Two propositions relating to the interpretation of size-exclusion chromatograms (SEC) of coal-derived materials in 1-methyl-2-pyrrolidinone (NMP) have been examined. These were (i) that signal peaks showing up at exclusion (short retention time) limits of SEC columns are due to sample polarity alone and (ii) that shifts in SEC chromatograms to longer retention times, observed upon addition of LiBr to the eluent (NMP), are due to dissipation of ionic binding forces, causing disaggregation of polar clusters that would otherwise have appeared at retention times appropriate to larger molecular masses. In our experiments, effects due to polarity and molecular mass have been isolated by using two nonpolar samples (a naphthalene mesophase pitch and a mixture of fullerenes). In the presence of LiBr, precipitation of solute out of solution and shifts of chromatograms to longer retention times, unrelated to sample polarity, have been observed. A partial breakdown of the size exclusion mechanism was identified by the observed extension of chromatograms beyond the permeation limit of the column, similar to those observed when using eluents of insufficient solvent strength (e.g., THF, chloroform). Dosing LiBr into NMP sharply reduces the solvent power of NMP for coal-derived solutes. In the absence of LiBr, SEC chromatograms of the fullerene mixture, the naphthalene mesophase pitch, and its fractions separated by planar chromatography clearly showed significant signal under the “excluded” peak, entirely due to nonpolar material. The damage caused to the SEC column arising from precipitation of sample, in the presence of LiBr, was not permanent as had originally been feared. The balance of the evidence suggests that polarity of some molecules may cause shifts in their elution times to shorter values (larger apparent molecular masses) and that these may overlap with signal from large molecular mass material.

A number of samples which eluted at unexpectedly short retention times during size exclusion chromatography have been characterized. Soot and tar samples likely to show similar behavior have been examined. Distinct peaks from about 6 min were observed, compared to 9−10 min for more usual samples. Molecular masses of the early eluting material appear to be large, although extrapolation of existing calibrations does not seem appropriate. A naphthalene mesophase pitch also gave peaks at short elution times. It appears reasonable to interpret chromatograms of this nonpolar material as a direct indication of the presence of large molecular mass material, and to infer that excluded peaks of SEC chromatograms do not necessarily result from the presence of clusters of polar molecules. GC-MS and probe-MS examination of the samples showed only very limited proportions of the samples to have small molecular masses. MALDI-MS spectra of the samples indicated the presence of signal up to 20 000 u. Taken together, data from SEC and the three MS techniques indicated the presence of very large molecular mass materials in this set of samples. Tar deposits recovered from entrained, combusting coal particles have also been examined, providing direct evidence for the presence of large molecular mass material in combustion environments. The observation contrasts with mathematical models of coal burners, where rates of combustion of volatiles are assumed similar to rates of combustion of methane. The nature of the early-eluting material is not known but repeated microfiltration and TEM indicate that it may correspond to molecular diameters in the region of 20 nm. The soots and other samples appear to be in true solution rather than in colloidal suspension.

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