1.3. Zone Migration . 6

The process from the sample is introduced to the point where sample is detected happens in the mobile phase.

When the sample passes through the column, there are three possible types of zone migration in the column.

- Frontal analysis( also called the basis of solid-phase extraction techniques): When solutes reach the columns and pass through it, the equilibrium is obtained between solutes and stationary phase. Each solute(A,B,C) is retained at different extent. The least retained solute(A) will exit the column first following it by B(less retained than A). A is extended from (A,B,C) farthest. The signals from the detector is compromised by a series of rectangular steps of increasing height. The advantages of this method is that it is used to determine sorption isotherms for single component or simple mixtures and to isolate a less strongly retained trace component from a major component. It also is used to collect contaminants from airs and water by sorption onto short sorbent beds. Otherwise, it is rarely used for separation because it causes contamination to columns and it is hard to determine the quantitative aspect of sample components.
- Displacement: a substance called displacer from the mobile phase is higher affinity for the stationary phase than any of sample components. The displacer continuously passes through the column and pushes sample components down the column(if the column is long enough, the ''steady state will be reached). Each sample component displaces the component ahead of it, and the last and most strongly retained component(A) will be forced by the displacer. This method is used in preparative and process in Chromatography.
- Elution: The most convenient method, samples components pass through the column with discrete bands, and the equilibrium is reached for samples components between the mobile phase and stationary phase. To achieve this the distribution constants(in mobile phase, from mobile phase to stationary phase, and in stationary phase) for sample components must be different.

Finally we have response signals and information obtained from elution