The
size distributions of sulfur (S), vanadium (V), and nickel (Ni) compounds
in four crude oils, two residues, and their saturate, aromatic, resin,
and asphaltene (SARA) fractions were determined using gel permeation
chromatography (GPC) coupled to inductively coupled plasma high-resolution
mass spectrometry (ICP HR MS). The results show trimodal distributions
of V, Ni, and S compounds in the crude oils and residues. V and Ni
compounds are present in both resins and asphaltenes. Trimodal distributions
are clearly apparent in the resins but not apparent in the asphaltenes.
In the latter, the predominant compounds have a high molecular weight
(HMW), even when the solution of asphaltenes is diluted by 40000-fold.
In the resins, compounds with a medium molecular weight (MMW) were
expected; however, HMW compounds were observed, indicating that nanoaggregates
or large molecules exist in both the asphaltenes and resins. Low-molecular-weight
(LMW) compounds are predominantly present in the resins and do not
represent more than 22% of V and Ni present in crude oil. These compounds
appear to have molecular weights similar to simple metalloporphyrins.
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