Jim Hoyer, age 12, of Rice Lake, Wisconsin, for his question:
What is the science of chromatography?
We ordinary folk know when a heavy smog smarts our eyes. But it takes an expert in chromatography to pinpoint the percentage of this or that smoggy pollutant in the air. This expert also can identify pollutants and other chemicals in water and other liquids. Chromatography techniques also can be used to sort and separate the various mineral compounds in a rock sample.
The word chromatography means identification by color and the substances identified are chemical compounds. Actually the color may be just the icing on the chemical cake. Many compounds have distinctive personal colors that a trained chemist can identify at a glance. Others are murky or colorless and further tests are needed to tag them.
Chromatography is used to separate mixtures of molecular compounds in solids, gases and watery or oily liquids. Testing each sample requires special treatment and success depends on a trained chemist who knows what various chemicals do to each other. The basic toil is done by special substances that sort and separate the various ingredients in the sample mixture of compounds.
This is done by groups of chemicals called absorbents and solvents, sometimes assisted by eluants or rinsers. The absorbents attract verious chemicals at different speeds. A chemist uses his know how to select the right one for the test sample. It may be charcoal or cellulose, aluminia or lime, starch or sucrose or silica gel. Sometimes the job is done by a strip of thick absorbent paper.
Usually the absorbent is packed into a glass tube. The test sample is mixed in a selected solution. This liquid must be able to dissolve the test ingredients without affecting the absorbent. Chemical activity starts when the solution is poured into one end of the tube.
The ingredients absorbed fastest stay near the top. Those absorbed more slowly pass farther down the glass .tube. This is the secret that sifts, separates and sorts the various compounds in the sample. If only a small helping of the solution is used, the banded layers may be too thin and crowded to form a clear picture. Then a suitable solvent is added to enlarge them and wash them through the tube.
Some or perhsps all the bands of sorted chemicals may be colored and the chemist can identify them at a glance. When the absorbent is white and all the bans are colored, the science of chromatography earns its glamorous name.
The saturated absorbent may be poked from its tube and various chemical tests made to identify the bands of colorless ingredients. The width of the different bands reveals the proportion of this or that compound in the test sample. Somewhat similar techniques are used to pinpoint the compounds in gaseous mixtures and oily petroleums.