Curious to know if the simple technique of paper chromatography that we learned in school, is really useful? Then you need to look no further! This article speaks about the various paper chromatography uses, and tells a little about how this technique works.
Chromatography is a method used to separate different components of a mixture. There are different types of chromatography, but all of them are based on the same principle: the different molecules or ions in the mixture will interact differently with the stationary phase of the chromatograph, and get separated in the process. The different techniques of chromatography use different substances as the stationary and mobile phases. Chromatography can either be analytical or preparative. Analytical chromatography is used for determining the relative proportions of the different components in the mixture, while the separation of the different components is what preparative chromatography is used for.
Chromatography techniques are generally classified on the basis of the mechanism of separation. The types of chromatography, based on the mechanism of separation, are adsorption chromatography, partition chromatography, ion exchange chromatography, molecular exclusion chromatography, and affinity chromatography. Paper chromatography is based on the principle of partition chromatography.
Uses of Paper Chromatography
Paper chromatography is a simple chromatography technique which has many applications. Its main advantage is that it is not very expensive to perform, and provides clear results. Given below are some important uses of paper chromatography.
Separating Colored Pigments
Paper chromatography is an effective technique for separating colored pigments from a mixture. A few drops of the mixture of colored pigments are placed on the filter paper (stationary phase) and it is then slowly submerged into a jar of solvent (mobile phase). As the solvent rises up the paper, it dissolves the molecules present in the mixture, their solubility depending on their polarity. Because of different polarity, molecules of each pigment leaves the solution at different places, as the solvent continues to rise up the stationary phase. Thus, each pigment rises up to a particular level on the chromatography paper, and gets separated in the process.
★ Used in the separation of plant pigments.
Obtaining Pure Compounds
Paper chromatography is used to obtain pure compounds from a mixture. This is done by cutting out and redissolving the patterns formed by each constituent. Also, this technique can be effectively used to remove impurities from chemical compounds. Due to the process of paper chromatography, the impurities get separated from the compound and the pure compound can be obtained.
★ Used in the separation of proteins into homogenous groups for use in medicine.
Paper chromatography is one of the methods of qualitative analysis, to analyze or separate the different constituents of a mixture. It is a useful tool for separating polar as well as nonpolar solutes. Pharmaceutical companies use this technique to analyze the different compounds in drugs.
★ Used in the testing of antibiotics and determining the pollutants in water.
Pathology and Forensic Science
Paper chromatography is useful in the field of forensic science, for investigation of crime. This is because this process can be successfully carried out even with very small quantities of material. Samples from crime scenes are collected to be analyzed and identified, using this technique.
★ Used in DNA and RNA fingerprinting. Pathological laboratories use paper chromatography to detect the presence of alcohol or chemicals in blood.
Analyzing Complex Mixtures
Paper chromatography is used to detect the presence of, or identify certain organic compounds such as carbohydrates and amino acids, from a complex mixture of organic compounds.
★ Used in the separation of amino acids and anions.
How Does Paper Chromatography Work
To understand the principle of paper chromatography, we must learn what is partition chromatography. As the name suggests, partition chromatography is a method of separating the components of a mixture in which the constituents of the mixture are partitioned or separated between two liquid phases, one of which is supported by a solid and is termed as the stationary phase. The other liquid phase is the solvent in which a small amount of the mixture (that is to be separated) is dissolved. In paper chromatography, the solid in question is a filter paper, and the stationary phase is water in the pores of the filter paper. The following are the steps to perform paper chromatography.
Step 1: Take a long rectangular piece of filter paper and draw a straight line on it using a pencil, a few centimeters above one of its shorter edges. This is your start line. Place a drop of the mixture on the start line, using a capillary tube.
Step 2: Take a glass jar and pour a small amount of the solvent liquid into it. Now, place the filter paper inside the glass jar such that the part of it below the start line, is submerged in the solvent. Do not disturb the setup and you shall see that the solvent in the jar slowly rises up due to the capillary action of the paper. Wait for around 15-20 minutes, till the solvent nearly reaches the top of the paper.
Step 3: Remove the filter paper from the jar and mark the highest point on the filter paper till which the solvent has risen. You shall see that the different components of the mixture have been carried to different levels, by the solvent. In the above diagram, you can see two colored spots formed by two different solutes, A and B. This is due to the difference in the affinity of the solutes present in the mixture, to the filter paper (stationary phase). So, while one solute (solute B) is easily carried farther away by the solvent, the other is not. This results in the solutes getting separated from the mixture.
Step 4: When the filter paper has dried, note the distance covered by each constituent from the start line. Now, calculate the retardation factor (Rf value) by the following formula. This value can never be more than 1, which implies that a solute can never travel ahead of the solvent.
|Retardation Factor (Rf)||=||Distance traveled by the solute from the start line
Distance traveled by the solvent from the start line
Note: If the spot formed by a component is irregular, you need to measure the distance from the middle of the spot to the start line.
This was all about paper chromatography and its uses. However, while performing an experiment on paper chromatography, you need to follow each step carefully in order to get the desired results.