Page 57 - Subyek Encyclopedia - Encyclopedia of Separation Science

52 I / CHROMATOGRAPHY/ Derivatization

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 ef-
Rciency, resulting in faster separations and more com-
pact 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-Sow
development, indicating that under capillary-control-
led Sow conditions the optimum potential perfor-
mance is currently never realized in full. Under for-
ced-Sow 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 mobile-
phase 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-
Sow development, resistance to mass transfer is an
order of magnitude more signiRcant for layers than
for columns. The large value for resistance to mass
transfer for the layers may be due to restricted dif-
fusion 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-Sow TLC are that separ-
ations will be slower than for columns and fast separ-
ations at high Sow rates will be much less efR-

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
1
the mobile phase (0.07 cm s ) 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
efRciency and separation speed the possibilities
for forced-Sow development are signiRcantly better
than those of capillary-controlled separations
Figure 13 Plot of the reduced plate height (H/d P ) against the (Figure 14).
reduced mobile-phase velocity (ud P /D M ) for a high performance
and a conventional TLC layer using forced-flow development
superimposed on a curve for an ideal LC column. (Reproduced Separation Quality
with permission from Fernando WPN and Poole CF (1991) Jour-
nal of Planar Chromatography 4: 278, copyright ^ Research The general object of a chromatographic separation
Institute for Medicinal Plants.) is to obtain an acceptable separation (resolution)