Which statement describes the polarity relationship in reverse-phase HPLC?

• A. The mobile phase is more polar than the stationary phase
• B. The mobile phase is less polar than the stationary phase
• C. The mobile phase is nonpolar
• D. The stationary phase is more polar than the mobile phase

Answer: (A)

In reverse-phase HPLC, the stationary phase is nonpolar (hydrophobic) while the mobile phase is polar. This arrangement means the mobile phase is more polar than the stationary phase. Polar solvent components carry along the analytes and help move them through the column, while the nonpolar parts of molecules interact with the hydrophobic surface of the stationary phase and are retained longer. As a result, polar compounds tend to elute sooner, and nonpolar compounds are held back longer, which is the hallmark of reverse-phase separation. The other descriptions would imply the opposite polarity arrangement or a nonpolar mobile phase, which describes a different (normal-phase) mode.