In the laboratory, no matter whether it is an inorganic experiment, organic experiment, or physical and chemical experiment, overkill operation is essential. How to choose a membrane filter? Filtering reasonably and quickly is a skill that every experimental worker must master. A laboratory filtration selection guide is what we should know.
Based on the membrane filtration principle, recognize the importance of filtering. Filtering protects chromatographic systems and columns, prolongs their service life, and improves the accuracy of data. Filtration eliminates pressure fluctuations due to friction particles and baseline fluctuations due to irregular impurities. The interference of bubbles in the detection system can be eliminated.
Therefore, a membrane filter is usually used to filter the mobile phase and a syringe filter is used to filter the sample. Membrane manufacturers provide related parameters and common sense, let us know first.
Filter membrane selection
Made of a thin layer of a semi-permeable material, the membrane can separate substances when a driving force is applied. Membrane filtration is a good method to remove bacteria, microorganisms, particulates, and natural organic material.
Before you identify which filter material is suitable to use, you should find out the properties of the fluid or gas type which is the key element to achieve the best result of filtration in the lab.
When you are going to analyze the fluid coming through the membrane filter, membranes that have lower extractability and high flow rates can achieve your request best. If you’re working on the left behind a membrane or the retentate, membranes with a smooth surface and proper structure will be perfect for the analytical application.
In both conditions, the nature of your fluid will also play a role in your selection. Membranes with very high binding of proteins, peptides, or nucleic acids are easy to be clogged. It will interfere with the result by removing desired materials from the filtrate or obscuring what you need in the retentate.
How to select the right membrane filter? You should consider the following filter membrane selection guide factors of filter material of the filter membrane, the filter membrane compatibility chart, The pore size of the filter membrane, and the characteristics of samples.
1. Filter material of the filter membrane
When choosing a filter membrane, chemical compatibility should be considered first. Is the filter resistant to acid, alkali, organic solvents, etc?
The material of the filter membrane, filter membrane has different materials. the filter membrane is used to filter different liquids. It can be selected according to the similar compatibility of the chemical.
For example, if there are strong acids or alkalis in the liquid, it is generally used PTFE filter membranes.
If you don’t have acid or alkali, you can choose mixed cellulose ester filter membranes or polycarbonate filter PVDF filter membranes.
If there is a weak acid or weak, PVDF membranes can also be considered for the base.
PTFE membrane is compatible with almost all solvents, acids, and alkalis.
PP membrane has good compatibility, stable chemical performance, acid, and alkali resistance.
Nylon 66 membrane is compatible with most organic solvents and water-based solvents; but when used with strong acids, dichloromethane, and DMF are not recommended.
The cellulose acetate membrane is not compatible with organic solvents. They are very suitable for aqueous solutions and are especially recommended for protein and protein-related samples.
2. Filter Membrane Compatibility Chart
3. The pore size of the filter membrane
To determine the pore size of the filter membrane, such as the commonly used 0.22um filter sterilization, 0.45um bacteriological analysis, particle monitoring, and biological analysis.
For chromatographic column systems using fillers of 3um or greater size, membrane filter 0.45 or filter membrane can be used. For chromatographic systems with fillers less than 3um or involving microbial growth, a 0.22um filter is recommended. For the difficult turbidity solution, a 1-5um filter membrane can be used for pre-filtration, and then the corresponding filter membrane can be used for further filtration.
Remove the requirements of extremely fine particles in the sample and mobile phase; it can meet the requirements of 99.99% sterilization specified by GMP or Pharmacopoeia. It is suitable for the processing of high-demanding solvents and samples, such as ion-pairing reagents for chromatography and ultra-pure. Samples of water, mass spectrometry solvents, etc. are filtered. For color chromatography systems with fillers smaller than 3μm, 3μm, or larger.
Filter out most bacterial microorganisms, suitable for the routine sample and mobile phase filtration, and can meet general chromatographic requirements
Removal of most insoluble particles, suitable for UV and other analysis and detection of samples with relatively low requirements, higher efficiency than 0.45μm filter membrane, especially suitable for insoluble excipients, suspending agent, and binder dosage, and the use of UV detection of oral solid dosage dissolution testing.
Filtration of impurities with larger particles or pretreatment for difficult-to-handle turbid solutions can be performed first with a 1-5 μm filter and then with a corresponding filter.
4. Characteristics of samples
Highly corrosive organic solvents: generally use a hydrophobic membrane.
Water-borne sample: water-wet diaphragm is selected. It has an affinity for water and is suitable for filtering water-based solutions. Available filter membranes are mixed cellulose membrane, polyethersulfone (PESM), NylonM, etc.
Protein solution: choose the filter membrane with low protein adsorption, such as the PVDF filter membrane.
Ion chromatography: PES membrane is generally considered to be suitable for the filtration of solutions with low inorganic ions.
In atmospheric detection: it is generally used as a quartz filter PTFE filter.
Water quality analysis: it is usually a mixed cellulose or polycarbonate filter membrane, if the cleanliness test is commonly used, the material is nylon.
The size of the filter membrane, the most commonly used filter membrane diameter is 25mm, 47mm, 13mm, 90mm, 142mm, 293mm, and so on. You also should consider the sample size when using a syringe filter(that is, the size of the syringe filter to choose): usually, if the sample size is less than 2ml, a miniature filter head with a diameter of 4mm should be selected. The sample size is between 2 and 10ml, and a filter head with a diameter of 13mm is selected. When the sample size is greater than 10ml, a filter head with a diameter of 25mm is selected.
Related parameters and common sense of membrane filter
In order to better understand syringe filter membrane selection, now we first learn the relevant parameters and common sense of the lower membrane filter.
1. Air flux
Is a method of measuring air passing through a filter. That is, in the case of different pressure, different porosity, and different device area, the flow of air through.
2. The bubble point
In the microporous membrane industry, the use of a specific liquid to wet the roll membrane, and at a specific temperature, the need to squeeze out of the membrane hole liquid minimum pressure.
3. Absolute aperture
The absolute pore size class is defined by identifying pore size by 100% interception of challenging bacteria of a particular size under very stringent test conditions. Among the conditions that must be specified are the measurement of organic (or molecular) size and concentration, measurement of pressure, and inspection.
4. Filter function
Filters are defined in terms of their overdose and blocking particle size at specific pressures. In general, the lower the obstruction and pressure, the more effective the filter.
5. Hydrophobic and hydrophilic membrane
Hydrophilic membrane filter usually has a special chemical layer that allows the membrane to be infiltrated by water; Hydrophobicity is a reference for repulsion to water. Hydrophobic membrane filters rarely do not absorb water at all. In observation, small droplets of water can be seen to stay on the surface of the filter membrane without being absorbed by the surface and diffused into the water surface. The size of hydrophobicity depends on the aperture of the filter material and the characteristics of the membrane filter material.
The maximum lifetime of the 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.
7. Flow rate and pore rate
Flow rate is the amount of filtrate passing through the membrane per unit of time at a specific temperature and pressure. The flow rate is closely related to the surface properties of the membrane filter. Flow rate and pore rates are two important parameters of filter material and design performance. This performance depends on the following aspects :
(1) Pressure difference is the pressure difference between inlet and outlet infiltration. When the filter is fully loaded, the filtration pressure difference increases. (2) Viscosity: viscosity determines the flow of the liquid. The higher the viscosity of the liquid (at a given temperature and pressure), the lower the flow rate. The higher the pressure required to achieve the same flow rate. (3) Pore rate: refers to the proportion of the volume of all holes in the filter membrane to the total volume of the filter membrane. Usually, the filter membrane has a 50-90% pore area, and the flow rate is directly related to the pore rate of the membrane.