Liquid chromatography is a qualitative and quantitative analytical instrument with high precision and sensitivity. During the use of liquid chromatography to detect the samples, the purity requirements for the sample and mobile phase are quite high. The filter membrane plays a key role in the removal of impurities. It is necessary to properly choose the filter membrane depending on the mobile phase solution.
Filter membranes have different types and apertures for removing bacteria and impurities, which is important to protect the chromatographic system and column. As we all know, the filter membrane generally used in the lab is the microporous filter membrane, which is an essential product in the precise filtration process. It is necessary for the customers to carefully choose the correct filter membrane for an accurate filtration result in terms of its material and aperture.
Factors to be considered in the selection of membranes
The first thing to consider is chemical compatibility, that is, whether the filtration membrane is acid, alkali, organic solvents, and so on.
1. Membrane Material
Depending on the different materials used, there are mainly five different types of filter membranes, including PTFE, PES, PVDF, Nylon, RC, and GF material. Basically, the PTFE filter membrane is available for all the solvents, such as acids and salts, due to its superior chemical compatibility. The PES membrane is suitable for the filtration of aqueous samples rather than the polar solution and strong acid solutions, like ketones, and ester solutions.
The PVDF membrane could be specially applied in the purification of suspended particles in the air and sterilization in the fermentation industry with its high flow rate feature. The Nylon filter membrane could be used for most organic solvents, aqueous solutions, and strong acids as well, but not for dimethylformamide. The RC membrane is available for aqueous samples and organic solutions. The GF filter membrane is often used as a pre-filter before a more accurate filtration.
2. The apertures of the filter membrane
There are three most common aperture sizes provided for different filtration purposes – pore size 0.22μm, 0.45μm, and 1.0μm. The 0.22μm membrane is suitable for the filtration of extremely fine particles in the samples. The 0.45μm membrane could filter most microorganisms, which can satisfy the filtration of regular samples and the general chromatographic requirements. The 1.0μm membrane is used for the filtration of most insoluble particles.
3. Characteristics of the filter sample
a. Water system filter membrane Uniform pore size, high porosity, no media shedding, thin texture, low resistance, fast filtration speed, low cost, but not resistant to organic solutions and strong acids and bases.
b. Aqueous Filter Membrane
The aqueous filter membrane is very suitable for water-based solutions. It has high chemical and thermal stability, which could highly resist to the strong acid and alkaline solutions. Meanwhile, it possesses high mechanical strength and a fast flow rate. It also has low adsorption capacity to proteins. Nevertheless, it is not resistant to organic solvents.
c. Organic Filter Membrane
The organic filter membrane is a hydrophobic membrane, which is used for organic solvents and air filtration. It is resistant to acids, alkaline, and antioxidants because of its stable chemical properties. Therefore, it is not only perfect for aqueous solutions containing acid and alkaline, but also for organic solvents such as alcohols, hydrocarbons, lipids, phenols, ketones, etc.
d. Nylon filter membrane The Nylon filter membrane has good hydrophilicity and quite high resistance to acid and alkaline solvents. Like the organic filter membrane, it is usable for both aqueous solutions containing acid and alkali and organic solvents. Good temperature resistance, high-pressure sterilization of saturated steam for 30 minutes, the highest working temperature 60 ℃, good chemical stability, can withstand dilute acid, dilute alkali and other organic and inorganic compounds and solvents.
e. PVDF filter membrane Great mechanical strength and tensile strength, superior heat resistance and chemical inertness, low protein adsorption, strong hydrophobicity.
f. PTFE filter membrane Wide chemical compatibility, good temperature resistance, strong acid, and alkali resistance, suitable for strong chemical corrosive solvents and oxidants.