Paper chromatography is a way to separate substances from a mixture. The mixture is dissolved in a suitable solvent. The solution is allowed to travel across paper first of the substances in the solution travel at different speeds first of the separate because of the different solubility in the solvent and attraction to the paper the travel over. The more soluble a substance is the more faster it travels. The more attracted it is to paper the less it will travel. It is used to find out how many substances are present in a mixture. It check the purity of a substance, if the impurities are present they are separated out. Chromatography can be used to identify amino acids from a sample of protein. The diagram shows the chromatogram obtained when four samples of amino acids were analyzed. The paper was sprayed with ninhydrin.
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Chromatography is used to separate mixtures of substances into their components. All forms of chromatography work on the same principle. They all have a stationary phase (a solid, or a liquid supported on a solid) and a mobile phase (a liquid or a gas). The mobile phase flows through the stationary phase and carries the components of the mixture with it. Different components travel at different rates. We'll look at the reasons for this further down the page. In paper chromatography, the stationary phase is a very uniform absorbent paper. The mobile phase is a suitable liquid solvent or mixture of solvents.
Producing a paper chromatogram
You probably used paper chromatography as one of the first things you ever did in chemistry to separate out mixtures of colored dyes - for example, the dyes which make up a particular ink. That's an easy example to take, so let's start from there.
Suppose you have three blue pens and you want to find out which one was used to write a message. Samples of each ink are spotted on to a pencil line drawn on a sheet of chromatography paper. Some of the ink from the message is dissolved in the minimum possible amount of a suitable solvent, and that is also spotted onto the same line. In the diagram, the pens are labeled 1, 2 and 3, and the message ink as M.
paper1.gif
The paper is suspended in a container with a shallow layer of a suitable solvent or mixture of solvents in it. It is important that the solvent level is below the line with the spots on it. The next diagram doesn't show details of how the paper is suspended because there are too many possible ways of doing it and it clutters the diagram. Sometimes the paper is just coiled into a loose cylinder and fastened with paper clips top and bottom. The cylinder then just stands in the bottom of the container.
The reason for covering the container is to make sure that the atmosphere in the beaker is saturated with solvent vapour. Saturating the atmosphere in the beaker with vapour stops the solvent from evaporating as it rises up the paper.
paper2.gif
As the solvent slowly travels up the paper, the different components of the ink mixtures travel at different rates and the mixtures are separated into different colored spots.
The diagram shows what the plate might look like after the solvent has moved almost to the top.
paper3.gif
It is fairly easy to see from the final chromatogram that the pen that wrote the message contained the same dyes as pen 2. You can also see that pen 1 contains a mixture of two different blue dyes - one of which might be the same as the single dye in pen 3.
Rf values
Some compounds in a mixture travel almost as far as the solvent does; some stay much closer to the base line. The distance travelled relative to the solvent is a constant for a particular compound as long as you keep everything else constant - the type of paper and the exact composition of the solvent, for example.
The distance travelled relative to the