All chromatographic methods -with the exception of TLC- use columns for the separation process. Column chromatography has found its place in many laboratories for preparative purposes as well as for reaction control in organic syntheses. The importance of column chromatography is mainly due to following factors:

• Simple packing procedure
• Low operating pressure
• Low expense for instrumentation

Column chromatography is separated into two categories, depending on how the solvent flows down the column. If the solvent is allowed to flow down the column by gravity, or percolation, it is called gravity column chromatography. If the solvent is forced down the column by positive air pressure, it is called flash chromatography, a "state of the art" method currently used in organic chemistry research laboratories. The term "flash chromatography" was coined by Professor W. Clark Still because it can be done in a “flash."

The Adsorbent
Silica gel (SiO2) and alumina (Al2O3) are two adsorbents commonly used by the organic chemist for column chromatography. These adsorbents are sold in different mesh sizes, as indicated by a number on the bottle label: “silica gel 60” or “silica gel 230-400” is a couple examples. This number refers to the mesh of the sieve used to size the silica, specifically, the number of holes in the mesh or sieve through which the crude silica particle mixture is passed in the manufacturing process.

Adsorbent particle size affects how the solvent flows through the column. Smaller particles (higher mesh values) are used for flash chromatography; larger particles (lower mesh values) are used for gravity chromatography. For example, 70–230 silica gels are used for gravity columns and 230–400 mesh for flash columns.

Typical Data of Silica gel Column Grade Adsorbents
Iron Content	<0.02%
Chloride Content	<0.10%
Loss on Drying	<3%
pH (10% suspension)	7 ± 0.5
Surface Area	500–600