Page 96 - Multidimensional Chromatography

88 Multidimensional Chromatography

closure, or a pressure change. Depending on the demands of the analysis, the target
solutes might be only those transferred to the 2D, or might be peaks recorded at both
detectors. There is no guarantee that both detectors respond to an equivalent degree.
The use of modern electronic gas controls certainly adds to the precision of the
MDGC experiment and should increase method robustness, but if there is a flow bal-
ancing problem between the two exit paths from the I/V device, the heartcutting
event might be no longer reliable.
LMCS should provide an almost equivalent experimental result for the basic
MDGC experiment described above. We have recently shown that within the single-
capillary GC analytical run, a number of modes can be performed, including nor-
mal GC, targeted GC and comprehensive gas chromatography (22). The in-line
cryotrap, which is capable of ‘modulation’ or pulsing of sections of effluent from
column 1 to column 2, is conceptually the same as the heartcutting process in con-
ventional MDGC, as described above. However, in order to create a workable sys-
tem the second column must be operated under fast GC conditions (compared with
the first column elution times) so that during the analysis of sequential sections of
the first column zones, there is no overlap of adjacent pulsed zones. Instead of
excising heartcut zones from column 1 to column 2, in this case we collect the
desired zone in the cryotrap, and isolate it from the neighbouring zones by simply
pulsing the collected zone to column 2. Any solute following this target zone in the
first column is held back in the cryotrap while the previous zone is being analysed
on column 2, and is retained in the cryotrap until itself is pulsed to column 2. If the
column 2 analysis is fast (e.g. 0.2–0.5 min) and the column selectivity is sufficient,
then many repeated pulsed zones can be analysed with target analytes resolved.
Figure 4.7 shows how this concept can be viewed, with zones 1–9 collected and
separately pulsed though to column 2. Dimension 2 is a fast separation and all
peaks transferred to column 2 are eluted before the next pulse is delivered to col-
umn 2. Thus the three peaks in zone 3 will be resolved. This demands that these
solutes have sufficient chemical difference to allow the phase on column 2 to differ-
entiate them into individual peaks. Note that events 4 and 8 are blank (or dummy)
events, and are inserted so that any trace components which may be collected since
the previous zone are cleaned out of the cryotrap before the next peak is trapped.
This could also be done to remove the buildup of column bleed in the cryotrap.
Note that on the diagram, zone 1 is pulsed to dimension 2 when zone 2 is being col-
lected, and so this explains why each peak or peak group is offset by one event step
from the upper event sequence. The above procedure has been implemented in this
laboratory for a number of applications such as semi-volatile aromatic hydrocar-
bons (23) and sterols (24).
It is again clear that the two benefits of increased sensitivity and better resolution
are both achieved, where these arise from zone compression and phase selectivity,
respectively. However, since this mode of analysis is relatively new, it has yet to be
tested for a wide range of applications; such studies will be required to fully demon-
strate its general utility. It is unclear whether this operational mode of selective
MDGC constitutes a mode which is consistent with the definition of comprehensive