it is also necessary to consider the sample volume (that is, what size syringe filter is selected): when the sample volume is less than 2ml, a 4mm diameter microfilter is used. When the sample volume is between 2-10ml, use a 13mm diameter filter head. When the sample volume is greater than 10ml, use a 25mm diameter filter head.