Page 410 - Multidimensional Chromatography

402 Multidimensional Chromatography

Nevertheless, a number of gas chromatographic applications exist, epecially those
for the determination of crude oil indicators. Such indicators are used as geochemi-
cal parameters for the thermal history of the crude as well as to indicate the possible
relationship between crudes from different wells. These indicators comprise a num-
ber of isomeric aromatic species, such as the individual alkylnaphthalenes (44, 45),
the individual C 10 -mono-aromatics or the individual C 9 -mono-aromatics. The ratio
between these isomers gives a definite indication of the crude oil. In general, these
systems use a Deans switching unit to make a heart-cut, which then is focused, re-
injected and separated on a second column with a different polarity.

14.5.1 A DETAILED HYDROCARBON ANALYTICAL SYSTEM
COUPLED TO A SIMULATED DISTILLATION PROCESS

The main characteristic of a crude oil for processing is its boiling range, which is gen-
erally determined by (single-column) gas chromatography, and is designated as ‘sim-
ulated distillation’. Because this analysis is performed with a ‘high-temperature’
(short, thin film, highly temperature stable, etc.) column, the lower part of the boiling
range is not too well separated and defined. In order to determine the economic
value, as well as to predict and control the optimum (crude) distillation cut points,
this low-boiling part can be determined with a valve switched pre-column. The ‘high
temperature’ column is fitted in the GC oven, which is temperature programmed up
to temperatures of around 400 °C. In a separate valve oven, a second, high resolution
capillary column with a valve is accommodated. This column will separate isother-
mally the light end (in which an internal standard, 3,3-dimethyl-1-butene, is
included) of the sample, e.g. up to C 9 , as shown in Figure 14.23. The remaining part
is back-flushed to vent. A separate injection is performed on to the ‘high-tempera-
ture’ column for the simulated distillation. A software program then enables the
incorporation of the two data sets into a final report.

14.5.2 AN SEC–LC–GC SYSTEM FOR THE ANALYSIS OF
‘LOW BOILING’ MATERIALS IN RESIDUAL PRODUCTS

A more complicated, but flexible, system has been reported by Blomberg et al. (46).
Here, size exclusion chromatography (SEC), normal phase LC (NPLC) and GC were
coupled for the characterization of restricted (according to size) and selected
(according to polarity) fractions of long residues. The seemingly incompatible
separation modes, i.e. SEC and NPLC, are coupled by using an on-line solvent-evap-
oration step.
Another interesting, but rather complex system, which couples flow injection
analysis, LC and GC has been recently reported (47). This system allows the deter-
mination of the total amount of potentially carcinogenic polycyclic aromatic com-
pounds (PACs) in bitumen and bitumen fumes. This system could also be used for
the analysis of specific PACs in other residual products.