Abstract:
The Fischer–Tropsch (FT) process produces a variety of compounds over a wide carbon number range
and the synthetic crude oil produced by this process is rich in highly valuable olefins and oxygenates,
which crude oil only contains at trace levels. The characterization of these products is very challenging
even when using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass
spectrometry (GC × GC–TOF-MS). The separation between cyclic paraffins and olefins is especially difficult
since they elute in similar positions on the GC × GC chromatogram and since they have identical
molecular masses with indistinguishable fragmentation patterns. Previously, a high performance liquid
chromatography (HPLC) fractionation procedure was used prior to GC
× GC–TOF-MS analysis to distinguish
between alkenes and alkanes, both cyclic and non-cyclic, however, there was co-elution of the
solvents used in the HPLC fractionation procedure, and the volatile components in the gasoline sample
and the dilution introduced by the off-line fractionation procedure made it very difficult to investigate
components present at very low concentrations. The hyphenation of supercritical fluid chromatography
(SFC) to GC × GC is less complicated and the removal of the supercritical CO2 can be easily achieved
without any loss of the volatile sample components, eliminating the introduction of co-eluting solvents
as well as the dilution effect. This paper describes the on-line hyphenation of SFC to a GC × GC system
in order to comprehensively characterize the chemical groups (saturates, unsaturates, oxygenates and
aromatics) in an FT sample.
