Page 184 - Multidimensional Chromatography

Multidimensional Planar Chromatography 177

Figure 8.5 Schematic illustration of 2-D multiple development on a bilayer with the same
mobile phase (S T1 ; S V1 ).

In the first version with a mobile phase of constant composition and with single
developments of the bilayer in both dimensions, a 2-D TLC separation might be
achieved which is the opposite of classical 2-D TLC on the same monolayer station-
ary phase with two mobile phases of different composition. Unfortunately, the use of
RP-18 and silica as the bilayer is rather complicated, because the solvent used in the
first development modifies the stationary phase, and unless it can be easily and quan-
titatively removed during the intermediate drying step or, alternatively, the modifica-
tion can be performed reproducibly, this can result in inadequate reproducibility of
the separation system from sample to sample. It is therefore suggested instead that
two single plates be used. After the reversed-phase (RP) separation and drying of the
plate, the second, normal-phase, plate can be coupled to the first (see Section 8.10
below).

8.6 MULTIPLE DEVELOPMENT IN ONE,
TWO OR THREE DIMENSIONS

When multiple development is performed on the same monolayer stationary phase,
the development distance and the total solvent strength and selectivity values (16)
of the mobile phase (17) can easily be changed at any stage of the development
sequence to optimize the separation. These techniques are typically fully off-line
modes, because the plates must be dried between consecutive development steps;
only after this can the next development, with the same or different development dis-
tances and/or mobile phases, be started. This method involves the following stages:
• Repeated chromatography of the sample in the same direction with the chro-
matographic plate being dried before all re-developments.
• Repeated chromatography of the sample in the first direction, followed by
repeated development in the second direction, at right angles to the first. Again,
the chromatographic plate is dried between re-developments.
• Repeated chromatography in a third dimension after completion of two-dimen-
sional development. Here, development in the first, second, and third dimensions
can be envisaged as occurring on three ‘plates’arranged in the form of a cube; the
plate is again dried between developments.