Chemical Analysis By Color

Popular Science, heinäkuu 1942

Simple Method Makes "Spectrum" of a Complex Mixture

Activated alumina, or other adsorbent, is poured into a tube set into a filter flask...
...and packed uniformly by tapping the sides and tamping down from the top as shown above.
With a disk of filter paper above the packed adsorbent, gentle suction is applied and a solution of the material to be analyzed is poured in. De. H. A. Frediani, of Eimer & Amend, demonstrated here

By means of bands of color, adsorbed by columns of alumina, magnesia, precipitated chalk, or even powdered sugar, chemists can now readily separate the constituents of complex chemical mixtures that not long ago defied division. By the same simple technique they can also purify chemicals; determine the structure of molecules; concentrate such substances as vitamins, hormones, and pigments from extremely dilute solutions; identify and compare drugs, dyes, and food products almost instantly. Although still employed chiefly as a tool in the research laboratory, experiments are being conducted to adapt the method to the isolation, purification, and testing of materials in the chemical and drug industries.

This "new" technique, named chromato-graphic-adsorption analysis by the invertor, was first developed back in 1906 by the Russian botanist M. Tswett, who was investigating the pigments in plant leaves. In his pioneer experiment, duplicated in principle in the photographs above, a chemist's dream was realized: the ingredients of a complex mixture were spread out for investigation like the colors of light in a spectrum. Still more amazing, they could be cut apart with a knife!

The reason for the separation of mixtures into distinct bands on an adsorption column is now considered to be this: molecules of different substances travel down a columb of absorptive material at different rates, depending upon their individual affinity to the adsorbent. Substances that have a strong affinity for the adsorbent travel down slowly. Substances that have less affinity travel down faster.

Tswett's simple but revolutionary method of chemical separation was little noticed for 25 years. In 1931 it came suddenly into prominence when Kuhn and Lederer passed carotene through an adsorption column and found that instead of being a single substance it was made up of a number of substances.

Solid pigments are held near the top of the column while the solvent flows through
To develop the chromatogram, clear solvent is pouredin. This spreads out the color bands and separates them
For further analysis, the core of adsorbent is partially dried and pushed out of the tube (at top). Then the various segments are cut apart carefully along the divisions of the color bands.

From the separation of plant pigments, it was only a step to the isolation and purification of vitamins. Some of the first pure specimens of Vitamin A, for instance, were made by chromatography. Vitamins D, E, K, B1, B2, and C were separated and purified by this method.

Means were soon found to isolate bands of chemical substances that were colorless or very faintly colored. By using quartz tubes in place of glass, and viewing the column by ultraviolet light, many ordinarily colorless chemicals were found to fluoresce with different colors.

Colorless Vitamin D3 was isolated from fish-liver oils by means of an indicating pigment. Vitamin A, in a column, was detected by painting a streak down the column with the Carr-Price reagent, which turns dark blue in the presence of the vitamin.

The chemical substance in each band is washed nad filtered out of the adsorbent. Colorless bands on an adsorption column often can be made visible by fluorescence under ultraviolet light, or by painting with a reagent as below

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