Explain the difference between normal-phase and reversed-phase chromatography.

Explain the difference between normal-phase and reversed-phase chromatography.

• A. Normal-phase uses polar stationary phase (e.g., silica) with nonpolar mobile phase; reversed-phase uses nonpolar stationary phase (e.g., C18) with polar (water-containing) mobile phase.
• B. Normal-phase uses nonpolar stationary phase with polar mobile phase; reversed-phase uses polar stationary phase with nonpolar mobile phase.
• C. Both use the same stationary phase; differences are in detector type.
• D. Normal-phase is used for ionizable compounds; reversed-phase for nonpolar compounds.

Answer: (A)

The main idea is how polarity is arranged between the stationary and mobile phases and how that controls which compounds stick around longer. In normal-phase chromatography, the stationary phase is polar (think silica or alumina) and the mobile phase is relatively nonpolar. Because polar analytes interact strongly with the polar surface, they are retained longer, while nonpolar compounds move with the nonpolar mobile phase and elute earlier. In reversed-phase chromatography, the stationary phase is nonpolar (such as C18) and the mobile phase is polar (water mixed with an organic solvent like methanol or acetonitrile). Here, nonpolar compounds interact more with the nonpolar surface and are retained longer, whereas polar compounds prefer the polar mobile phase and come off the column sooner. This explains why the provided statement is correct: it accurately describes polar stationary with nonpolar mobile for normal-phase, and nonpolar stationary with polar mobile for reversed-phase. The other ideas—such as using the same stationary phase in both modes, or relying on detector type to change separation, or associating normal-phase with ionizable compounds—don’t reflect how retention is governed by the phase polarity and its interaction with the analyte.

The main idea is how polarity is arranged between the stationary and mobile phases and how that controls which compounds stick around longer. In normal-phase chromatography, the stationary phase is polar (think silica or alumina) and the mobile phase is relatively nonpolar. Because polar analytes interact strongly with the polar surface, they are retained longer, while nonpolar compounds move with the nonpolar mobile phase and elute earlier.

In reversed-phase chromatography, the stationary phase is nonpolar (such as C18) and the mobile phase is polar (water mixed with an organic solvent like methanol or acetonitrile). Here, nonpolar compounds interact more with the nonpolar surface and are retained longer, whereas polar compounds prefer the polar mobile phase and come off the column sooner.

This explains why the provided statement is correct: it accurately describes polar stationary with nonpolar mobile for normal-phase, and nonpolar stationary with polar mobile for reversed-phase. The other ideas—such as using the same stationary phase in both modes, or relying on detector type to change separation, or associating normal-phase with ionizable compounds—don’t reflect how retention is governed by the phase polarity and its interaction with the analyte.