Question

Suppose you are using adsorption chromatography to elute a mixture of compounds bound to alumina gel. The mobile phase is cyclohexane. Some compounds remain bound to the column; what change of mobile phase might you use to elute the compounds? Explain your answer.

Answer 1

Since your mobile phase is cyclohexane, a non-polar solvent, so change of mobile phase to polar solvent might help to elute the compounds. Slowly replace the 100% hexane mobile phase with 95% hexane - 5% ethyl acetate if your compound does not elute still then use 90% hexane - 10% ethyl acetate, so on. Polar solvents can be ethyl acetate, methanol, or some drastic conditions even water also used. A less polar solvent will not compete well, allowing the compounds to remain bound to the stationary phase, resulting in slow elution. A polar solvent will compete well with molecules and will occupy sites on the stationary phase. This will force compounds into the mobile phase, and result in faster elution/increased travel distance.

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