Page 21 - Multidimensional Chromatography

10 Multidimensional Chromatography

Figure 1.3 Peak capacity of a 2D system (reproduced with permission from reference (30)).

diagonal (Figure 1.4) (31). Such a system would, however, be effective for alkylated
PAHs, where the two separations are less correlated (more similar retentions to the
parent PAHs in HPLC, but better separated from the parents in GC) (Figure 1.4).
Giddings pointed out (32) that separated compounds must remain resolved
throughout the whole process. This situation is illustrated in Figure 1.5, where two
secondary columns are coupled to a primary column, and each secondary column is
fed a fraction of duration t from the eluent from the first column. The peak capacity
of the coupled system then depends on the plate number of each individual separa-
tion and on t. The primary column eliminates sample components that would other-
wise interfere with the resolution of the components of interest in the secondary
columns. An efficient primary separation may be wasted, however, if t is greater
than the average peak width produced by the primary column, because of the recom-
bination of resolved peaks after transfer into a secondary column. As t increases,
the system approaches that of a tandem arrangement, and the resolution gained in
one column may be nullified by the elution order in a subsequent column.
Two-dimensional separations can be represented on a flat bed, by analogy with
planar chromatography, with components represented by a series of ‘dots’. In fact,
zone broadening processes in the two dimensions result in elliptically shaped ‘spots’
centred on each ‘dot’. Overlap of the spots is then possible, but Bertsch (30) also
showed how the contributors to the overall resolution, R, along the two axes, R z and
R y contribute to the final resolution according to the following:

2
2
R R R y 1 2 (1.5)
z
If the resolution is greater than 1 along either axis, then the final resolution will be
also greater than 1. It follows that the isolation of a component in a two-dimensional