Chromatography techniques Knowledge-Based Download PDF

Questions

1. 1.

   In gas chromatography (GC), which statement correctly describes the primary role of the mobile phase?

   • A) It chemically reacts with analytes to alter their volatility and retention times.
   • B) It selectively solvates polar analytes to dictate differential partitioning along the column.
   • C) It serves solely as an inert carrier to transport vaporized analytes through the column without chemically interacting with them.
   • D) It establishes an adsorption equilibrium with the column wall to stabilize the thermal gradient.
2. 2.

   Which statement correctly describes the fundamental retention and elution mechanism of cation-exchange chromatography?

   • A) A positively charged stationary phase retains negatively charged analytes, which are subsequently eluted by decreasing the mobile phase ionic strength.
   • B) A negatively charged stationary phase retains positively charged analytes, which are subsequently eluted by increasing the mobile phase ionic strength or pH.
   • C) A neutral nonpolar stationary phase retains hydrophobic analytes, which are subsequently eluted by increasing the organic solvent concentration.
   • D) A porous inert stationary phase retains smaller analytes within its internal pores, which are subsequently eluted ahead of excluded macromolecules.
3. 3.

   In normal-phase thin-layer chromatography (TLC) using a silica gel stationary phase, what is the effect on the retention factor ($R_f$) of a polar analyte when the polarity of the mobile phase is increased?

   • A) The $R_f$ value decreases because a higher polarity mobile phase reinforces the adsorption of the analyte onto the stationary phase.
   • B) The $R_f$ value remains unchanged because the retention factor depends exclusively on the chemical identity of the analyte.
   • C) The $R_f$ value increases because the more polar mobile phase competes more effectively for the polar sites on the stationary phase and solvates the analyte better.
   • D) The $R_f$ value drops to zero because polar solvents deactivate analyte migration along the plate.
4. 4.

   Which statement correctly describes the separation mechanism and elution order of molecules in size-exclusion chromatography?

   • A) Smaller molecules elute first because they interact more strongly with the hydrophobic interior of the porous beads.
   • B) Molecules are separated based on their net surface charge, with highly charged species eluting first through electrostatic repulsion.
   • C) Larger molecules penetrate deeper into the internal pores of the stationary phase, resulting in longer retention times than smaller molecules.
   • D) Larger molecules are excluded from the internal pore volume of the stationary phase and elute first, whereas smaller molecules enter the pores and elute later.
5. 5.

   In reversed-phase high-performance liquid chromatography (RP-HPLC), which combination correctly describes the relative polarities of the phases and the resulting elution order?

   • A) A nonpolar stationary phase and a polar mobile phase, resulting in more polar analytes eluting first
   • B) A polar stationary phase and a nonpolar mobile phase, resulting in more polar analytes eluting first
   • C) A nonpolar stationary phase and a polar mobile phase, resulting in less polar analytes eluting first
   • D) A polar stationary phase and a nonpolar mobile phase, resulting in less polar analytes eluting first