In biochemistry, large molecules (like proteins or DNA fragments) are frequently separated by gel chromatography. Since they are usually invisible, one needs to stain them. One row in the picture represents the stained separated fragments of a large molecule. The more intense the color, the higher the concentration found at that spot. This Demonstration shows the variation in intensity in a chromatogram.
After color inverting the original chromatogram, we count black pixels and thus determine how much of the substance is located at each rectangular area on the film. By using the sliders, you can specify your area of interest. On the left the whole image is shown, whereas on the right your selection is magnified and centered. The percentage refers to the selected fraction of black pixels in that region. The slider for lets you rotate both images and the slider for lets you control the counting tolerance.
Chromatography (from Greek meaning "writing with colors") is a chemical or physical technique used to separate molecules. The simplest experiment is to let a felt pen drain onto a piece of wet and porous paper so that different colors run out and separate.
More sophisticated techniques have become standard procedure, which can be applied to separate even large numbers of different molecules.
In all cases, there is always a stationary phase (e.g. paper) and a mobile phase (e.g. water) whether the experiment is performed in a tube or in two dimensions. Chromatography is not restricted to liquid systems; it can also be used with molecules in the gas phase.
For planar chromatography, you put a small amount of your mixture on one side and put the 2D medium into the mobile phase, so that the mobile phase does not touch the mixture immediately. The liquid gradually rises until all the paper is submerged. If you have colored compounds you will see a row of different spots.
This Demonstration shows three rows, created by three different mixtures, so the resulting spots have not run through the same length.