In reverse-phase high-performance liquid chromatography (HPLC)
CHEMISTRY · Single correct · A Level · Chemistry · Chromatography · Chromatography
Problem
In reverse-phase high-performance liquid chromatography (HPLC), the stationary phase is nonpolar and the mobile phase is a water–acetonitrile gradient becoming more nonpolar over time. Elution times for four analytes are listed:
• benzoic acid: 3.2 min
• benzaldehyde: 4.5 min
• toluene: 6.8 min
• ethylbenzene: 7.5 min
Which of the following best explains why benzoic acid elutes first under these conditions?
(a) Benzoic acid is the least polar analyte and therefore interacts least with the non-polar stationary phase, shortening its retention time.
(b) Benzoic acid is the most polar analyte and therefore partitions more strongly into the polar mobile phase, shortening its retention time.
(c) Benzoic acid has the smallest molar mass and therefore diffuses through the stationary phase most rapidly, shortening its retention time.
(d) At the pH of the mobile phase, benzoic acid is ionized; the resulting negative charge is repelled by the non-polar stationary phase, causing it to elute first.