Planar Chromatography

The consequence of the suboptimal mobile-phase velocity in planar chromatography obtained by capillary-controlled flow is that zone broadening is dominated by diffusion. Since the mobile-phase velocity varies approximately quadratically with migration distance, solutes are forced to migrate through regions of different local efficiency and the plate height for the layer must be expressed by an average value (Figure 12). Each solute in the chromatogram experiences only those theoretical plates over which it migrates, with solutes close to the sample application point experiencing very few theoretical plates and those close to the solvent front experiencing up to an upper limit of about 5000. High performance layers, with a nominal average particle size of about 5 | m, provide more compact zones than coarser particles, provided that the solvent front migration distance does not exceed about 5-6 cm; beyond this point zone broadening exceeds the rate of zone centre separation. When the development length is optimized the separation performance of conventional layers (average particle size about 10 |im) is not very

Figure 12 Variation of the average plate height as a function of the solvent front migration distance for conventional and high performance silica gel layers with capillary-controlled and forced-flow development. (Reproduced with permission from Poole CF and Poole SK (1997) JournalofChromatographyA 703: 573, copyright © Elsevier Science B.V.)

Figure 12 Variation of the average plate height as a function of the solvent front migration distance for conventional and high performance silica gel layers with capillary-controlled and forced-flow development. (Reproduced with permission from Poole CF and Poole SK (1997) JournalofChromatographyA 703: 573, copyright © Elsevier Science B.V.)

different from that of the high performance layers; the primary virtue of the latter is that a shorter migration distance is required to achieve a given efficiency, resulting in faster separations and more compact zones that are easier to detect by scanning den-sitometry. The minimum in the average plate height under capillary-controlled conditions is always greater than the minimum observed for forced-flow development, indicating that under capillary-controlled flow conditions the optimum potential performance is currently never realized in full. Under forced-flow conditions the minimum in the plate height is both higher and moved to a lower velocity compared with values anticipated for a column in LC, (Figure 13). Also, at increasing values of the mobilephase velocity, the plate height for the layer increases more rapidly than is observed for a column. At the higher mobile-phase velocities obtainable by forced-flow development, resistance to mass transfer is an order of magnitude more significant for layers than for columns. The large value for resistance to mass transfer for the layers may be due to restricted diffusion within the porous particles or is a product of heterogeneous kinetic sorption on the sorbent and the binder added to layers to stabilize their structure. The consequences for forced-flow TLC are that separations will be slower than for columns and fast separations at high flow rates will be much less effi-

Figure 13 Plot of the reduced plate height (H/dP) against the reduced mobile-phase velocity (udP/D) for a high performance and a conventional TLC layer using forced-flow development superimposed on a curve for an ideal LC column. (Reproduced with permission from Fernando WPN and Poole CF (1991) Journal of Planar Chromatography 4: 278, copyright © Research Institute for Medicinal Plants.)

Figure 14 Separation of polycyclic aromatic hydrocarbons by forced-flow TLC with online detection (elution mode). A silica gel high performance layer, migration distance 18 cm, with hexane as the mobile phase (0.07cms~1) was used for the separation. (Reproduced with permission from Poole CF and Poole SK (1994) Analytical Chemistry 66: 27A, copyright © American Chemical Society).

Figure 13 Plot of the reduced plate height (H/dP) against the reduced mobile-phase velocity (udP/D) for a high performance and a conventional TLC layer using forced-flow development superimposed on a curve for an ideal LC column. (Reproduced with permission from Fernando WPN and Poole CF (1991) Journal of Planar Chromatography 4: 278, copyright © Research Institute for Medicinal Plants.)

Figure 14 Separation of polycyclic aromatic hydrocarbons by forced-flow TLC with online detection (elution mode). A silica gel high performance layer, migration distance 18 cm, with hexane as the mobile phase (0.07cms~1) was used for the separation. (Reproduced with permission from Poole CF and Poole SK (1994) Analytical Chemistry 66: 27A, copyright © American Chemical Society).

cient than for columns, although in terms of total efficiency and separation speed the possibilities for forced-flow development are significantly better than those of capillary-controlled separations (Figure 14).

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