Principle, Plate and Rate Theories of Chromatography

Chromatography is an essential technique in analytical chemistry. M.S Tswett first developed it in 1906 to obtain the separation of coloured substances into their components. Chromatography helps in separating, identifying, and purifying components from a mixture. The plate and rate theory of chromatography are two crucial concepts that help understand how the separation is obtained. During the chromatography process, sample components are separated into discrete bands, producing a peak in the reading. The topic breaks down the rate and plate theory of chronography and the basic principle behind the technique. 

Principle of Chromatography

The basic principle of chromatography is based on the concept of distribution of components of a mixture of organic compounds between two phases which are stationary and mobile. The two aspects of chromatography involve adsorption and separation. Here are a few things to know about chromatography:

• The components are coloured, and the coloured solutes are soluble in the same solvent. 
• One of the components should be more soluble in the solvent for obtaining the separation.
• The stationary phase is the one that is attached to the column or plate of the chromatography. The most common material used for the same is silica gel, calcium carbonate, activated charcoal, etc. It is primarily porous. 
• Mobile phases are the mixture of the solute dissolved in the solvent, consisting of the solute to be separated. As the mobile phase flows along with the stationary phase, it adsorbs on the stationary phase and attains separation. 

The separation is based on the principle of chromatography that components in the mobile phase move at different rates along with the stationary phase. The solutes in the mobile phase have a distinct affinity for the stationary phase and hence move at different rates. It is based on the phenomenon of adsorption and partition. 

Rate Theory of Chromatography

• The rate theory of chromatography is important for chromatographic analysis. The theories help understand how the analytes move in the stationary phase as the mobile phase flows through it. 
• The rate theory of chromatography defines the activity in a chromatography column. It shows that when solute elutes out of the column, it impacts the band shape and is affected by the elution rate. 
• The rate theory provides information about the shape and breadth of the elution bands as the mobile phase migrates and flows through a column. It helps understand the process of peak dispersion and factors impacting band broadening. 
• It gives a realistic explanation of the process occurring in a chromatographic column. It considers and measures the time taken by the solute to equilibrate between the stationary and mobile phase. 
• It considers the rate of elution on the resulting band shape or the chromatographic peak. 
• The rate theory of chromatography is expressed mathematically by the van Deemter equation. The equation helps calculate the variance per unit length of a column in terms of mobile phase velocity and analyte properties. The relationship between a column’s efficiency and the mechanism behind band broadening is described by the Van Deemter equation. 
• It is represented by 

            HETP = A + B/u (C_s + C_m). u

 Where

A = Eddy diffusion parameter

B = diffusion co-efficient of eluting particles in the longitudinal direction C = Resistance to mass transfer coefficient of the analyte between the stationary and mobile phase

 u = speed

C_m = dispersive convection in the mobile phase

C_s = sorption, and desorption of the solute from the stationary phase

 HETP is the measure of zone broadening.

Plate Theory of Chromatography

• The plate theory of chromatography is an older concept and does not give a realistic view of column efficiency and what is happening in the column. The theory was integrated into the chromatography technique in 1941 by Martin and Synge. 
• According to the model, the chromatographic column consists of separate layers known as the theoretical plates. These plates are hypothetical zones or stages that help in establishing an equilibrium between two phases. 
• Unlike the rate theory of chromatography, it gives a hypothetical picture of the separation of the analytes in a chromatographic column. These plates provide separation equilibrium of the sample between the mobile and stationary phase. 
• As the mobile phase passes through the stationary phase in a column, the analytes in the mobile phase are distributed between the two phases establishing an equilibrium. 
• Once the equilibrium is obtained, the solute from the mobile phase is carried from one plate to another and continues till it is eluted out of the column. The nature and solute type determine the retention time and peak width. 
• As the theoretical plate number increases in the column, it narrows the solute peak and provides better resolution between different components in the sample. 
• The number of the theoretical plate in a column is represented by N. The efficiency of the column is represented by HETP (Height Equivalent to a Theoretical Plate). A measure of the efficiency of a chromatography column is the height equivalent to a theoretical plate or plate height.

Conclusion

The principle of chromatography essentially focuses on the distribution and partition based on the different affinities. As the mobile phase moves down along the stationary phase, it is adsorbed on the stationary phase surface depending on its affinity and hence helps in partitioning and separation. The plate and rate theory of chromatography provides information on what is happening inside the column and how the separation is attained. Both theories are critical to chromatographic separation techniques. The plate theory describes the properties of chromatographic separation and hence determines the number of theoretical plates. The rate theory is more realistic and accurate and describes chromatographic separation by comparing the rate of analyte eluting through the column. In summary, both concepts are crucial for the chromatographic process.

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