Question

Experimental Organic Chemistry 1

HPLC Questions

- How do you predict the elution order of compounds?

-What's the difference between the mobile phase and the stationary phase?

Answer 1

In normal phase column where silica is the stationary phase non-polar compounds elute first. Silica is polar. It holds the polar compounds tightly to itself. The non-polar compounds are less tightly held. Therefore, non-polar compounds elute first.

In reverse phase column where silica surface is modified with long chain hydrocarbons, the polar compounds elute first. The long chain hydrocarbons make silica surface hydrophobic. They hold non-polar molecules more tightly than the polar compounds. Therefore, the polar compounds elute first.

The mobile phase is the solvent or mixture of solvents which is passed through the stationary phase which does not move. Stationary phase is the column of HPLC which is either packed with silica (normal phase) or modified silica with n-octylsilyl (C-8) or n-octadecylsilyl (C-18) moieties on silica (reverse phase).