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Abstract

Rationale:
Structural characterization of individual compounds in very complex mixtures is difficult to achieve. One important step in structural elucidation is understanding the mass spectrometric fragmentation mechanisms of the compounds present in such mixtures. Here, different individual compounds presumed to be present in a complex crude oil mixture have been synthesized and structurally characterized by tandem mass spectrometry (MS/MS) studies.

Methods:
Model compounds with different aromatic cores and various substitutents were synthesized. Major effort has been put into producing isomerically pure compounds to better understand the fragmentation pattern. Each synthesized compound has been subjected to MSⁿ studies using either a triple quadrupole or a linear ion trap mass spectrometer with electrospray or atmospheric pressure photoionization. The results are used to analyze individual compounds from a complex vacuum gas oil (VGO).

Results:
The synthesized compounds and a chromatographically simplified vacuum gas oil were used for structural analysis. The major fragmentation mechanism is the benzylic cleavage of the aliphatic side chain. Each side chain can be separately removed from the aromatic core by using MSⁿ methods. At the end of a series of fragmentations, the base aromatic core structure remains and can be chararcterized.

Conclusions:
By defining the fragmentation mechanism in complex oil samples it was possible to structurally characterize individual compounds present in a chromatographically simplified VGO. The compounds consist of an aromatic core with aliphatic side chains. Cleavage of all side chains can be achieved by MSⁿ measurements, allowing characterization of the remaining core structure.