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Multidimensional High Resolution Gas Chromatography 49

a single portion of primary column flow is reanalysed using the second column
selectivity. This naturally has a very significant impact on the application of the tech-
nique, in that only a limited fraction of the total sample is analysed with the full reso-
lution of a two-dimensional separation. This is not to say, however, that this limits
the usefulness of the technique, as later example applications of two-dimensional
GC will show.
Efforts have been made, however, to extend the range or extent of samples that
can be analysed by using a two-dimensional separation when used in heart-cut
mode. This has been reported to include the use of numerous parallel micro-traps to
essentially store the primary column eluent fractions ready for second-column sepa-
ration, and the use of parallel second-dimension columns.
The ultimate extension of two-dimensional GC was introduced in the early 1990s
(4) and involves the reanalysis of all components from the primary dimension on a
secondary column. To enable this, the peak capacity of the secondary column is
often very much smaller than the primary thus allowing completion of the separation
in a time that may be considered insignificant as a fraction of the time required to
complete the primary stage. This is discussed in much further detail in Chapter 4.

3.2.1 EXPERIMENTAL CONFIGURATIONS

The most basic classification of GC couplings is into off-line and on-line interfacing.
Off-line is described as the manual collection of effluent from a column prior to
manual re-injection to a second column. While this can be relatively simple to per-
form, there are very significant problems in handling volatile species, and the poten-
tial for artefact generation exists. Perhaps more importantly, the reproducibility of
manual handling of samples is poor and automation is clearly not practical. Far more
commonly used is a direct on-line coupling between systems. This may be described
as a system where the collection and transfer of fractions between columns is per-
formed within a sealed analytical system. In practice, this is enabled by the auto-
matic diverting of column flows via mechanical or pressure-driven switching
devices. Automation and reproducibility are greatly increased, and the introduction
of chemicals external to the procedure are eliminated.
Figure 3.1 shows several potential on-line modes of two-dimensional GC opera-
tion. These couplings demonstrate HRGC–HRGC performed by using a single
heart-cut from the primary to the secondary column, multiple heart-cuts, transferred
to multiple intermediate traps, and heart-cuts transferred to a multiple parallel sec-
ondary column configuration.
Although the ability to generate separation systems with significantly enhanced
peak capacities is the most obvious practical usage of two-dimensional GC, there are
several ancillary benefits which are often also achieved when analysis is performed
using this approach.
• Time for analysis. The analysis of complex samples when performed on single
columns generally requires very long separation periods, commensurate with the