SOLUBILITY LIMITATIONS IN THE DETERMINATION OF MOLECULAR-MASS DISTRIBUTIONS OF COAL-LIQUEFACTION AND HYDROCRACKING PRODUCTS - 1-METHYL-2-PYRROLIDINONE AS MOBILE-PHASE IN SIZE-EXCLUSION CHROMATOGRAPHY

The use of tetrehydrofuran (THF) as mobile phase in size exclusion chr omatography (SEC) has been found to lead to partial loss of sample and to give anomalous results in the characterization of a liquefaction e xtract and its hydrocracking products. The problem has been resolved b y using NMP (1-methyl-2-pyrrolidinone) as mobile phase in SEC, showing significant fractions of sample eluting at the exclusion limit of an identical SEC column. This fraction has not previously been observed i n SEC chromatograms obtained in THF. Comparison of SEC chromatograms o btained by UV-absorption and UV-fluorescence detection (in NMP) sugges ts that the material observed at the exclusion limit of the column cor responds to larger, more complex polynuclear aromatic ring systems tha n those present in material separated by the column. In NMP, samples p roduced during progressively higher temperature hydrocracking experime nts eluted, as expected, at longer times, indicating progressive molec ular size reduction with increasing intensity of the reaction. These d ata are consistent with the W-fluorescence spectra and TGA-derived boi ling point distributions of the set of samples. A twofold mechanism fo r loss of material in THE-based SEC may be proposed: (i) not all the s ample dissolves in THE and (ii) some of the larger/more polar molecule s apparently soluble in THF tend to deposit on column packings and do not elute through the column. Considerable caution therefore appears n ecessary in using THF as mobile phase in SEC work for the characteriza tion of complex coal-derived liquids.