Page 200 - Multidimensional Chromatography

Multidimensional Planar Chromatography 193

Naturally, several other possibilities can be used to increase the number of dimen-
sions. Between the first and second developments, or sample, the characteristics of
the chromatographic plate or the properties of the sample can also be modified.
Although interfacing of on-line OPLC with one- or two-dimensional TLC is not par-
ticularly difficult, it is not yet widely practiced. It must be concluded that full
exploitation of the versatility of MD-PC is at an early state of development; as a con-
sequence several significant changes in practice might be expected in the next few
years (10).

8.14 COUPLING OF PC WITH OTHER
CHROMATOGRAPHIC TECHNIQUES

Another means of realizing multidimensional separation is combination of two com-
plementary separation techniques which use different methods of separation. In such
multi-modal separation, different techniques can be coupled in which PC is used as
the second dimension and another separation method, as the first. Some possible
variations are as follows:

• combination of different extraction methods (e.g. thermo and solid phase) with
TLC;
• combination of GC and TLC;
• combination of supercritical fluid chromatography (SFC) and TLC;
• combination of HPLC and TLC;
• combination of OPLC and TLC;
• combination of capillary electrophoresis (CE) and TLC;
• combination of different counter-current chromatographic (CCC) methods with
TLC.
Current interest is, however, mainly in the coupling of HPLC and TLC, to which
considerable attention has been devoted for the solution of difficult separation
problems. Since Boshoff et al. (39) first described the direct coupling of HPLC and
TLC, several papers (40–43) have been published describing the on-line coupling of
liquid chromatographic methods and PC, usually with different interfaces, depend-
ing on the first technique applied. If PC is used as the second method, all the MD
methods discussed above can be applied to increase the separating power.

8.15 FURTHER CONSIDERATIONS

Multidimensional planar chromatographic separations, as we have seen, require not
only a multiplicity of separation stages, but also that the integrity of separation
achieved in one stage be transferred to the others. The process of separation on a
two-dimensional plane is the clearest example of multidimensional separations. The
greatest strength of MD-PC, when properly applied, is that compounds are dis-
tributed widely over two-dimensional space of high zone (peak) capacity. Another