This paper presents work on the size-exclusion chromatography (SEC) of petroleum-derived fractions, using a Mixed D column (Polymer Laboratories, U.K.), where pure 1-methyl-2-pyrrolidinone (NMP), as well as several NMP/CHCl3 mixtures were used as eluents. The column calibration was performed using a variety of polymeric molecular mass standards and model polycyclic aromatic hydrocarbon (PAH) and heterocyclic compounds. Mixing CHCl3 into the eluent was observed to reduce the structure dependence of elution times of sample polymers compared to NMP alone. Elution times of a set of PAHs and heterocyclic compounds gave a line with a different slope for these smaller mass model compounds (<500 units). The mixing of CHCl3 with NMP as the eluent appears to extend the elution times of these aromatic and polar compounds, possibly because of the marginal promotion of surface effects. However, the mixed solvents allowed elution of the fractions of petroleum asphaltenes that were insoluble in NMP alone, indicating that these materials were both aromatic and of a larger size than the NMP-soluble fractions. The excluded peak observed in chromatograms of asphaltene samples became more intense when using the mixed solvent compared to SEC using pure NMP as eluent. Synchronous UV-fluorescence spectra confirmed that petroleum-derived components with the largest chromophores are insoluble in NMP but soluble in chloroform and observable by UV-fluorescence spectroscopy. The fraction insoluble in NMP was of larger aromatic cluster size and presumably attached to very large aliphatic groups that rendered the material insoluble in NMP but soluble in both chloroform and the mixed solvent. The work has shown that careful consideration of solvent properties may be instrumental in bringing together data from structurally very different polymeric standards. However, we have seen that the combination of NMP with chloroform has generated a SEC system, with its corresponding calibration, able to dissolve and elute petroleum asphaltene samples that were only partly characterized when using pure NMP as eluent.

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Methyl-2-pyrrolidinone (NMP) is widely used as a solvent for coal-derived products and as eluent in size exclusion chromatography. It was observed that sample contamination may take place, through reactions of NMP, during extraction under refluxing conditions and during the process of NMP evaporation to concentrate or isolate samples. In this work, product distributions from experiments carried out in contact with air and under a blanket of oxygen-free nitrogen have been compared. Gas chromatography/mass spectrometry (GC-MS) clearly shows that oxidation products form when NMP is heated in the presence of air. Upon further heating, these oxidation products appear to polymerize, forming material with large molecular masses. Potentially severe levels of interference have been encountered in the size exclusion chromatography (SEC) of actual samples. Laser desorption mass spectrometry and SEC agree in showing an upper mass limit of nearly 7000 u for a residue left after distilling "pure" NMP in contact with air. Furthermore, experiments have shown that these effects could be completely avoided by a strict exclusion of air during the refluxing and evaporation of NMP to dryness.

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