First, the importance of filtration in pre-chromatography processing Due to the precision of the chromatographic system and the accuracy requirements of the analysis of the results, the filtration of the sample is particularly important.
Filtration protects the chromatographic system and column, extending column life and improving data accuracy. Filtration eliminates pressure fluctuations caused by friction-generating particles and baseline fluctuations caused by irregular impurities. It is possible to eliminate interference with the detection system due to the presence of air bubbles. Therefore, a microporous membrane is usually used to filter the mobile phase, and a needle sample filter is used to filter the sample.
Second, the relevant parameters and common sense of the filter
1. Absolute Aperture: Absolute Aperture Rating refers to the distinction between pore size by challenging bacteria that are 100% cut under very stringent test conditions to a specific size. The conditions that must be specified are: test organism (or molecular) size and concentration, test pressure and detection method.
2. Air Flux: A method of measuring the passage of air through a filter. That is, the flow rate of air flowing under different pressures, different porosity and different filter areas.
3. Bubble Point: In the microporous filter industry, the specific liquid is used to wet the filter membrane, and at a specific temperature, it is required to discharge the minimum pressure of the liquid in the pores of the filter membrane.
4. Filter efficiency: The filter defines its filtration efficiency at its specific pressure with its total amount of filtration and the size of the barrier particles. In general, the lower the degree of obstruction and the lower the pressure, the greater the effectiveness of the filter.
5. Filter life: The maximum operating time of the filter under certain operating conditions. It depends on many factors, such as the nature of the filtrate, the operating temperature, the choice of filter material, and the like.
6. Hydrophilic: Hydrophilic membranes usually have a special chemical layer that allows the membrane to be wetted by water; hydrophilicity is a reference to the repulsion of water. Hydrophobic filters rarely absorb water or do not absorb water at all. In observation, it can be visually observed that the small water droplets stay on the surface of the filter membrane without being adsorbed by the surface and diffused into the water surface. The size of the hydrophobicity depends on the pore size of the filter material and the characteristics of the filter material.
7. Flow rate and flow rate: The flow rate is the total amount of filtrate passing through the filter per unit time at a specific 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 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 in the filtration. When the filter is fully loaded, the filtration pressure difference increases.
3) Porosity: refers to the ratio of the volume of all pores on the membrane to the volume of the entire membrane. Usually, the membrane has a pore area of ​​50-90%, and the flow rate is directly related to the porosity of the membrane.
Third, the choice of needle sample filter to consider factors The main function of the microporous membrane is to remove particles, bacteria and other impurities from the gas phase or liquid phase, in order to achieve the purpose of separation, purification, purification. Therefore, the following factors should be considered when selecting a filter:
1. Material of the filter (chemical compatibility): When selecting a filter, first consider the chemical compatibility. Whether the filter is resistant to acids, alkalis, organic solvents, etc.
2. Diameter of the membrane: For a column system using a particle size of 3 μm or larger, a 0.45 μm needle sample filter or membrane may be used; for a chromatography system with a filler of less than 3 μm, or a chromatography system involving microbial growth It is recommended to use a 0.2μm filter. For refractory turbid solutions, pre-filtration can be carried out using a 1-5 um filter and then continually filtered with the corresponding filter.
3. Characteristics of the sample:
1) Hydrophilic sample: A hydrophilic membrane is used. It has an affinity for water and is suitable for filtering water as a matrix solution. Useful membranes are: mixed cellulose esters, polyethersulfone (PEs), Nylon, and the like.
2) Strong corrosive organic solvent: a hydrophobic film is generally used. Filters such as polytetrafluoroethylene (PTFE), polypropylene (PP), etc.
3) Protein solution: Select a low protein adsorption filter, such as a polyvinylidene fluoride membrane (PVDF).
4) Ion Chromatography: Polyethersulfone (PEs) membranes are generally considered to be suitable for filtration of solutions with low inorganic ions.
4. When selecting the needle sample filter, in addition to considering the above factors, consider the volume of the sample (ie, what size of the needle sample filter is selected): Generally, when the sample amount is less than 2 ml, a 4 mm diameter micro filter is used. The sample volume is between 2-10ml, and a 13mm diameter filter head is used; when the sample amount is more than 10ml, a 25mm diameter filter head is selected.
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