Gradient chromatography is an elution mode in which a mobile-phase variable that controls retention is changed continuously or stepwise during a chromatographic run. The programmed variable is commonly organic-solvent fraction, ionic strength, or pH; its change alters solute distribution between mobile and stationary phases and causes compounds with different retention strengths to migrate within a practical time window ^[1]. Unlike isocratic chromatography, the mobile-phase elution strength is not constant. A gradient is characterized by its initial and final compositions, duration, shape, flow rate, and dwell volume between solvent mixing and the column inlet ^[2]. Peak compression occurs because the rear of a migrating band encounters a stronger mobile phase than its front, affecting peak width and separation performance ^[3]. The concept excludes flow programming without chemical change, temperature programming in gas chromatography, and nonchromatographic stepwise extraction. It is independent of the specific stationary-phase chemistry and can operate with reversed-phase, normal-phase, ion-exchange, or other retention mechanisms.