This article will introduce the selection of filter membranes when performing mobile phase filtration in chromatography experiments. Before that, first, understand the purpose of filtering.
Importance of filtering
Due to the precision of the chromatographic system and the accuracy requirements for the analysis of the results, the filtration of the sample is particularly important.
Filtering protects the chromatographic system and column, prolongs the life of the column, and improves the accuracy of the data. Filtering eliminates pressure fluctuations due to friction-generated particles and baseline fluctuations due to irregular impurities. The interference to the detection system due to the presence of air bubbles can be eliminated. Therefore, the mobile phase is usually filtered with a microporous membrane, and the sample is filtered with a syringe filter.
Related parameters and common sense of filter membrane
Absolute pore size:
The absolute pore size rating refers to the distinction of pore size by retaining 100% of challenge bacteria of a certain size under very stringent test conditions. Among the conditions that must be specified are test organism or molecular size and concentration, test pressure, and detection method.
Air flux:
A method of measuring air passing through a filter. That is, the flow rate of air flowing through under different pressures, different porosity, and different filter areas.
Bubble point:
In the microporous membrane industry, the minimum pressure required to expel the liquid from the membrane pores is required to wet the membrane with a specific liquid and at a specific temperature.
Filter Efficiency:
A filter at its specific pressure, and its total filtration and hindering particle size define its filtration efficiency. In general, the lower the degree of obstruction and the lower the pressure, the more effective the filter will be.
Filter life:
The maximum use time of the filter under specific operating conditions. It depends on many factors, such as the nature of the filtrate, the operating temperature, the choice of filter material, etc.
Hydrophilic:
The hydrophilic filter membrane usually has a special chemical layer that allows the membrane to be wetted.
Hydrophobicity:
Hydrophobicity is a reference to the repulsive force against water. Hydrophobic filters rarely completely not absorb water. In the observation, it can be seen that the small water droplets stay on the surface of the filter membrane and will not be adsorbed by the surface and diffuse to form a water surface. The size of the hydrophobicity depends on the pore size of the filter material and the original characteristics of the filter membrane.
Flow rate:
The 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 is an important parameter for filter media and design performance. This performance depends on the following aspects:
- Viscosity: Viscosity determines how easily a liquid can flow. The higher the difficulty of the liquid (under certain temperature and pressure conditions) the lower the flow rate.
- Pressure difference: The pressure difference between the inlet and the outlet during filtration. When the filter is fully loaded, the filtration pressure will increase.
- Porosity: Refers to the proportion of the volume of all pores on the membrane to the volume of the entire membrane. Usually, the filter membrane has a porosity of 50 to 90%, and the flow rate is directly related to the porosity of the membrane.
Factors to consider when choosing a filter membrane
The main function of the microporous filter membrane is to retain particles, bacteria, and other impurities from the gas phase or liquid phase to achieve the purpose of separation, and purification. Therefore, the selection of filter membranes should consider the following factors:
- Membrane material (chemical compatibility): When selecting a filter membrane, chemical compatibility must be considered first. Whether the filter membrane is resistant to acid, alkali, organic solvent, etc.
- The pore size of the filter membrane: For chromatography column systems using 3μm or larger particle size packings, a filter membrane with a pore size of 0.45μm can be used; for chromatography systems using fillers less than 3μm, or those involving microbial appreciation, it is recommended to use 0.22 μm filter. For difficult-to-handle turbid solutions, pre-filter with a 1-5 μm pore size filter, and then filter with the corresponding filter.
- Characteristics of the samples
- Aqueous solution samples: select hydrophilic membranes. It has an affinity for water and is suitable for filtering water-based solutions. Available filters are MCE filter membrane, polyethersulfone filter membranes, nylon filter membrane, etc.
- Strong corrosive organic solvent: the hydrophobic membrane is generally used. Such as PTFE, PP, and other materials.
- Protein solution: choose a filter with low protein adsorption, such as PVDF.
- Ion chromatography: PES filter membranes are generally considered to be more suitable for the filtration of solutions with low inorganic ions.
Several commonly used filter membranes
Nylon membrane
Features: good temperature resistance, can withstand 121°C saturated steam autoclave for 30mins, maximum working temperature 60°C, good chemical stability, can withstand dilute acid, dilute alkali, alcohol, ester, oil, carbon Various organic and inorganic compounds such as hydrogen compounds, halogenated hydrocarbons, and organic oxides.
Application: electronics, microelectronics, semiconductor industry water filtration, tissue culture medium filtration. Liquid filtration, beverage filtration, and high-purity chemical filtration. Filtration of aqueous and organic mobile phases.
Polyvinylidene fluoride (PVDF)
Features: Membrane has high mechanical strength, high tensile strength, good heat resistance, and chemical stability, low protein adsorption rate; strong negative electrostatic and hydrophobicity; hydrophobic and hydrophilic two forms. Intolerant of acetone, DMSO/THF/DMF/dichloromethane, chloroform, etc.
Application: Hydrophobic PVDF filter membrane is mainly used in gas and steam filtration, high-temperature liquid filtration;
Hydrophilic polyvinylidene fluoride membrane is mainly used in the purification and filtration of tissue culture medium, additives and other sterilizing filtration solvents and chemical raw materials, aseptic treatment of reagents, filtration of high-temperature liquids, etc.
Mixed Cellulose Esters (MCE)
Features: relatively uniform pore size, high porosity, no medium shedding, thin texture, low resistance, fast filtration, minimal adsorption, low cost of use, but not resistant to organic solutions and strong acid and alkali solutions.
Application: The pharmaceutical industry needs autoclaved water injection, and large infusion to filter out particles.
For the sterilization of heat-sensitive drugs (biochemical preparations such as insulin ATP, coenzyme A, etc.), use a 0.45-micron or 0.22-micron filter.
Analysis and determination are of particles and oil insoluble in solution, determination of water pollution index.
Applied to somatic hybridization and mitochondrial complementation to predict heterosis research, etc.
Polypropylene (PP)
Features: without any binder, stable chemical properties, flexible, not easy to break, high-temperature resistance, can withstand high-pressure sterilization.
Non-toxic, tasteless, acid, and alkali-resistant.
Application: It is suitable for all kinds of coarse and fine filters and pleated filter elements.
Suitable for plate and frame filter press membranes in beverage, pharmaceutical, and other industries.
It is suitable for the support and pretreatment of reverse osmosis membranes and ultrafiltration membranes.
The PP filter membrane is non-toxic and can be widely used in medicine, the chemical industry, food, beverage, and other fields.
Hydrophobic, especially for gas filtration.
Polyethersulfone (PES)
Features: The hydrophilic membrane has the characteristics of a high flow rate, low extractability, good strength, no adsorption of proteins and extracts, and no pollution to samples.
Application: Low protein adsorption and high drug compatibility, specially designed for biochemical, inspection, pharmaceutical, and sterilization filtration devices.
Polytetrafluoroethylene (PTFE)
Features: The broadest chemical compatibility, resistant to strong solvents such as DMSO, THF, DMF, dichloromethane, chloroform, etc.
Application: Filtration of all organic solutions, especially strong solvents that cannot be tolerated by other membranes.
Introduced here, presumably, everyone has a certain understanding of how to choose a filter membrane, pay attention to HAWACH, we will share more laboratory tips from time to time.
