Page 403 - Multidimensional Chromatography

Multidimensional Chromatographic Applications in the Oil Industry 395

Figure 14.16 Typical chromatograms of LC (a) and SFC (b) analysis of aromatics in diesel
fuel. Peak identification is as follows: 1, total saturates; 2, total aromatics; 3, mono-aromatics;
4, higher-ring aromatics.

retention gap. Here, the heptane will start the ‘solvent-effect’, thus focusing the satu-
rates into a narrow band. The heptane on the front of the plug will evaporate and
leave the system through the SVE valve. As soon as the saturates have completely
left the LC column, i.e. at the start of the mono-aromatics fraction, the LC pump is
stopped. The remaining fractions in the column will not broaden, since the diffusion
coefficients in the liquid phase are very low. Programming the GC oven will then
separate the focused fraction in the GC system. After cooling down the GC oven, the
whole process subsequently starts again for the mono-aromatics, di-aromatics, tri-
aromatics, etc. Clear separated chromatograms of all the fractions can be obtained,
as can be seen in Figure 14.18, for the mono-aromatics, and in Figure 14.19, for the
di-aromatics. No overlap from one fraction into the other is detectable. This means
that not only a quantitative group-type separation can be provided, but also that
within the various groups a clear carbon distribution can be obtained.

14.4.3 ON-LINE COUPLED LC–GC-FID-SCD FOR
SULFUR COMPOUND CHARACTERIZATION

Since the majority of middle distillates are used as a fuel, combustion of these prod-
ucts will contribute to SO 2 /SO 3 air pollution and acid rain. However, in catalytic pro-
cesses of petroleum fractions sulfur levels are also important. For instance, quantities