Question

Reverse-phase chromatography involves the use of a stationary phase that is

• A. Nonpolar
• B. Polar
• C. Neutral
• D. Electrically charged

Hint:

• Normal Phase Stationary Phase = Polar; Mobile Phase = Nonpolar. (Polar compounds stick longer).
• Reverse Phase Stationary Phase = Nonpolar; Mobile Phase = Polar. (Nonpolar compounds stick longer).
• Reverse-phase HPLC (High-Performance Liquid Chromatography) is very widely used. Common stationary phases are C18 or C8 bonded silica.

Answer 1

The Correct Option is A

Chromatography is a separation technique. In liquid chromatography, there are two main modes based on the relative polarities of the stationary phase and the mobile phase: 1. Normal-Phase Chromatography (NPC):

• Stationary phase: Polar (e.g., silica gel, alumina).
• Mobile phase: Nonpolar (e.g., hexane, dichloromethane).
• Principle: Polar analytes interact more strongly with the polar stationary phase and elute later. Nonpolar analytes elute earlier.

2. Reverse-Phase Chromatography (RPC):

• Stationary phase: Nonpolar (hydrophobic). Commonly silica gel modified with nonpolar alkyl chains (e.g., C18 - octadecylsilyl, C8 - octylsilyl).
• Mobile phase: Polar (e.g., water, methanol, acetonitrile, or mixtures).
• Principle: Nonpolar (hydrophobic) analytes interact more strongly with the nonpolar stationary phase and are retained longer, eluting later. Polar analytes interact more with the polar mobile phase and elute earlier.

The question asks about reverse-phase chromatography. In this mode, the stationary phase is nonpolar. Option (a) "Nonpolar" is correct. Option (b) "Polar" describes the stationary phase in normal-phase chromatography. Option (c) "Neutral" refers to pH or charge, not directly polarity in this context. Option (d) "Electrically charged" describes ion-exchange chromatography stationary phases. \[ \boxed{\text{Nonpolar}} \]