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?