On the basis of mechanism of separation of components of the given mixture, chromatography can be classified into four distinct classes given as follows
(i)
Adsorption chromatography
(ii)
Ion-exchange chromatography
(iii)
Partition chromatography
(iv) Gel permeation/size exclusion chromatography
1. Adsorption chromatography:
Adsorption chromatography is one of the most common
and oldest type of chromatography among all the other types of chromatography. The
stationary phase is a solid adsorbent with high adsorbing power. Most commonly
used adsorbent in this type of chromatography is silica gel while Alumina and
Cellulose powder are also widely used. The mobile phase can be in the form of
liquid or gas according to requirements of the analysis. When mobile phase
moves through the stationary phase along with mixture to be separated, the
components of the given mixture adsorbed on the surface of solid adsorbent
depending upon the adsorbing powers, thus resulted in separation of different
components of the mixture. The phenomenon of adsorption solely depends upon the
force of attraction among sample components and the active adsorbing sites of
the stationary phase that is further highly dependent upon the polarity of the
solvent used. This technique is mostly used for the separation and purification
of organic samples. Common examples of this technique are GC and TLC.
2. Ion-exchange chromatography:
Ion-exchange chromatography is commonly used for the
identification and separation of ionic particles and charged molecules. Ionic
resins are used as a stationary phase to which cationic or anionic groups are
covalently bonded on the surface. The mobile phase used must also contain
charged particles. When mobile phase runs through the resins used as stationary
phase, the charged particles of sample components compete with charged
particles of the mobile phase and interact with stationary phase according to
their affinity to opposite charges of stationary phase and thus attached with
it. Compounds with positive charges like mixture alkaloids can be separated
through an anionic resin, while the compounds having negative charges like
organic acids with negative charges can be separated using cationic exchangers.
Common applications of ion-exchange chromatography are isolation of proteins,
separation of bio-molecules and desalination of water.
3. Partition chromatography:
The stationary phase in this type of chromatography
is made up of thin film of liquid on an inert solid which serves as a support. Wet
silica gel and cellulose powder are most widely used supports for stationary
phase. The mobile phase is also in liquid form but differs in polarity and
should be immiscible with each other. The liquid selected for stationary phase
is more polar than the liquid used as mobile phase. The components of the
sample distribute themselves among two liquids. It is mostly used for the
analysis and purification of polar compounds. Paper chromatography is the
common example of this technique.
4. Gel permeation/size exclusion chromatography:
The stationary phase in this type of chromatography
is specially designed with specified and controlled pore size while mobile
phase is in the form of an organic liquid. Thereby, large molecules eluted
first then smaller ones and at the end the smallest molecules separated as
these pass through all the pore sizes. Therefore, it is also known as gel
filtration chromatography. In this technique separation of components of the
mixture solely depends upon the size of the molecules consequently it is named
as size exclusion chromatography. Isolation of proteins is the most common
application of gel permeation/size exclusion chromatography.
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