Theory of Gas Chromatography

As mentioned earher, the plate theory has played a role in the development of chromatography. The concept of plate was originally proposed as a measmement of the performance of distillation processes. It is based upon the assumption that the column is divided into a number of zones called theoretical plates, that are treated as if there exists a perfect equilibrium between the gas and the Hquid phases within each plate. This assumption imphes that the distribution coefficient remains the same fi-om one plate to another plate, and is not affected by other sample components, and that the distribution isotherm is hnear. However, experimental evidences show that this is not true. Plate theory disregards that chromatography is a dynamic process of mass transfer, and it reveals httle about the factors affecting the values of the theoretical plate number. In principle, once a sample has been introduced, it enters the GC column as a narrow-width band or zone of its composite molecules. On the column, the band is further broadened by interaction of components with the stationary phase which retains some components more than others. Increasing 

The van Deemter rate theory identified three major factors that cause band or zone broadening during the chromatographic process the eddy diffusion or the multi-path effect (A-term), longitudinal diffusion or molecular diffusion of the analyte molecules (B-term), and resistance to mass transfer in the stationary phase (C-term). The broadening of a zone was expressed in terms of the plate height, H, and was described as a function of the average linear velocity of the mobile phase, u. 

The C-term is a measure of the resistance to mass transfer in the hquid phase. Because there is a continuous flow in the column, the concentration of the analyte (i.e. mass transfer of analyte molecules) in the liquid phase keeps changing from one zone to the next one. The whole process is a dynamic non- 

Taking into consideration aU of the effects discussed above, the van Deemter equation can be formulated in a more detailed form as foUows  

Dg is the diffusion coefficient of the analyte in the carrier gas k is the capacity factor  

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It is these two points of view which are used in the Chapter 2 to discuss the theory of gas chromatography. Linear, nonideal chromatography may be visualized by the relationships shown in Figures 1.12 and 1.13. 

D. L. Peterson and F. Helfferich, Towards a generalized theory of gas chromatography at high solute concentrations. J. Phys. Chem.,... 

Many excellent books have been written on the theory of gas chromatography so we will not discuss the subject in detail in the present context. Briefly it is based on the differences in the energy required for each component of a mixture to pass from the surface of a liquid phase, onto which it has been adsorbed, back into the vapor phase. 

The nomenclature also reflects the origin of the concept the distillation theory of gas chromatography. In this concept, a certain length of the chromatographic column was occupied by a theoretical plate. This length was the height equivalent to the theoretical plate. If we divide the column length... 

Before approaching the theories of gas chromatography, the processes of carrier gas flow and lon tudinal diffusion that take place in the chromatographic column during separation need to be considered. The mass transfer rate will be dealt with in the section concerned with the theories of gas chromatography. 

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