Page 98 - Multidimensional Chromatography

90 Multidimensional Chromatography

Figure 4.8 The GC GC experiment can be considered to be a series of fast second chro-
matograms conducted about five times faster than the widths of the peaks on the first dimen-
sion. The 1D elution time is the total chromatographic run time, while the 2D time is the
modulation period (e.g. 4–5 s). This figure shows two overlapping peaks A and B, with the
zones of each peak collected together. When these slices are pulsed to the second column, they
are resolved. Here, we show peak B eluting later on column 1, but earlier on column 2, with
the 2D peak maxima tracing out a shape essentially the same as the original peak on 1D.

column 2 (e.g. using a more selective stationary phase) then each peak becomes a
series of related second-column peaks. These secondary peaks will increase and then
decrease according to how the primary peak distribution varies on the first column.
Note that peak B has a longer retention time t R on column 1, but is drawn as a shorter
t R on column 2. This would suggest that compound B has a smaller distribution coef-
ficient (K) than compound A on column 2, and consequently a smaller retention fac-
tor, k, and thus a smaller retention time than A.
The peaks are revealed in the 2D separation space as oval-shaped peaks in a con-
tour plot format, reproduced as a schematic diagram in Figure 4.9. The contour
peaks are now completely separated in the 2D space, whereas they were severely
overlapping on the first column.
This novel manifestation of the gas chromatographic separation demands that
our fundamental understanding of the GC method–invariably of single-dimensional
scope – is challenged as follows: concepts of column efficiency and separation are
now supplanted by a need to compare the performances of two columns operating