Bile acid is a kind of cholanic acid in bile. It is a white powder, odorless, bitter taste, and its alkali metal salts are soluble in water and alcohol.  Naturally occurring bile acids are usually associated with glycine or taurine by peptide bonds and with sodium and potassium ions to form bile salts present in bile.  The bile acids in normal bile can be divided into two groups according to their structure:  One was free bile acid, including cholicacid, deoxycholicacid, chenodeoxycholicacid and a small amount of lithocholic acid.  The other is the above free bile acid and glycine or taurine binding products, called conjugated bile acid.  It mainly includes glycine cholic acid, glycine goose deoxycholic acid, taurocholic acid and taurocholic acid, etc.  Generally, conjugated bile acids are more water-soluble than free bile acids, and this conjugation makes bile salts more stable and less likely to precipitate in the presence of acid or Ca2+.  Bile acids can be divided into primary and secondary bile acids according to their sources in the body.  In liver cells, bile acids synthesized directly from cholesterol are called primary bile acids, including cholic acid and deoxycholic acid.  Cholic acid and deoxycholic acid are both cholic acid derivatives with 24 carbon atoms.  The only difference between the two chemical books is the number of hydroxyl groups. Cholic acid contains three (3α, 7α, 12α) and deoxycholic acid contains two (3α, 7α).  Secondary bile acids Primary bile acids produced by 7α dehydroxylation by bacteria in the intestinal tract are secondarybile acids, including deoxycholic acid and shicholic acid.  The three-dimensional configuration of bile acids has two sides of hydrophilic and hydrophobic, which makes bile acids show strong interfacial activity.  It can reduce the surface tension between lipid and water phases and promote the formation of mixed lipid microclusters, which play an important role in the digestion and absorption of lipid substances and the maintenance of cholesterol dissolution in bile.  Cholic acid, dehydrocholic acid, deoxycholic acid, ursodeoxycholic acid, isodeoxycholic acid and cholic acid, etc. Thin layer chromatography method: chromatography plate, silica gel G plate;  The sample was prepared into 1mg/ mL solution with methanol.  Development agent, isooctane - ether - glacial acetic acid - n-butanol - water (10:5:5:3:1);  Color developing agent: 30% sulfuric acid solution, after spraying, bake at 105℃ for 10min, and observe fluorescence under 365nm uv lamp.  In the liquid chromatography expert system proposed by Chen Nong et al., reversed-phase liquid chromatography was recommended for the analysis of bile acid samples, and the action index was used for qualitative study of bile acid.