Running silica gel column chromatography by hand: choosing the mesh and the column dimensions from the separation, packing a bed with no channels, and loading in a way that does not undo the plate you spent an hour optimising

Packing and running the column

1. Choose the mesh from how you will run it. Coarser silica flows under gravity and gives poorer resolution; the finer flash grade around two hundred and thirty to four hundred mesh needs pressure and gives much better separation. Buying the wrong grade for your setup produces either a column that will not flow or one that separates badly.
2. Size the bed from the difficulty, not just the mass. Twenty times the sample mass in silica is generous for a well separated pair; fifty times or more is needed for close-running compounds. A short wide column is fast and resolves poorly; a tall narrow one is the opposite.
3. Pack a bed with no channels and no air. Slurry the silica in the starting solvent, pour it in one go, and tap the column while it settles. A dry-packed bed wetted afterwards traps air, and a channel is invisible until the band running down it arrives early and broadened.
4. Keep the bed covered, always. A bed that runs dry cracks, and a cracked bed cannot be recovered. Add a sand layer on top so pouring solvent does not disturb the surface, and never leave a running column unattended near the end of a reservoir.
5. Dry load anything that will not dissolve in a weak solvent. Liquid loading in a strong solvent pushes the sample down the column before separation begins. Adsorbing the crude onto a small amount of silica and loading it dry gives a tight starting band and fixes most disappointing separations by itself.

Handling silica safely

Fine silica is a respirable dust and weighing it out generates airborne material. Weigh and slurry it in a fume hood, and prefer pre-packed cartridges where the work is routine, which removes the exposure entirely.

Solvent handling belongs in the hood as well, and the waste stream from a column is substantial. Plan the waste route before running a large column rather than after.

Compounds that silica does not suit

Strongly basic compounds streak badly on acidic silica and often need a base additive in the eluent or a different stationary phase. Acid sensitive compounds can decompose on the column, which presents as poor recovery rather than poor separation.

Where recovery is low and the plate looked fine, suspect the stationary phase rather than the technique. Alumina or a reversed phase cartridge may be the honest answer.

Recovering the fractions

Check fractions by thin layer plate before combining, and combine on the evidence rather than on the tube numbers you expected. Pooling by plan rather than by plate is how a pure fraction gets mixed with a shoulder.

Concentrate promptly. Compounds sitting in dilute solution in a fraction rack overnight give a surprising amount of decomposition and evaporation loss.

Common questions

What mesh silica should I use for column chromatography?

Around 230-400 mesh flash grade where you can apply pressure, which is most modern practice. Coarser grades flow under gravity and separate noticeably worse, and are worth using only when no pressure source is available.

How much silica do I need?

Sized to the difficulty of the separation: roughly twenty times the sample mass for an easy one and fifty times or more for close-running compounds. Sizing on mass alone while ignoring the plate is the commonest reason fractions overlap.

Why did my column separate badly?

A channel in the bed, an overloaded column, or a load applied in too strong a solvent. Repacking properly, using more silica and dry loading address almost all of it, in that order of likelihood.

When should I use an automated flash system instead?

When the separation is repeated, when the record matters, or when the volume of work makes hand collection tedious. An automated system gives a chromatogram, which is evidence about purity that a rack of tubes is not.

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