Comparison of fractionation methods for the structural characterization of petroleum residues

Two petroleum residues have been fractionated using solvent (heptane) separation, planar and column chromatography. The residues and the separated fractions have been characterized by size exclusion chromatography (SEC), MALDI (matrix-assisted laser desorption/ionization) mass spectrometry, and by W-fluorescence spectroscopy (UV-F), MALDI mass spectrometry has indicated both residues to contain material with molecular mass ranges up to 15 000 u. The upper mass ranges indicated by size exclusion chromatography using polystyrene standards were higher; the earliest eluting material from both distillation residues eluted at times corresponding to polystyrene standards of MMs above 1.85 million u. Data from UV-F suggests that the heptane solubility separation method was the most successful for the separation of the largest molecular mass-and also probably the most polar-materials in these residues, However, all three fractionation methods produced similar trends, showing greater polarity of the fractions to correlate with increasing molecular mass. The shift of maximum intensity of fluorescence toward longer wavelengths (in UV-fluorescence) with increasing molecular size, as indicated,by SEC, strongly suggests that the fluorescing molecules are large rather than aggregates of small molecules.