*3.7.4 High performance capillary electrophoresis (HPCE)*

It is an instrumental analysis method developed in the late 1980s combining classical electrophoresis with modern microcolumn separation technologies. In pharmaceutical analysis, the most commonly used separation modes are capillary zone electrophoresis, micellar electrokinetic capillary chromatography, and capillary gel electrophoresis. It is an efficient separation technology of large and small molecules in a hollow and thin inner diameter capillary (10–200 μm). The two ends of the capillary are immersed in a buffer solution and electrodes connected with a high voltage power supply are inserted separately. The voltage makes samples migrate along the capillary. According to the charge and volume of the separated substances, various molecules are separated under high voltage. In zone capillary electrophoresis, separation could be achieved by the movement of electrophoresis and electroosmotic flow. The strength of electroosmotic flow depends on the strength of electric field, PH value of electrolyte, composition of buffer solution, ionic strength, internal friction, and so on. Sample injection could be accomplished by pressing the sample into a capillary tube by atmospheric pressure or voltage.

HPCE has the advantages of high efficiency, microamount, economy, high automation, and wide application. However, it has the disadvantages of poor preparation ability, low sensitivity, and poor separation reproducibility. For example, see [23].

### *3.7.5 Affinity chromatography (AC)*

Affinity chromatography is a unique chromatographic separation method based on the principle of reversible combination of high affinity and specificity between molecules. By simulating the reversible and specific interaction between biological molecules, affinity chromatography uses the adsorption medium coupled with affinity ligands as the stationary phase to adsorb target compounds. It is a development of adsorption chromatography. This method can selectively separate and analyze specific chemical constituents from complex samples. Firstly, ligands that can specifically bind to the target compounds are fixed on the filler carrier to make the chromatographic column. Then the mixture containing the target compounds is passed through the column. Only the target compounds which show affinity with the ligands can bind to the ligands and remain in the column. Finally, the adsorbed target compounds are eluted by changing the composition of the mobile phase and are separated from other chemical constituents. AC is mainly used for the separation and purification of proteins, especially enzymes, antigens, and antibodies. Its application range has been expanding along with the continuous development of technology in recent years. For example, see [24].
