GRADES 5-8
CHROMATOGRAPHY

BACKGROUND INFORMATION:

Chromatography is a process used to separate mixtures. The word chromatography is derived from the Greek words "khroma" and "graphein" meaning "color" and "to write" or "to represent". Although there are several different types of chromatography, in each case a substance is placed onto or into a medium and a solvent is passed through the test substance. In chromatography science, the solvent is called "the mobile phase" or "the carrier fluid" and the medium is called "the stationary phase".

In this experiment, the medium is coffee filter paper, the solvent is water and our test substance is ink. Ink is a mixture; it is made of different substances mixed together. Parts of the test substance (the ink) may be attracted to the solvent (water) and follow it up the medium (the coffee filter paper).

As the solvent and the mixture move through the medium, different types of molecules in the ink mixture spread out (elute) at different speeds, causing the mixture to separate. Molecules with an affinity (attraction) to the medium travel slower than the molecules with less of an affinity to the medium. In paper chromatography, colored bands of the separated substance are created.

Other porous materials (a material liquid can penetrate) like paper towels, newspaper, or even cloth can be used as the medium. Wax and writing paper are not suitable mediums in paper chromatography. Wax paper is coated and generally, writing paper is treated to stop ink from bleeding.

More than 150 years ago, chromatography was used in the dyeing industry. Workers tested a dye's quality and strength by placing a drop on an absorbent cloth or paper. As the dye formed colored rings, the worker checked the dye's strength. In the mid-1800's, a German dye chemist, F. F. Runge, determined that other solutions (or materials made into a solution) could be separated as they were drawn through a porous material like paper.

Today, chromatography is a major chemistry field and important analysis tool used in industry and in medicine. Chromatography is used to detect, separate or purify different substances. Using this process food, drugs, blood, soil, water, air, fuel and petroleum and radioactive-fission products are analyzed. For example, in medicine, chromatography can determine the presence of drugs in a person's blood. In water and air quality analysis chromatography is used to isolate pollutants.

Chromatography also is used to synthesize new products since it can separate a pure substance from a complex mixture with great precision. Operating conditions generally are not severe and safe for (do not damage) even delicate products. The techniques have been used in the petroleum industry to separate and purify products and make petroleum jelly.

Have you ever mixed paint, crayons or food dye colors to create new colors? How do you make the color orange? Orange is made by mixing yellow and red together. Yellow, red and blue are primary colors (yellow, red and blue are not created by combining other colors). As you study the chromatographs and separate the ink colors in your chromatography test, keep this information in mind. What do you think a chromatogram of orange or brown ink will look like?

REFERENCES:

Williams, T. I., The Elements of Chromatography, Blackie & Sons Limited, London, 1955.

"Intro to Biochemical Engineering", Carrier, R., Bordonaro, J., Yip, K, http://www.rpi.edu/dept/chem-eng/Biotech-Environ/CHROMO/chromintro.html, 1994 & 1997.

"Chromatography", Blaszczynski, S., Daiker, B., Zipprich, N. http://www.iit.edu/~smile/ch9123.html.

"Paper Chromatography", Townsend, V., http://www.iit.edu/~smile/ch9414.html.