2.2A: Overview of Chromatography

The first uses of chromatography involved separating the colored components of plants in the early 1900's. The pigments in a plant can be separated into yellow, orange and green colors (xanthophylls, carotenes and chlorophylls respectively) through this method. The Greek name for color is chroma, and graphein is 'to write,' so chromatography can be thought of as "color writing."

The general idea of chromatography can be demonstrated with food dyes in your kitchen. Commercial green food dye does not contain any green colored components at all, and chromatography can show that green food dye is actually a mixture of blue and yellow dyes. If a drop of green food dye is placed in the middle of a paper towel followed by a few drops of water, the water will creep outwards as it wets the paper (Figure 2.1). As the water expands, the dye will travel with it. If you let the dye expand long enough, you'll see that the edges will be tinted with blue (Figure 2.1d). This is the beginning of the separation of the blue and yellow components in the green dye by the paper and water.

Figure 2.1: Time-lapse sequence of a drop of green food dye on a paper towel, followed by a drop of water.

A complete separation of the green food dye can be accomplished using paper chromatography. A dilute sample is deposited on the bottom edge of a piece of paper, the paper is rolled in a cylinder, stapled, and placed vertically in a closed container containing a small amount of solvent\(^1\) (Figure 2.2a). The solvent is allowed to wick up the paper through capillary action (called "elution," Figure 2.2b), and through this method complete separation of the blue and yellow components can be achieved (Figure 2.2d).

Figure 2.2: Paper chromatography of diluted green food dye: a) Before elution, b) After elution, c) Zoom in before elution, d) Zoom in after elution.

\(^1\)The solvent used in this separation is a solution made from a 1:3:1 volume ratio of \(6 \: \text{M} \: \ce{NH_4OH}\):1-pentanol:ethanol.