Page 250 - Multidimensional Chromatography

244 Multidimensional Chromatography

The 2D chromatograms reveal additional components of the natural mixtures. They
also give a ‘map’of the essential oil, which is helpful in the identification of the com-
ponents by the position and the characteristic colours of the derivatives on the plate.
A further, considerable improvement in the separation performance can be obtained
by using overpressured layer chromatography (OPLC). Härmälä et al. (70) used 2D
OPLC for the separation of coumarins from the genus Angelica. Figure 10.15 shows
the one-dimensional (a) and two-dimensional (b) OPLC separations of 16
coumarins.
Another way to improve separation in TLC is the use of gradient elution.
Programmed multiple development (PMD) is a technique in which the plate is devel-
oped over increasing distances with several eluents of decreasing eluent strength,
with the mobile phase being evaporated after each development. Other variations of
this technique are automated multiple development (AMD) automated version, and
the reversed PMD method, multiple gradient development (MGD), where a gradient
of increasing strength is applied over decreasing distances. Figure 10.16 shows the

Figure 10.16 Densitograms obtained for four subsequent developments of the extract from
Radix rhei: (a) first development, 10% (vol/vol) ethyl acetate/chloroform, distance 9 cm; (b)
second development, 50% (vol/vol) ethyl acetate/chloroform, distance 9 cm; (c) third
development, 100% ethyl acetate, distance 8 cm; (d) fourth development, 15% (vol/vol)
methanol/ethyl acetate, distance 5 cm. Reprinted from Chromatographia, 43, G. Matysik,
‘Modified programmed multiple gradient development (MGD) in the analysis of complex
plant extracts’, pp. 39–43, 1996, with permission from Vieweg Publishing.