There are two different modes of separation in high-performance liquid chromatography (HPLC) called isocratic and gradient. Both methods are commonly used in reversed-phase chromatography, normal phase chromatography, and ion-exchange chromatography.Isocratic means that your mobile phase is constant over the full analysis time. Using a gradient elution involves that the solvent mixture is changed throughout the analysis and therefore affects the retention of the components.
The main difference between isocratic and gradient HPLC elution system is that during the process of separation in isocratic elution, the same composition is used and the same polarity of the mobile phase is maintained. In gradient elution, it gradually increases or decreases the polarity of the mobile phase throughout the process of separation.