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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.

A method of sequential liquid–solid extraction (leaching) has been developed to extract the V, Ni, and S compounds present in asphaltenes (n-C7) according to their molecular weight distribution. For the high-molecular-weight (HMW) compounds, two new families of compounds were extracted, labeled as HMW1 and HMW2, where the latter was smaller than HMW1, and together represented approximately 85% of the asphaltene mass according to the mass balance obtained after the extractions. The compounds associated with HMW1 were asphaltenes that were insoluble in hot dimethylformamide (DMF), while the compounds associated with HMW2 were soluble in DMF but insoluble in the second leaching step based on hot acetone. The third family of obtained compounds was the medium-molecular-weight (MMW) compounds, which were soluble in hot acetone but insoluble in acetonitrile (ACN). The last fraction to be obtained was the low-molecular-weight compounds, which were soluble in ACN. The results reported here represent a new method that allows the extraction of different types of aggregated asphaltenes according to their molecular weights. With respect to the temperature and number of extraction steps, it was observed that an increase in both parameters increased the extraction efficiency.

A method using liquid–liquid extractions has been developed for matrix simplification and evaluated by gel permeation chromatography coupled with inductively coupled plasma and mass spectrometry. In this method, maltenes were dissolved in n-heptane and extractions with methanol (MeOH), acetonitrile (ACN), and dimethylformamide (DMF) were performed. The extraction with ACN is more efficient than that with MeOH for the removal of compounds with low-molecular weights (LMW) containing V (our reference element) and more selective than that with DMF (with this solvent, compounds with LMW and medium-molecular weights (MMW) are extracted). Thus, a sequential extraction was performed by applying ACN to selectively remove LMW compounds, followed by extractions with DMF of the resulting maltene to separate the MMW compounds from the high-molecular weights (HMW) compounds remaining in the final remnant maltene. The results show, for the first time in the literature, that it is possible to separate the three V and Ni species of compounds present in maltenes based on their molecular weights (HMW, MMW, and LMW).

Gel permeation chromatography inductively coupled plasma high-resolution mass spectrometry (GPC ICP HR MS) was used for the understanding of the evolution of two crude oil cut samples after their dilution. We first studied different method parameters to compare two GPC procedures already published [flow rate, column set, and presence or not of the tetrahydrofuran (THF) stabilizer]. Thus, the principal parameters affecting the molecular size distribution and its evolution were demonstrated. The column set and flow rate can drastically affect the molecular size distribution of the sample. Moreover, an evolution of the size distribution of the complexes of vanadium, nickel, and sulfur into higher molecular weight compounds was observed over time. The study led to the recommendation of the sample preparation to the same day of the analysis to obtain reproducible data.