Sample desalting technology In immunoenzymatic technology, antigen, antibody, enzyme, and enzyme-labeled antibody (or antigen) often encounter the experiment of desalting during separation and purification. After desalting, unnecessary inorganic salt molecules in the protein solution are removed, thereby further purifying and stabilizing the protein. Commonly used desalination techniques are as follows: 1. Dialysis is suitable for protein solutions that are not too large in volume, it is simple and convenient, and the effect is ideal but time-consuming. 2. Gel filtration is suitable for medium-volume protein solutions. It is simple and fast, and the effect is ideal, but the volume of protein solution after desalting increases more. 3. Ultrafiltration is suitable for large protein solutions. It has a fast salt removal rate and has a concentration effect. However, due to the membrane adsorption effect, more samples are lost. 4. The commonly used desalination method in dialysis laboratories is dialysis. The dialysis bag used in the dialysis method is a bucket membrane. The semi-permeable membrane has the property of allowing small molecules or ions to pass through, while protein molecules cannot pass through. The protein solution is put into a dialysis bag and soaked in distilled water. Due to the difference in the concentration of diffusible ions inside and outside the membrane, the inorganic salt molecules in the protein solution diffuse into the distilled water outside the membrane. If the distilled water is changed frequently, all the salt molecules in the protein solution can be removed. This is the principle of the desalting dialysis of the semi-permeable membrane. If it is not distilled water but a buffer solution outside the membrane, it can change the inorganic salt composition of the protein solution through the interdiffusion of ions inside and outside the membrane. This is the principle of equilibrium dialysis of the semi-permeable membrane. Balanced dialysis is often performed before ion exchange chromatography. The semi-permeable membrane dialysis desalting process is as follows: according to the amount of sample, cut a section of semi-permeable membrane, immerse in distilled water for a period of time, and then rinse with distilled water. When in use, fold one end of the semi-permeable membrane, close it to make it the bottom end, and add distilled water with a dropper to check for water leakage. Fill a large 1000-2000ml beaker with water, and place a glass rod on the mouth of the beaker. Pour out the distilled water in the membrane, and then add protein sample solution (not too full to avoid bursting), tie the upper end of the tube with a rubber band, hang it in the center of the glass rod, and put it into the distilled water beaker. Place the beaker of distilled water in a 4 ° C refrigerator for dialysis for 2-3 days, and change the water more than 3 times a day. If placed on a magnetic stirrer to stir dialysis, the desalting effect is better. Always check the tightness of the dialysis bag to prevent leakage. According to the requirements of different samples, different inorganic salt ions can be removed, and a specific indicator can be used to determine whether the distilled water used in the final dialysis still contains such ions. 5. Discussion (1) The concentration of protein samples should be determined after dialysis, because after semi-permeable dialysis, the osmotic pressure inside the membrane is higher than the outside of the membrane, and water molecules will enter the membrane, so after desalting, the volume of the protein solution will increase accordingly. (2) Sometimes precipitation occurs in the dialysis bag. Generally speaking, the removal by centrifugation does not affect the properties of the protein sample. (3) After the dialysis bag is dipped in normal saline, it can be used continuously.

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