permeation material supported on glass plates, plastic sheets or aluminium foil; development tanks;
components sometimes examined by reflectance or transmittance densitometry or removed for
spectrometric analysis.
Applications
Very widespread use, largely for qualitative purposes and for both organic and inorganic materials;
especially useful for checks on purity, to monitor reactions and production processes and to characterize
complex materials.
Disadvantages
Migration characteristics very sensitive to conditions; thin layers easily damaged; quantitative precision
only moderate: 5–10%.
The difference between this technique and GC or HPLC is that the separation process occurs on a flat
essentially two-dimensional surface. The separated components are not usually eluted from the surface
but are examined in situ. Alternatively, they can be removed mechanically for further analysis. In thin-
layer chromatography (TLC), the stationary phase is usually a polar solid such as silica gel or alumina
which is coated onto a sheet of glass, plastic, or aluminium. Although some moisture is retained by the
stationary phase, the separation process is predominantly one of surface adsorption. Thin layers are
sometimes made from ion-exchange or gelpermeation materials. In these cases the sorption process
would be ion-exchange or exclusion.
Samples are applied as spots or streaks close to one edge of the plate and the mobile phase is allowed to
travel from that edge over the samples and towards the opposite edge. In so doing, the components of
the sample move across the surface at rates governed by their distribution ratios and therefore separate
into individual spots or bands. This procedure is called development. After development, coloured
substances are immediately visible on the surface. Those which are colourless can be visualized by
treatment with a chromogenic reagent, detected by fluorescence under a UV lamp or by using
radioactive tracers. A typical thin-layer chromatogram is shown in Figure 4.48.
Diffusion and mass transfer effects cause the dimensions of the separated spots to increase in all
directions as elution proceeds, in much the same way as concentration profiles become Gaussian in
column separations (p. 86). Multiple path, molecular diffusion and mass transfer effects all contribute to
spreading along the direction of flow but only the first two cause lateral spreading. Consequently, the
initially circular spots become progressively elliptical in the direction of flow. Efficiency and resolution
are thus impaired. Elution must be halted before the solvent front reaches the opposite edge of the plate
as the distance it has moved must be measured in order to calculate the retardation factors (Rf values) of
separated components (p. 86).