Page 110 - Multidimensional Chromatography

102 Multidimensional Chromatography

marker components and also absolute peak amounts that allow the comparison of
spills with their likely sources. The goals of some studies were to provide the sepa-
ration of target groups of compounds from gasoline (e.g. benzene, toluene,
xylenes, C 9 ,C 10 and C 11 benzenes, naphthalene and C 11 and C 12 naphthalenes),
quantitate these compounds by using internal standards, and then establish the fea-
sibility of such studies by using GC GC. They concluded that the GC GC
approach produced results directly comparable with ASTM methods. The ranges
of individual compounds were of the order of 0.1–24% for calibration data, while
the RSD% values were from 0.6–5% for individual components, and up to 14% for
grouped components such as C 12 naphthalenes. The ability of the GC GC
method to give the required data in a single run, using a new technique, with a
result that was not too dissimilar to the ASTM result, was laudable. The GC GC
method was also able to produce data of sufficient reliability to analyse quantita-
tively the levels of target compounds in oil spills, for comparison with possible
discharge sources.
Again in the petroleum realm, Beens et al. (39) compared the analytical results
obtained by the GC GC method against both single-column capillary GC and an
LC–GC hyphenated system. A test mix of a variety of typical analytes gave essen-
tially equivalent results in the first comparison, and the analysis of heavy gas oil by
LC–GC and GC GC for grouped classes of components (e.g. saturates and
mono- di- and tri-aromatics) again appeared acceptable. These authors concluded
that GC GC was the technique of choice, although they acknowledged that the
thermal sweeper had a restricted useable upper temperature limit. This work also dis-
cussed the data handling program ‘Tweedee’ that had been developed to provide
some automated reporting capability of peak areas.
There are reported to be a number of important characteristics of GC GC that
permit more reliable peak response quantitation over single-column GC analysis.
These are as follows:
(i) There is less chance of peak overlap, which means that peak areas/heights will
more reliably give these parameters of a pure peak rather than have contribu-
tions from minor unresolved constituents.
(ii) Related to this is the fact that the regions of the 2D space that do not contain
any peaks will represent the true detector baseline rather than a chemical base-
line comprising unresolved components.
(iii) GC GC gives sharp peak pulses at the detector and so these peaks should be
more reliably measured than broader peaks where the baseline construction
may be less precise.
(iv) Quantitation requires calibration (response versus amount), and detection
limit information should be available. Zone compression gives sensitivity
improvement, and hence lower detectable amounts.
(v) Faster data acquisition (e.g. 50/100 Hz versus 5 Hz for normal capillary GC)
does lead to greater detection noise (by 3–4 times, respectively), and this
means that detection limit increases are not quite as good as might be
expected just on the basis of peak signal increases.