selection of eluting agents & mobile phase in phenomena of adsorption

How eluting agents or mobile phase are selected in the process of adsorption?

In the selection of eluting agent or mobile phase as solvent following two main points should be kept in mind:

First consideration is that either solvent meets the requirement of practical factors such as

Suitable viscosity

Stability

Compatibility with detection

Solubility with respect to the sample

Suitable purity

And ease of removal.

Second is that does the solvent provide maximum resolution for the separation of sample in a given time.

Above given two considerations are also applicable in selection of suitable liquid mobile phase in partition chromatography.

How polarity factor is involved in co relation of selection of eluting agents?

Commonly a polarity related related factor can also be used to co relate the solvents as eluting phases in adsorption chromatography.

Eluotropic series

A group of solvents in order of chromatographic strength is known as eluotropic series.

Commonly used typical solvents

Light petroleum (Pet ether,hexane,heptane)

Cyclo hexane

Carbon tetra chloride

Toluene

Benzene

Chloroform

Ethyl ether

Ethyl acetate

Acetone

n-propanol

Ethanol

Methanol

Water

Pyridine

Organic acids

Inorganic acids and bases

Both eluting and polarity are increasing at the top and less at the bottom.

Relation of polarity of adsorbents and mobile phase

Greater will be the polarity of the adsorbed solute. Larger will be the polarity of mobile phase and this case happens at that time when it goes to the suitable eluting agent.due to this reason adsorption is not generally used for the separation of polar compounds,because binding is too much strong which is not compatible with the polarity of eluting agent to overcome this binding.

Use of mix solvents

Mix solvents can also be used by providing that they are miscible.

Example

Ethanol and chloroform can also be used by a gradual change in the ratio which is incorporated into the eluting mixture.

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