Q1(h). Gradient elution

Answer in brief

The mobile-phase composition is changed during a run, usually to increase eluting strength with time.

Key points

• Early weak eluent resolves weakly retained components. Later stronger eluent releases strongly retained components without excessively long retention or broad peaks.
• In reversed-phase HPLC, a typical gradient increases the organic-solvent fraction of an aqueous mobile phase.
• The gradient may be linear, stepped or otherwise programmed. Temperature programming in GC serves an analogous retention-control purpose.
• Re-equilibrate the column before the next run; mixing delay and gradient reproducibility affect retention times.