As illustrated below, in chromatography we pass a sample-free phase, which we call the mobile phase, over a second sample-free stationary phase that remains fixed in space. We inject or place the sample into the mobile phase. As the sample moves with the mobile phase, its components partition themselves between the mobile phase and the stationary phase. A component whose distribution ratio favors the stationary phase requires more time to pass through the system. Given sufficient time, and sufficient stationary and mobile phase, we can separate solutes even if they have similar distribution ratios.
Shown here are four examples of interactions between a solute and the stationary phase: (a) adsorption on a solid surface, (b) partitioning into a liquid phase, (c) ion-exchange, and (d) size exclusion. For each example, the smaller, green solute is more strongly retained than the larger, red solute.
