Chromatography is a separation process involving two phases,
stationary and mobile phase. Mixture to be analyzed is adsorbed in
stationary phase and mobile phase is passed into it, eventually
compounds of mixture get separated on basis of rate of adsorption and
solubility. Both are physical properties. Depending on this,
chromatography is classified into two groups.
1)Adsorption chromatography:
a)Column Chromatography:
Generally, In this chromatography, a glass tube is filled with adsorbent (alumina or silica gel) up-to one third of it's length. Then, it is soaked with selective solvent. Sometimes, column is filled with slurry (adsorbent + solvent). The column should not have any space. Such a column is stated as 'well packed column'. In this method, less polar compound will be eluted first. Because, less polar compound will be less adsorbed in polar stationary phase. Eventually, more polar compound will come out.
b) Thin layer chromatography:
In TLC, a plate (glass/plastic) is coated with a thin layer of solid adsorbent. A small drop of mixture is spotted near the bottom of plate. Then, plate is placed in solvent chamber in such way that only bottom part gets dipped into solvent (mobile phase). This liquid slowly rises up to TLC. In this method, separation is measured by RF value. Separated compounds move to different distance, which is expressed by retention factor (RF value). Compound of lower polarity will have higher RF value than more polar ones.
2)Partition chromatography:
a) Paper chromatography:
In this chromatography, stationary phase is water adsorbed in paper and mobile phase is mixtures of different organic solvent and water. Any drop of organic solvent on a filter paper gets partitioned between water and solvent. Then, this paper is dipped into number of solvent mixtures and chromatograms are developed. Ascending and descending, this two types of development generally take place. Like TLC, in this method also, separation is expressed by RF value. Compounds with higher RF value has lower polarity and vice versa.
b)Gas chromatography:
This is the most modern technique of chromatography. It is commonly used in analytical chemistry. In this method, sample (vaporized without decomposition ) is injected into column. The sample is moved through this column by the flow of mobile phase. Here, mobile phase is inert carrier gases (Ex- He or nitrogen). Column is coated with different stationary phases. So, basically, elements of analyzed mixture are partitioned between solid stationary phase and mobile gas. Each compounds elute at a different time, which is called retention time. Compounds eluted at different retention time then get detected in various detectors. Finally, these are recorded in a recorder and chromatograms are obtained.
1)Adsorption chromatography:
a)Column Chromatography:
Generally, In this chromatography, a glass tube is filled with adsorbent (alumina or silica gel) up-to one third of it's length. Then, it is soaked with selective solvent. Sometimes, column is filled with slurry (adsorbent + solvent). The column should not have any space. Such a column is stated as 'well packed column'. In this method, less polar compound will be eluted first. Because, less polar compound will be less adsorbed in polar stationary phase. Eventually, more polar compound will come out.
b) Thin layer chromatography:
In TLC, a plate (glass/plastic) is coated with a thin layer of solid adsorbent. A small drop of mixture is spotted near the bottom of plate. Then, plate is placed in solvent chamber in such way that only bottom part gets dipped into solvent (mobile phase). This liquid slowly rises up to TLC. In this method, separation is measured by RF value. Separated compounds move to different distance, which is expressed by retention factor (RF value). Compound of lower polarity will have higher RF value than more polar ones.
2)Partition chromatography:
a) Paper chromatography:
In this chromatography, stationary phase is water adsorbed in paper and mobile phase is mixtures of different organic solvent and water. Any drop of organic solvent on a filter paper gets partitioned between water and solvent. Then, this paper is dipped into number of solvent mixtures and chromatograms are developed. Ascending and descending, this two types of development generally take place. Like TLC, in this method also, separation is expressed by RF value. Compounds with higher RF value has lower polarity and vice versa.
b)Gas chromatography:
This is the most modern technique of chromatography. It is commonly used in analytical chemistry. In this method, sample (vaporized without decomposition ) is injected into column. The sample is moved through this column by the flow of mobile phase. Here, mobile phase is inert carrier gases (Ex- He or nitrogen). Column is coated with different stationary phases. So, basically, elements of analyzed mixture are partitioned between solid stationary phase and mobile gas. Each compounds elute at a different time, which is called retention time. Compounds eluted at different retention time then get detected in various detectors. Finally, these are recorded in a recorder and chromatograms are obtained.
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