Frontal analysis:

Therefore, when the sample components have a wide range of distribution coefficients, graded eluents are used. While utilizing graded eluents, the usual procedure is to begin with the weakest solvent capable of eluting one of the sample components and to change increasingly stronger solvents until the entire sample have been eluted. The change from one member of the series to the next is made by a stepwise increase in concentration of the stronger solvent.  This technique is called as gradient elution analysis. An gradient elution technique can be used not only with mixed solvents but also with any phenomena that will give a solvent gradient in the column, e.g. pH or ionic strength.

Figure: (a) Frontal analysis; (b) Displacement development; and (c) Elution analysis