Gravity and Flash Column
Chromatography
Flash
chromatography is a type of preparative liquid chromatography used for
the separation of organic compounds. This is adsorption chromatography
for the routine purification of organic compounds. By using the flash
technique chromatographers can scale up normal phase chemistries from
thin layer chromatography (TLC) helping to satisfy the demands of the
pharmaceutical and biotech industries in the transition to large scale
purification of organic compounds and peptides. The technique utilizes
an air pressure driven hybrid of medium pressure and short column
chromatography optimized for particularly rapid separations.1
Flash is very similar to traditional column chromatography except that
solvent is driven through the column by applying positive pressure.
Resolution is measured in terms of the ratio of retention time (r) to
peak width (w, w/2). The technique simply uses a set of chromatography
columns and flow
controller valves. Modern flash chromatography systems are very
convenient, being sold as prepackaged plastic cartridges with solvent
being pumped through the cartridge.
Column chromatography (which is the basis for flash chromatography)
follows the same principles as thin layer chromatography (TLC). The main
difference is that TLC separates miniscule amounts of material whereas
column chromatography can be used to separate large amounts of material.
If the solvent flows down the column by gravity orpercolation the
technique is called gravity column
chromatography. If the solvent is forced down the column by positive air
pressure it is called flash chromatography. The term flash
chromatography was first used by Dr. W. Clark at Columbia University
because the technique allows organic compounds to be purified “in a
flash”.
Column chromatography involves stationary and mobile phases. In
column
chromatography the stationary phase (a solid absorbent) is placed in a
vertical column and the mobile phase (liquid) is added to the top and
flows down through the column by either gravity or external pressure. In
column chromatography the stationary phase is most commonly either
silica (Si02) or alumina (Al2O3). The columns packed with
silica usually
have a defined particle size of 40-60 microns. The mobile phase is
normally a mixture of hexane and ethyl acetate. Mobile phases with low
viscosity require smaller particle sizes. The stationary phase is
normally more polar than the mobile phase.
By increasing the polarity of the solvent system all components of the
mixture move faster. By lowering the polarity all components move more
slowly.
The eluting power of organic solvents
The highest polarity being the most powerful eluters (at the
top of the list)
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