up correctly, the solvent will creep up the plate by capillary action, carrying the various compounds
in the sample with it at varying rates. When the solvent front nears the top of the plate, the plate is
removed from the chamber and allowed to dry.
As mentioned before, TLC is often done with silica gel, which is polar and hydrophilic. The mobile
phase, on the other hand, is usually an organic solvent of weak to moderate polarity, so it doesn’t
bind well to the gel. Because of this, nonpolar compounds dissolve in the organic solvent and move
quickly as the solvent moves up the plate, whereas the more polar molecules stick to the gel. Thus,
the more nonpolar the sample is, the further up the plate it will move, as shown in Figure 12.5.
Figure 12.5. Thin-Layer Chromatography
Samples are placed at the “X” marks. As the nonpolar solvent moves up the plate via capillary action,
the samples that are nonpolar move further up the plate along with the solvent, while the samples
that are polar do not move as far.
Reverse-phase chromatography is the exact opposite. In this technique, the stationary phase used
is nonpolar, so polar molecules move up the plate quickly, while nonpolar molecules stick more
tightly to the stationary phase.
The spots of individual compounds are usually white, which makes them difficult or impossible to
see on the white paper or TLC plate. To get around this problem, the developed plate can be placed
under ultraviolet light, which will show any compounds that are ultraviolet-sensitive. Alternatively,
iodine, phosphomolybdic acid, or vanillin can be used to stain the spots, although this will destroy
the compounds such that they cannot be recovered.