Principles and Practice of Chromatography - The Development Process > Elution Development > Page 6
This type of chromatographic development is rarely used and probably is of academic interest only; it and can only be effectively employed in a column distribution system. The sample is fed continuously onto the column as a dilute solution in the mobile phase in contrast to displacement and elution development, where discrete samples are placed on the system and the separation subsequently processed. Frontal analysis can only separate part of the first compound in a relatively pure state, each subsequent component being mixed with those previously eluted. Consider a three component mixture, containing solutes (A), (B) and (C) as a dilute solution in the mobile phase that is fed continuously onto a column. The first component to elute, (A), will be that solute held least strongly in the stationary phase. Then the second solute, (B), will elute but it will be mixed with the first solute. Finally, the third solute (C), will elute in conjunction with (A) and (B). It is clear that only solute (A) is eluted in a pure form and, thus, frontal analysis would be quite inappropriate for most practical analytical applications. This development technique has been completely superseded by elution development.
Elution development is best described as a series of absorption-extraction processes which are continuous from the time the sample is injected into the distribution system until the time the solutes exit from it. The elution process is depicted in Figure 1. The concentration profiles of the solute in both the mobile and stationary phases are depicted as Gaussian in form. Equilibrium occurs between the two phases when the probability of a solute molecule striking the boundary and entering one phase is the same as the probability of a solute molecule randomly acquiring sufficient kinetic energy to leave the stationary phase and enter the other phase. The distribution system is continuously thermodynamically driven toward equilibrium. However, the moving phase will continuously displace the concentration profile of the solute in the mobile phase forward, relative to that in the stationary phase which, in a grossly exaggerated form, is depicted in Figure 1. This displacement causes the concentration of solute in the mobile phase at the front of the peak to exceed the equilibrium concentration with respect to that in the stationary phase. As a consequence, a net quantity of solute in the front part of the peak is continually entering the stationary phase from the mobile phase in an attempt to re-establish equilibrium. At the rear of the peak, the reverse occurs. As the concentration profile moves forward, the concentration of solute in the stationary phase at the rear of the peak is now in excess of the equilibrium concentration.