Principles and Practice of Chromatography - The Development Process > Displacement Development > Page 5
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 detail in due course.
Displacement development is only effective with a solid stationary phase where the solutes are adsorbed on its surface. The sample mixture is placed on the front of the distribution system, and the individual solutes compete for the immediately available adsorption sites. Initially, all the nearby adsorbent sites will be saturated with the most strongly held component. As the sample band moves through the system the next available adsorption sites will become saturated with the next most strongly adsorbed component. Thus, the components array themselves along the distribution system in order of their decreasing adsorption strength. The sample components are usually held on the stationary phase so strongly that they are eluted very slowly or even not at all. Consequently the solute must be displaced by a substance more strongly held than any of the solutes (called the displacer). The displacer, contained at a low concentration in the mobile phase, first displaces the most strongly held component. In turn this component will displace the one next to it. Thus, the displacer forces the adsorbed components progressively through the distribution system, each component displacing the one in front until they are all pass through the system. The solutes will be characterized by the order in which they elute and the amount of each solute present will be proportional to the length of each band, not the height. In displacement development the solutes are never actually separated from one another. The solutes leave the system sequentially and in contact, each somewhat mixed with its neighbor. This type of development is not used in analytical chromatography and only very rarely in preparative LC. However, displacement effects can occur in overloaded distribution systems and in the development of thin layer plates with multicomponent solvents.