Question

When is it a good idea to use flash chromatography?

Answer 1

Flash column chromatography is a quick and easy way to separate complex mixtures of compounds. We will be performing relatively large scale separations of compound. Columns are often smaller in scale than this and some of you will experience these once you move into the research lab. Column chromatography uses the same principles, but can be used on a preparative scale. We are running flash columns since we will use compressed air to push the solvent through the column.

In flash column chromatography the most commonly used stationary phases are silica gel (SiO₂) and alumina (Al₂O₃).

Neat: If your compound is a non-viscous oil, it is easiest to apply it neat. You can use a long Pasteur pipet to apply the liquid to the column and then rinse any traces of it into the column using the predetermined solvent system.

In Solution: Neat application can sometimes lead to column cracking, so a more common method for liquids, as well as solids, is to dissolve the sample in a solvent and apply it to the column in solution. Another option for liquids as well as solids is to dissolve it in a solvent and apply it to the column in solution. In the best case, all of the components of your mixture should have an Rf of zero in this solvent - usually pentane or hexane. In many situations this is not possible, so a solvent that moves only one compound in the mixture can be used, or you can simply use your chosen eluent. Keep in mind that these last two options are risky for difficult purifications.

Adsorption onto Silica: The final technique that is useful for some liquids and all solids is to deposit (adsorb) the compound on silica. Caution: silica gel is acidic and this procedure can destroy compounds that are acid-sensitive and normally survive on silica gel columns