Paper chromatography

Paper chromatography definition-

Paper chromatography is an analytical method and it is used to separate colored chemicals or substances. The chromatography technique which uses paper sheets or strips as the stationary phase of the adsorbent and the solution is made to pass through the paper sheet, is known as paper chromatography. It is an inexpensive way to separate dissolved chemical substances on sheets of paper with different migration rates. It is a powerful analytical tool that uses a very small amount of material. this chromatography was discovered in 1943 by Sing and Martin. Such variant, two-dimensional chromatography, involves using two solvents and rotating the paper 90° in the middle. It is useful for separating complex mixtures of compounds with similar polarity, for example, amino acids. The setup consists of three components. The mobile phase is a solution that travels to the stationary phase due to capillary action. The mobile phase is typically a mixture of non-polar organic solvent, while the stationary phase is the polar inorganic solvent water. Here paper sheets are used to support the stationary phase, water. The polar water molecules are inside the vacuole of the cellulose network of the host paper. The difference with respect to thin layer chromatography  is that the stationary phase in thin layer chromatography is a layer of adsorbent (usually silica gel, or aluminum oxide), and the stationary phase in this type is the less absorbent paper.

 Principle of paper chromatography-

There are two  sorts of  paper chromatography based on two different principles. The first  principle based on paper is adsorption chromatography which is based on varying degrees of interactions between molecules and the stationary phase. Molecules with higher affinity remain adsorbed for  a extended  period of time thereby reducing their speed of movement through the column. However, molecules with low affinity move at a faster rate, thus the molecules  are often  separated into different fractions. Adsorption chromatography between solid and liquid phases, wherein the solid surface of the paper is that the stationary phase and the liquid phase is the mobile phase.

Another  sort of  such chromatography is paper partition chromatography.  Such chromatography is based on the principle that the moisture on the cellulose paper acts as a stationary phase for the molecules moving along the mobile phase. Thus the dissociation of molecules  is predicated  on how strongly they adsorb on the stationary phase. An additional concept of ‘retention factor’ is applied during the separation of molecules in paper chromatography. The retention value for a molecule  is decided  as the ratio of the distance covered by the mobile phase to the distance traveled by the molecule.

Paper chromatography diagram-

paper chromatography diagram

The stationary phase is selected as a good quality cellulosic paper. Various combinations of organic and inorganic solvents are taken as the mobile phase. Approximately 2–200 μl of sample solution is injected into the paper base, and allowed to air dry. The sample loaded paper is then carefully immersed in a mobile phase not exceeding a height of 1 cm. After the mobile phase approaches the edge of the paper, the paper is pulled out. The retention factor is calculated, and the individual components are detected by different techniques.

 Types of paper chromatography-

types of paper chromatography

Ascending type-

The technique lives up to its name because the solvent moves in an upward direction.

Descending type-

Due to the gravitational pull and capillary action, the solvent flows downward, hence the name descending type of chromatography.

Ascending  and Descending type-

In this version of  chromatography, the movement of the solvent occurs in two directions after a particular point. Initially, the solvent travels upward on the paper which is folded on a rod and after crossing the rod it continues its journey in the downward direction.

Radial or Circular type-

The sample is deposited in the center of the circular filter paper. Once the spot has dried, the filter paper is then tied horizontally onto a Petri dish containing the solvent.

Two Dimensional type-

Substances which have the same rf value can be resolved with the help of two-dimensional chromatography.

Example of paper chromatography-

This chromatography has traditionally been used to analyze food dyes in ice cream, sweets, drinks and beverages, jams and jellies. Only food colorings are allowed to be used to ensure that no non-permissible coloring agents are added to foods. This is where quantization and detection come into play.

Application of paper chromatography-

  • This chromatography is performed to ascertain the purity of various pharmaceutical products.
  • It can also be used to detect contamination in various samples such as food and beverages.
  • This method can also be used to separate impurities from various industrial products.
  • The analysis of reaction mixtures is also done through this chromatography in chemical laboratories.

Advantages of paper chromatography-

The advantages of such chromatography are as follows:-

  • This  chromatography requires a minimum amount of quantitative material.
  • It is less expensive in comparison of other chromatography methods.
  • Such chromatography can identify both unknown  organic and inorganic compounds.
  • It takes up very little space as compared to other analytical methods or instruments.

Disadvantages of paper chromatography-

 Some  disadvantages are as given below-

  • This chromatography cannot handle large quantities of samples.
  • it is ineffective in quantitative analysis.
  • It cannot be used to separate complex mixtures.
  • Less accurate than HPLC or HPTLC

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