Comparison of fractionation methods for the structural characterization ofpetroleum residues

Two petroleum residues have been fractionated using solvent (heptane) separ ation, planar and column chromatography. The residues and the separated fra ctions have been characterized by size exclusion chromatography (SEC), MALD I (matrix-assisted laser desorption/ionization) mass spectrometry, and by W -fluorescence spectroscopy (UV-F), MALDI mass spectrometry has indicated bo th residues to contain material with molecular mass ranges up to 15 000 u. The upper mass ranges indicated by size exclusion chromatography using poly styrene standards were higher; the earliest eluting material from both dist illation residues eluted at times corresponding to polystyrene standards of MMs above 1.85 million u. Data from UV-F suggests that the heptane solubil ity separation method was the most successful for the separation of the lar gest molecular mass-and also probably the most polar-materials in these res idues, However, all three fractionation methods produced similar trends, sh owing greater polarity of the fractions to correlate with increasing molecu lar mass. The shift of maximum intensity of fluorescence toward longer wave lengths (in UV-fluorescence) with increasing molecular size, as indicated,b y SEC, strongly suggests that the fluorescing molecules are large rather th an aggregates of small molecules.