alumina) covered with thin layers of silica gel (see Fig. 3.19). The separation is
performed by stippling the compound mixture near the bottom of the sheet and by
positioning the sheet into a solvent in a way that the lower end is placed in the
solvent (without any contact of the substance mixture). Caused by capillary forces
the solvent starts to move upwards and serves in this manner as the mobile phase.
The individual substances become mobilized with different velocity depending on
the polarity of the solvent and their own polarity. Since silica gel represents a polar
phase, lipophilic compounds get mobilized relatively fast by nonpolar solvents,
whereas the more polar substances need more polar mobile phases for mobilization.
As already introduced, individual R f values can be determined for each separated
plastic/alumina
sheet or glass
plate
coating by SiO 2
glass chamber
starting line
sample
eluent (solvent)
turn 90°
Thin layer chromatography TLC
Simple 1D TLC
Radial TLC
2D TLC
Fig. 3.19 Principles and various technical realizations of thin layer chromatography (adopted from
and modified after Schwedt 2007)
3.3 Fractionation
35