Page 97 - Multidimensional Chromatography

Orthogonal GC–GC 89

Figure 4.7 The selective or targeted mode of LMCS operation allows selected peaks to be
collected sequentially in the cryotrap, and then pulsed rapidly to the second column. The
resulting peaks are narrow and tall; provided that the second column phase selectivity and effi-
ciency are adequate, they will also be resolved. The process is repeated as many times as
required during the analysis. On this diagram, the lower trace response scale will be consider-
ably less sensitive than on the upper trace.

gas chromatography as originally proposed, since this mode was not available until
recently, and so neither its introduction nor its capability were anticipated by earlier
chromatography researchers. However, it is probably better to only consider those
modes which produce the more familiar GC GC performance that should be
termed comprehensive gas chromatography, as described below.

4.3.3.2 Mode 2: Comprehensive Gas Chromatography (GC GC)

The process of GC GC requires fast analysis of multiple slices of each peak elut-
ing from column 1 to column 2. The cryogenic modulator was shown to produce this
result (25) with the pulsed peak faithfully following the peak profile. Peak
halfwidths of the order of 50 ms were reported, and a crude oil chromatogram pre-
sented. The process shown in Figure 4.8 illustrates the relative speed of modulation
with respect to the peak width on the first column, which enters into the cryotrap.
Thus about five or so slices are analysed successively on column 2. Figure 4.8
demonstrates that if two peaks are severely overlapping on the first column, and then
subjected to the cryotrap-pulsing process, we can therefore obtain separate resolved
peaks on column 2. By choosing conditions which are able to resolve the peaks on