The aim of the study was to obtain calibration curves for a pair of size exclusion chromatography (SEC) columns operating with 1-methyl-2-pyrrolidinone (NMP) as eluent. The dependence of the calibrations on sample chemical structures has been examined. The calibrations have been compared with elution times of several sets of standards. The level of agreement between SEC and MALDI-mass spectrometry has been evaluated. Molecular mass distributions of several complex samples have been examined in terms of these calibrations. The polystyrene (PS) and poly(methyl methacrylate) (PMMA) calibration curves were close, while a set of polysaccharides (PSAC) and other oxygenates eluted much earlier. However, numerous other samples eluted closer to the PS-PMMA line. To a first approximation, deviations between the PSAC and PS-PMMA lines may be treated as an upper limit to errors arising from structure-dependent variations in this SEC system. Below 15 000 u, MMs of oxygenated samples could be estimated to within a factor of ∼2-2.5. Other structural features gave rise to smaller deviations. Good agreement was observed up to about m/z 3000, between SEC and MALDI and LD-MS. The techniques are independent, suggesting that up to this limit, SEC may be considered as a quantitative tool. The accuracy of the measurement is subject to greater uncertainty with increasing molecular mass. The often-made assumption that high-mass materials are composed of aggregates has been examined. Furthermore, evidence from several analytical techniques provides indications of entirely different structural makeup (e.g., nature of fragments in mass spectrometry; trace element concentration) between fractions with different apparent molecular massessas determined by SEC. It is possible that some molecules adopt 3-dimensional conformations and show up as larger than they really are. While the "aggregates" assumption did not explain our experimental observations, structures of material appearing under the excluded peak in SEC require further careful study.

This paper describes the elution behavior of model compounds in a polystyrene-divinylbenzene SEC column with NMP as mobile phase, operating at high temperature (80°C). Model compounds covering polycyclic aromatic hydrocarbons, azaarenes, and other nitrogen and polar compounds have been studied. Most of the standard compounds eluted within one minute of the expected time indicated by the polystyrene calibration. The fractionation of a complex coal-derived sample (a coal tar pitch) using the same column has been achieved, with subsequent reinjection and analysis of the fractions by heated-probe mass spectrometry and UV-fluorescence. The probe-MS experiments were performed in order to show that the material of the excluded peak does not consist of small and polar molecular species, rather than larger-molecular mass material. All the fractions were reinjected and some of them gave small extra peaks in the SEC chromatogram. The earliest fractions showed very weak UV-fluorescence indicating the presence of very high molecular mass material. The later-eluting fractions showed relatively strong fluorescence intensities with the position of the fluorescence intensities shifting to shorter wavelengths as the SEC elution time increased, indicating that the smaller polynuclear aromatic ring systems elute in the late fractions. Probe mass spectra showed that only those fractions isolated from SEC at the long elution times gave signals characteristic of aromatic and nitrogen compounds; the molecular mass range decreased with increasing elution time. Since the structures of the material excluded from the column or even that near the exclusion limit are not known, it is impossible to select standard materials or standard polymeric compounds to represent the molecular mass range of coal-derived liquids. For this reason, we believe that the polystyrene calibration represents the most reasonable compromise for SEC in NMP solvent in our system.

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