Types of Chromatography

Chromatography is an essential analytical chemistry technique used for the separation, purification, and testing of different compounds in a mixture. The method helps in separating the solutes from a mix of components. It distributes the solute or compounds in a mixture between the mobile and stationary phases. The mobile phase consists of solutes dissolved in the solvent. The stationary phase is fixed on the column or plane through which the mobile phase runs. The solute from the mobile phase is adsorbed on the stationary phase, which helps in attaining the separation. There are different types of chromatography, out of which thin layer chromatography [TLC] is the simplest one. The topic explains chromatography and its different types.

Different Types of Chromatography 

Chromatography is based on general phenomena of adsorption and partition. The stationary phase is a solid phase that is attached to the column and adsorbs the solute, acting as a general basis for separation. The stationary phase is the liquid phase which has the solute be separated along with other components. It flows along with the stationary phase and hence is used as the basis for partition. Here are the different types of chromatography.

Thin-layer chromatography [TLC]

• Thin-layer chromatography [TLC] is also a type of adsorption chromatography. The technique is majorly used for the separation of non-volatile mixtures. 
• A very thin layer of adsorbent like silica gel, alumina, or cellulose is coated on a sheet of inert substrates like glass, aluminium foil or plastic. This layer is known as the stationary phase. 
• In thin layer chromatography [TLC], the solvent mixture known as the mobile phase is drawn on the plate via capillary action. The separation of the solute is obtained as the analytes move along the stationary phase at different rates.
• The plate used for thin layer chromatography [TLC] is known as a chrome plate. The plate has a size of around 5 x 2 cm. 
• The mixture with solute or the mobile phase is placed on the chrome plate using a fine capillary at a distance of about 2 cm from the bottom. 
• The plate is then placed in a chamber or TLC chamber containing a suitable solvent in a vertical position. The capillary action leads to the movement of the solute, and as the analyte component moves at different rates, it leads to separation. 

Paper chromatography [PC]

• It is one of the oldest types of chromatography, which was introduced by Schonbein in the year 1865. It works on the principle of capillary analysis or action. 
• In paper chromatography [PC], a strip of chromatography designed paper is used to achieve the separation between the components. The paper is the stationary phase, whereas the solvent mixture is the mobile phase.
• The mobile phase travels along with the filter paper at different rates through the mechanism of capillary action. 
• The filter paper or the stationary phase in paper chromatography [PC] is made of cellulose fibre with water molecules adsorbed on it. The solvent travels along the strip, with different solutes travelling at different speeds. 
• The solute mixture is partly adsorbed on the filter paper and partly portioned between the water molecules attached to the filter paper and solvent. Hence, the component travels at different rates due to both processes of absorption and partition, getting separated from one another. 

Column chromatography

• Column chromatography is a technique that helps separate solute components from a mixture using a column. The basic principle of chromatography is selective absorption and desorption. 
• The stationary phase is packed in a glass tube. Some of the examples of stationary phases used are activated charcoal, alumina, magnesia, silica gel, calcium carbonate etc. 
• The mixture to be separated is dissolved in a solvent which forms the stationary phase. The mixture is run along with the stationary phase for separation. 
• The moving solutes, while moving down along the stationary phase, are adsorbed on the column in a process called selective absorption. The adsorbed components are recovered through the process of elution by dissolving them in suitable solvents. Some common eluents are ether, chloroform, petroleum etc.
• As the components adsorbed on the column are separated one by one depending on the eluent used, it helps in achieving separation.
• Column chromatography is of three different types which are size-exclusion chromatography, ion-exchange and affinity chromatography. 

Gas Chromatography 

• Gas chromatography is used for the separation of gaseous as well as volatile substances from a mixture.
• The main principle behind gas chromatography is the partition of the volatile compounds between the gaseous and liquid phases. 
• It is majorly used for the separation of a range of organic components like metabolites, drugs, fatty acids, volatile gas etc. 

Conclusion 

Chromatography is an important technique used in chemistry and pharmaceutical processes for obtaining the separation of components in a mixture. It has a range of applications starting from isolation and characterisation of various compounds like amino acids, antibiotics, alcohol, amines, chemicals, level of pollutants in water, urine analysis, components in food etc. In summary, the principle behind the process of chromatography is adsorption and partition. It consists of the stationary phase, which adsorbs the components from a mixture. The mobile phase has a mix of solutes and solvent that flows through the stationary phase and helps in achieving separation through the elution process. The topic gives an overview of different types of chromatography like thin layer chromatography [TLC] and column and paper chromatography.