Gradient Elution:

It involves continuous change in the composition of the mobile phase by allowing a more polar solvent to flow into the reservoir containing a less polar one, whence the mixture flows to the column as illustrated in Figure. Thus, a complex mixture of solutes that cannot be separated by isocratic separation can be separated by gradient elution. It is especially meaningful for separating components that differ widely in polarity. For gradient elution using a low pressure mixing system, the solvents from different reservoirs are fed to a mixing chamber and then mixed solvent is pumped into the column.

Time proportionating electrovalves used in modern instruments are regulated by a microprocessor; therefore, the resolution for every chromatogram. It can decrease the run time and increase the sensitivity. Since the gradient develops, tailings are made to elute quicker.

The commercial liquid chromatographs are designed to mix two or more solvents in a progressive manner from 0 to 100% of one component. If one of the solvents gives an appreciable response at the detector after that the generation of a solvent gradient will also introduce a baseline drift in response. Within such a case, column will also need time to reproduce the starting solvent composition every time a fresh gradient run is started and ideally, a blank gradient is run between samples to avoid the occurrence of artifact peaks which can be observed. This could make gradient elution seem slower than literature values. It might be remembered that gradient elution produces affects same to temperature programming in gas chromatography.