Paper Chromatography Chromatography comes from the Greek word khromatos (color) and graphein (to write); i. e. paper chromatography is literally color writing on paper. This method tests the purity of compounds and identifies substances. This analytical process, despite having been replaced by the success of thin layer chromatography, still stands as a valuable teaching tool, and is nevertheless very common. This method is very useful because not only is it a relatively quick process, but also requires only small quantities of materials.
The substances being tested are first distributed between a stationary phase and a mobile phase. The stationary phase refers to a piece of high quality filter paper used for testing. The mobile phase references the developing solution that travels up the stationary phase, leaving the samples in its traces. The sample then separates according to two main concepts: how strongly the solution absorbs on the stationary phase versus how well they dissolve into the mobile phase.
This experiment also depends upon the polarity of the molecules, and how many are involved. When a colored sample is put through the stationary phase, the colors separate from the sample when one end of the paper is placed in a certain solvent. The solvent then diffuses the sample throughout the paper, leaving in its wake the dissolution of the various molecules in the same according to the polarities of the molecules and the solvent, respectfully. However, if the sample contains more than one color, it indicates that there is more than one molecule present.
Due to the different chemical structures of each kind of molecule, each one will have a slightly different polarity, giving each molecule a different solubility level when placed in the solvent. The unequal solubility levels from the various colors in the sample cause different color molecules to leave traces of the solution at different spots as the solvent moves up the paper. The more soluble a molecule is, the higher it will migrate up the paper. A nonpolar chemical, however, will not dissolve at all in a very polar solvent. The opposite of this statement is also in effect.
Nevertheless, it should be taken into account that when using H2O, which has high polarity, as a solvent, the less polar the color is, the higher the sample will rise on the paper during the experiment. As stated before, this method of experimenting has been replaced more by the thin layer chromatography, two-dimensional chromatography, and gas chromatography in laboratory testing throughout the years. Thin layer chromatography is performed on a sheet of glass or plastic, which is then coated with a thin layer of adsorbent material such as silica gel or cellulose.
The adsorbent layer is known as the stationary phase; the solvent (mobile phase) is then drawn up on the plate via capillary action. Two-dimensional, or two-paper chromatography, involves the exact same procedure as paper chromatography, except for the fact that it uses two solvent, with the paper being rotated 90° in between. This way the sample passes through two different separation stages. Regardless of any method used, chromatography has many real life applications.
Thin layer is used to detect pesticide or insecticide residues in food and also in forensics to analyze the dye composition of fibers. Gas chromatography, in which helium is used to move a gaseous mixture through a column of absorbent material, is a way to detect bombs in airports and identify and quantify drugs. Paper chromatography can be used to separate amino acids, anions, and histamines, and is also beneficial for RNA fingerprinting. In any way that it is approached, chromatography is useful for our lives – paper chromatography still being a commonly used method.