Page 402 - Multidimensional Chromatography

394 Multidimensional Chromatography

European countries, including the UK, and covers about 5% of the total European
diesel market. It contains less than 5 vol%. of mono-ring aromatics and less than
0.1 vol%. of di- and higher-ring aromatics. Furthermore it is low in sulfur. Both
LC and SFC methods have been developed for the rapid analysis of aromatics in
diesel fuel.

14.4.1 AN LC–SFC SYSTEM FOR THE ANALYSIS OF
TOTAL AROMATICS IN DIESEL FUEL

Different approaches utilizing multidimensional LC or SFC systems have been
reported for the analysis of middle distillates in diesel fuel. A method, based on the
LC separation of paraffins and naphthenes by means of a micro-particulate, organic
gel column has been described (23, 24). The complete system contained up to four
different LC columns, a number of column-switching valves and a dielectric
constant detector. However, the LC column for the separation of paraffins and
naphthenes, which is an essential part of the system, is no longer commercially
available.
In Figure 14.16, the chromatograms of a normal-phase LC analysis, using a
straight-phase silica column with back-flush are shown. The same method, using
carbon dioxide as the mobile phase and a cyanopropyl-modified silica column for
the SFC stage, is employed for the second separation. When the column is not back-
flushed, a clear distinction can be made between the mono-ring and higher-ring
aromatics.

14.4.2 ON-LINE COUPLING OF LC AND GC
FOR HYDROCARBON CHARACTERIZATION

A more sophisticated method, giving a much more detailed characterization,
involves the on-line coupling of LC and GC (LC–GC). Analysis schemes for middle
distillates (kerosine, diesel and jet fuels) combining LC and GC have been reported
by various authors (25–31). However, only Davies et al. (25) and Munari et al. (27)
have reported on the required automatic transfer of all of the individual separated
fractions from the LC unit the GC system. Davies used the loop-type interface and
Munari the on-column interface. Only Beens and Tijssen report a full quantitative
characterzation by means of LC-GC (31).
The system utilized is depicted schematically in Figure 14.17. In this set-up the
sample is injected onto the cyanopropyl-derivatized LC column. This column sepa-
rates hydrocarbons according to the number of -electrons in the molecules, which
is more or less according to the number of fused aromatic rings. The outlet of the
column is monitored by the UV detector for the start of eluting aromatics, and is con-
nected to the GC system by means of valve 1 (V1) through a narrow capillary into
the on-column injector. The effluent of the column–the mobile phase, n-heptane,
together with the first eluting fraction, the saturates–flows directly into the GC