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Procedures     

Types of Chromatography

Flash 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

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 or percolation 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)

  • Acetic acid
  • Alcohol
  • Acetone
  • Ethyl acetate
  • Diethyl ether
  • Halogenated hydrocarbons (methylene chloride)
  • Toluene
  • Alkanes (hexanes, petroleum ether)

Dry Column Chromatography

Dry column chromatography (DCC) is a versatile Prep LC method

Basically, any sample that can be separated on silica gel or neutral alumina TLC plate can also be separated by the corresponding DCC-setup. The dry-column procedure has been successfully applied for the preparation of dye-stuffs, alkaloids, and other heterocyclic substances which are known to be separated on other types of columns, but, with considerable difficulties. Lipids have also been successfully separated.

Dry column chromatography bridges the gap between analytical TLC and preparative classical column chromatography. The cost is much less than the cost incurred in instrumental pressure associated with preparative liquid chromatography.

 


dry column chromatography preplc photo

 


dry column chromatography preplc photo


The load sample versus adsorbent is maintained at approximately less than 1:500 in TLC while the ratio is 1:300 or even higher for dry column chromatography.

dry column chromatography preplc photo ratio chart

dry column chromatography prep lc techniqueThe Dry Column Technique

Bridges the gap between preparation column chromatography and analytical thin-layer chromatography.

Dry Column Chromatography

This is a unique and simple method for purifying material. If inexpensive and fast. It is single column elution technique. Below is a schematic form of the method.

Dry Column Chromatography DCC Compared to TLC
 

Chromatographic
Parameters 

TLC

DCC

Solvent Reservoir
Solvent Force
“Charge” Addition of Sample
Support
Adsorbent
Adsorbent Activity
Equilibrium with solvent vapor
Dimensions of width: thickness: length
Adsorbent bed
Detection
Techniques for Recovery
 

 

tank
capillary
pipette
glass, plastic
silica, alumina, polyamide
low
partial
(sometimes
controlled)
width: thickness: length

200 :1: 200
visible, UV
spray techniques
scrape off
elute

overhead
gravity
pipette

nylon tubes
silica. alumina
low
none
1 :1: 20

visible, UV


cut into sections
elute

 

 

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