It may be somewhat audacious to claim that some methods in GC×GC can be called non-classical — the technique has only been
around for about 15 years and one might argue that this is not long enough to establish a suite of classical methods.
However, it is possible to describe a general approach to GC×GC that held a predominant position in the years up to the early
2000s. This incorporated a conventional length 1D (first dimension; ~30 m) column interfaced to a short 2D column, typically 1–1.5 m long. The inner diameter ratio of the two columns (1i.d.:2i.d.) was about 2–3, and the modulation period (PM) was about 4–7 s. This period was selected based on the peak width generated using the first column, such that the modulation
ratio (ratio of peak width at base to modulation period) MR was about 3–4.
Figure 1
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Figure 1 presents the process used to generate first the modulated peaks, then the data manipulation and finally the 2D plot. Here the relative peak widths on each dimension are represented and the modulation period leads to about seven modulated
peaks, which may to some extent oversample the first dimension peak. Each 2D peak is a separate chromatogram on the second column, and data transformation treats this somewhat like a data matrix determined
by each of these chromatograms. This is illustrated in Figure 1.
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There are a few ways to operate the first column to satisfy these conditions, and a short but very thick film column will
also achieve the desired first peak width that allows multiple sampling of the peak. The 2D column is really only concerned with the elution temperature (Te) of the compounds as they pass from 1D to 2D. Te determines the retention factor (k) of the compounds on the 2D column, based on carrier flow-rate (α 1/tM) and film thickness (df), according to distribution constant (K):
Whilst the phase ratio (β) of the two columns is normally not much different, the short length and high carrier flow-rate
leads to very low retention times of a few seconds on the 2D columns.
Time, Peak Capacity and Speed
In much of the GC×GC literature, total analysis time has not been a critical concern. Generally, since we achieve such a high
peak capacity and very many peaks for GC×GC, most researchers have been interested in displaying the full compositional heterogeneity
of a sample, with as many resolved peaks as possible. Indeed, demonstrating the sheer complexity of many samples has been
a very attractive goal for researchers. The peak generation potential per minute for GC×GC is perhaps 5–10 times greater than
that of a single column analysis. However, with this available separation power, it is possible to sacrifice some of it to
produce a much faster analysis, but still with much more separation than a single column.
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