Page 192 - Multidimensional Chromatography

Multidimensional Planar Chromatography 185

3
Figure 8.14 Schematic diagram of the peak capacity of a 3-D (n ) planar chromatographic
system employing mobile phases of different composition; the number of cubes represents the
number of compounds which can theoretically be separated.

the third dimension, the separated compounds would be found on different sheets. It
might, however, also happen that the same spot could be found on more than one
sheet. Needless to say, the limitations of such a system would clearly include all the
disadvantages of 2-D TLC.
Botz et al. (29) also demonstrated, by scanning electron microscopy, that applica-
tion of overpressure increases the density of the layer, which could be one reason for
the higher separation efficiency. These results showed that Empore TM silica TLC
sheets enable extremely rapid separations (5–20 min) in one-dimensional OPLC,
and gave good resolution. Theoretically, for a 3-D OPLC separations development
times of 15–60 min would be required. The separation cube of sheets could be
especially useful for micropreparative separations (30).
Combination of 3-D OPLC with multiple development encompasses all of the
advantages of three-dimensional and forced-flow planar chromatography, and the
separating capacity of multiple development. Favourable conditions could be to start
the separation in the first dimension, and then reducing the total solvent strength
stepwise at constant mobile phase selectivity to achieve a crude separation on the
basis of the polarity of the compounds to be separated. In the second (perpendicular)
direction, multiple development could be performed at constant S T but with variation
of the mobile phase selectivity. The third dimension would enable a combination of
total solvent strength and mobile phase selectivity for improving the resolution of
complex matrices (see Figure 8.14).
Theoretically, 3-D OPLC in combination with multiple development is the most
powerful technique of instrumental planar chromatography. Unfortunately, suitable
instrumentation is at an early stage of development.