**2.1 Polypeptide extraction**

The separation and purification methods adopted in the serum peptidomics research depend on the nature of the extracted substance. The commonly used methods for polypeptide extraction and separation include: salting removal method, ultra-filtration process, gel filtration, isoelectric point precipitation method, ion-exchange chromatography, affinity chromatography, adsorption chromatography, countercurrent distribution and enzymolysis method. These methods often work together to separate and purify specific substances.
