How does changing mobile-phase polarity affect liquid chromatography?

In liquid chromatography, the polarity of the mobile phase is a critical factor in determining the rate at which solutes migrate through the system. Because chromatographic separation relies on the relative affinity of solutes for the stationary and mobile phases, adjusting the mobile phase's polarity directly influences these interactions.1

The impact of changing polarity depends on the specific chromatographic method:

• General Principles: Solutes are retained by the stationary phase based on their affinity for it. If the mobile phase is made more polar, it may increase the elution strength for polar compounds, causing them to be swept more rapidly from the column.2
• Gradient Elution: To address the general elution problem—where a sample contains both weakly and strongly retained solutes—chemists use gradient elution. By constantly changing the composition and polarity of the mobile phase during the separation, the concentration of well-retained solutes in the mobile phase is increased, allowing them to move through the column more efficiently.2
• Method Selection: In normal-phase chromatography, a less polar mobile phase is typically used with a polar stationary phase. Conversely, reverse-phase liquid chromatography utilizes a nonpolar stationary phase and a polar mobile phase.2