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80 Multidimensional Chromatography

analysis, unless the analyst was only specifically interested in increasing the separa-
tion of limited target zone(s).
This immediately also leads one to conclude that where a total sample should be
analysed by MDGC–due to the less-than-satisfactory resolution of components
achievable on a single column–or might be better analysed by MDGC applied over
the whole analysis, conventional MDGC fails to deliver the separation power
demanded. Wilkins and co-workers addressed this problem to some extent by using
multiple traps (5, 6) where they could isolate contiguous fractions from a petroleum
chromatographic separation on one column into different sorption traps, as shown by
the approach indicated in Figure 4.2(c). The lower part of the latter shows that each
‘trap’ is analysed separately, and here gives five independent 2D chromatograms.
Clearly, there is a physical and practical limitation to the number of traps that can be
employed for this purpose. We should view Wilkins’ work as an innovative transi-
tional technology, recognizing the limitations of MDGC and trying to develop a sys-
tem that offers a step towards maximizing the power of MDGC. Conceptually, we
should wish to apply Wilkins’ concept to the microcolumn environment, with mini-
mum zone widths and on a rapid time-frame, but without the multitude of traps. We
might thus seek to incorporate the idea of multiple-zone trapping into the column
itself.
While it is generally acknowledged therefore that MDGC can increase resolution
by transfering zones from one column to another, there is clearly a limited opportu-
nity to improve the total analysis through this approach.
Conventional multidimensional gas chromatography operation procedures should
now be reconsidered and redefined in light of the new method of comprehensive
GC GC technology, as discussed below.

4.2 INTRODUCTION TO GC GC SEPARATION
The comprehensive GC GC technique was introduced by Phillips and co-workers
(7–9) and has been reviewed by de Geus et al. (10). GC GC achieves the goal of
applying multidimensional gas chromatography separation over the total analytical
separation. The comprehensive GC GC experiment is also defined as a system
that allows all of the sample from the first column to be analysed on the second
column. The key to the experiment is the technical achievement of the interface
between the two dimensions, which is discussed in more detail below.
Notwithstanding the above discussion, GC GC is achieved precisely by the
method shown in Figure 4.2(b), except that we have to modify the diagram slightly,
and this is shown in Figure 4.2(d). The 2D analysis duration for the ‘heartcut’zone is
approximately the same as the zone duration taken or transferred from 1D. These
zones are a small fraction of the elution time of an individual chromatographic peak,
as shown in the figure. The transfer of solute from 1D to 2D involves, again, an inno-
vative approach that leads to zone compression of the chromatographic band
between the two dimensions. This is the ‘modulator’, and intervenes between the