# In-Depth Study of Several Filter Membrane Pore Size Tests

## First, the bubble point method

1. Test principle: When the channel is blocked by a liquid wetting agent, due to the surface tension of the wetting agent, if the hole is opened by gas at this time, it is necessary to apply a certain pressure to the gas, and the smaller the hole is, the more the hole is opened. The pressure required for the hole is greater. By comparing the relationship between the pressure and the gas flow rate of the porous material in the dry and wet state, the pore size distribution of the sample can be obtained by calculation according to a certain mathematical model.
2. Aperture test range: 20nm-500um
3. For the gas-liquid discharge method, since the gas-liquid interface tension is large, the smaller pore diameter can only be measured by increasing the gas pressure, but the high pressure is liable to cause a series of problems such as gas leakage, sample deformation, and pressure drop. The drawback of the bubble point method is that it is not suitable for measuring filter membrane materials with small pore sizes.

## Second, suspension filtration

Test principle: the spherical particle suspension is used as the medium, and the sample to be tested is used for cross-flow filtration. The pore size distribution can be calculated by comparing the particle size distribution of the original suspension and the permeate, and the largest particle in the permeate is calculated. The diameter is the maximum pore size of the porous material.

## Third, the liquid and liquid elimination method

1. The test principle is similar to the bubble point method, and is also used to measure the pore throat, except that another liquid that is incompatible with the wetting agent is used instead of the gas as the cell opener;
2. Test principle: 10nm-200um
3. Test filter membrane material pore size advantages and disadvantages: Because the liquid-liquid interface tension is small, only a small pressure is required to measure a large aperture, so the pressure measurement error is large, and the optimal measurement range is 10 nm to 200 μm.

Bubble pressure method (gas-liquid displacement) pressure-aperture correspondence
Calculation formula: D=4γCosθ/△P

## How to Check If the Filter Membrane Is Qualified?

1. Perform integrity test before filtration: This method requires the use of a syringe filter, first soaking the filter membrane with liquid, and then using a syringe to blow air into the filter. If the air cannot pass through the filter, the filter membrane is good.

2. If the filtration culture medium uses a large filter membrane, it is not feasible to perform the above-mentioned integrity test before filtration, and it can only be observed by the naked eye. Therefore, it is very important to do the post-filtration integrity test. The specific method is:

(1) After the filtrate is filtered, slowly open the filter to see if it has any problems such as cracks and small holes.
(2) Aspirate a small amount of culture medium for the test. A small amount (2 ~ 5ml) of the culture solution is added to the small culture flask, cultured in the incubator, for 24 to 48 hours without turbidity, or air bubbles, and can be used with confidence; or a small amount of culture medium for cell culture, 24 hours of good cell growth, indicating that this culture solution is no problem, and you can use it with confidence. There is no problem with the culture solution, indicating that the filter membrane is qualified, and the contamination during cell culture is greatly reduced.

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