Frontal Analysis 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

The transition from elution development to frontal analysis is shown experimentally in figure 5 where progressively larger charges are placed on the column starting with 10 ml and ending with 16 ml. Frontal analysis curves can also demonstrate the need to take certain precautions when employing an external loop valve for injecting the sample. As a result of the parabolic velocity profile of the sample and mobile phase as it is discharged from the sample loop onto the column, dispersion of the sample volume occurs resulting in the concentration profile of the sample entering the column taking the form of a tailing Poisson function as opposed to a rectangular slug. Thus, if the sample loop is

of the nature of the solute or its capacity ratio (k'). The peak dispersion towards greater retention is characteristic of volume overload which, as will be seen below, will not be true for mass overload. Sample volume overload distorts the normal elution profile derived from normal elution development to that of frontal analysis development. If elution development is carried out with sample of increasing volume, the distorted elution development concentration profile culminates in frontal analysis. After, J. Chromatogr., Ref. [3]   Figure 5. The Transition from Sample Volume Overload to Frontal Analysis Development

The Development Process A solute progresses through the chromatographic system, albeit through a column or along a plate, only while it is in the mobile phase. This process, whereby the substances are moved through the chromatographic system, is called chromatographic development. There are three types of chromatographic development, elution development, displacement development and frontal analysis. Elution development is now virtually the only development technique employed in both GC and LC although some displacement development is occasionally used in preparative LC.   In TLC, the development process is confused by the frontal analysis of the multicomponent solvent that occurs as the mobile phase moves through the system. In contrast, the solutes are transported across the plate by elution development. This apparent paradox will be explained in

(X) in the diagram. The now binary mixture continues to migrate along the plate and the next solvent component that interacts most strongly with the stationary phase (solvent B) is adsorbed as a layer on the surface corresponding to the area (Y). Finally, the remaining solvent (C) with the weakest interactions with the stationary phase continues to migrate and cover the surface with a layer of solvent (C) in the area (Z). It is seen that the distribution system, which results from the frontal analysis of the three mobile phase components is now quite complex. The solutes will interact during the separation process. In the first section (X) solutes will be distributed between the ternary solvent mixture (A), (B) and (C) and the surface covered with solvent (A). In the next section (Y) the solutes will be distributed between a binary solvent mixture of (B) and (C) and a surface covered with solvent (B). Finally, distribution will take place in section (Z) between pure

phase will randomly acquire sufficient energy (EA) to leave the stationary phase and enter the gas phase. Thus, the distribution coefficient of all solutes with respect to the stationary phase will be reduced as the temperature rises and it will be seen in due course that this will cause the band velocity of all the solutes to be increased. Elution Development in Thin Layer Chromatography The development processes that take place on a thin layer plate is complicated by the frontal analysis of the mobile phase itself. The mobile phases used to elute the solutes in TLC are usually multi-component, containing at least three individual solvents. If the plate is not pre-conditioned with solvent, there is an elaborate modification of the plate surface which is depicted, for a ternary solvent mixture, in Figure 3. The edge of the plate is dipped into a tray of the solvent mixture which begins to migrate along the plate, driven by surface tension forces. The