Theory of the chromatographic process

When a substance is fed into the separation column via a suitable injection system, it becomes distributed between the stationary and the mobile phases. A short time after injection of the sample, the concentration in the gas phase is very high and that in the stationary phase virtually zero. Some molecules of the substance then become dissolved in the separating liquid and a dynamic equilibrium is established, which means that per unit time the number of molecules desorbed is equal to the number absorbed. 

The position of the partition equilibrium is determined by the coefficient of partition Kq. 

It is also necessary that the mobile phase does not go into solution in the stationary phase, nor the stationary phase in the mobile phase. The former possibility can be excluded by using inert carrier gases. The latter eventuality may occur if the vapour pressure of the stationary phase is so high that the separating fluid evaporates. Strictly speaking, the concentration ratio for K ) also only applies if the molecular state of the substance is the same in both the gas phase and in the stationary phase. 

The ratio of the mole quantities ng/n is termed the capacity factor k, and the ratio of the volumes V /Vs the phase ratio B. 

The phase ratio B is a measure of the permeability of a separation column. By way of example. Fig. 42 shows a comparison of a packed column and a capillary column. It has been assumed for the purposes of this comparison that completely non-porous particles (support and phase) have been used for the packed column. 

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