Page 193 - Multidimensional Chromatography

186 Multidimensional Chromatography

8.10 COUPLED LAYERS WITH STATIONARY
PHASES OF DECREASING POLARITY

Chromatographic plates can be connected for both capillary-controlled and forced-
flow planar chromatography (FFPC), i.e. irrespective of whether capillary action or
forced-flow is the driving force for the separation. The first technique is denoted as
‘grafted’ planar chromatography (31), while the second is known as ‘long distance’
(LD) OPLC, which uses heterolayers (32, 33).

8.11 GRAFTED PLANAR CHROMATOGRAPHY

The idea of coupled TLC plates, denoted as graft TLC, was reported in 1979. Graft
TLC (31) is a multiple system with layers of similar or different stationary phases for
isolation of compounds from natural and/or synthetic mixtures on a preparative
scale. Two plates are grafted together and clamped in the fashion of a lap-joint with
the edges of their adsorbent layers in intimate contact, so that a compound from a
chromatogram developed on the first chromatographic plate can be transferred to the
second plate without the usual scraping of bands, extraction, and re-spotting. At that
time, the method could be used for preparative purposes only–because home-made
plates were employed, compounds could not be transferred quantitatively, as was
necessary for analytical separations.
Nowadays, almost all commercially available HPLC stationary phases are also
applicable to planar chromatography. In addition to the polar hydroxyl groups
present on the surface of native silica, other polar functional groups attached to the
silica skeleton can also enter into adsorptive interactions with suitable sample
molecules (34). Silica with hydrophilic polar ligands, such as amino, cyano, and diol
functions, attached to the silica skeleton by alkyl chains, all of which have been well
proven in HPLC, have also been developed for TLC (34).
A simple method for serially connected TLC on coupled chromatographic plates
coated with different stationary phases (6) is illustrated schematically in Figure 8.15.
An appropriate amount of stationary phase A (light shading in Figure 8.15(a)) must
be removed, as must a corresponding part of stationary phase B (dark shading in
Figure 8.15(b)). The amounts removed are such that when the two chromatographic
plates are turned face to face (Figure 15(c)) and are grafted (pressed) together
(Figure 15(d)), the remaining regions of the layers overlap.
The configuration illustrated in Figure 8.15 is an ultra-micro chamber in which
the vapour phase is practically unsaturated (35). The distance between the support-
ing glass plate and the stationary phase of the other chromatographic plate is the
layer thickness, which depends on the type of analytical (20–25 m) or preparative
(0.5–2.0 m) plate applied. The sample to be separated (black zone in Figure
8.15(a)) is applied to stationary phase A after removal of the region of the layer not
necessary for the graft TLC separation. The two plates must be clamped in lap-joint
fashion with the edges of their stationary phases in close contact (Figure 8.15(d)) so
that compounds from the chromatogram developed on the first chromatographic