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 work compares UV-fluorescence (UV-F) and UV-absorption (UV-A) as detection methods in the analysis of coal and petroleum-derived materials, using size exclusion chromatography (SEC). A UV-F spectrometer that was equipped with a flow cell was connected in series to an SEC chromatograph with a conventional UV-A detector. Samples were examined via SEC, using both UV-F and UV-A detectors that were operating in tandem. They included asphaltenes from heavy petroleum residues and three fractions of a coal tar pitch obtained by solvent solubility separation. The chromatogram of the lightest fraction of the coal tar pitch (the acetone solubles) showed a single peak, with close agreement between both detection systems. The rest of the samples showed an early-eluting peak that corresponded to material excluded from the column porosity, in addition to a retained peak. UV-F showed little sensitivity to material eluting under the excluded peak in any of the samples and also was less effective than UV-A in detecting the material eluting at shorter times under the retained peak, only responding to the smallest molecules. Number and weight averages of the molecular mass distributions calculated for the retained material from UV-A were significantly higher than those calculated from UV-F data. UV-F fails to detect the entire range of compounds present in these complex samples, and it is particularly insensitive to the heavier ends. It seems that detection by UV-F is more dependent on structural features than UV-A.

No abstract

The analysis of a Maya asphaltene using size-exclusion chromatography (SEC) and 1-methyl-2-pyrrolidinone (NMP), tetrahydrofuran (THF), and toluene as the eluent (separately used) showed radically different chromatograms. The SEC of fossil fuel hydrocarbons, e.g., asphaltenes, using NMP has shown a two-peak structure, which has been the source of debate, whether the peak excluded from the column porosity (appearing at the shorter retention times) contained asphaltenic aggregates or very large molecular masses of thousands of mass units. The SEC of asphaltenes using THF and toluene showed retained peaks at the longer retention times. In this work, atomic force microscopy (AFM), a technique capable of reliable and accurate detection of the interaction forces between various molecules and in different media, was exploited to investigate the interaction forces between polystyrene (PS) particles (SEC column packing) and asphaltenes in air and in NMP. AFM showed a strong interaction between the PS particles and asphaltenes, the outcome of which can be the aggregation of asphaltenes in the SEC columns. The use of solvents, such as toluene, NMP, and THF, which either damage the column packing or do not dissolve asphaltenes completely, the interaction between asphaltenes, and the SEC column packing and aggregation of asphaltenes in the SEC columns are the major limitations of SEC in analyzing asphaltenes. Furthermore, PSs, currently used to calibrate SEC columns, are significantly different from asphaltenes in their structure and solubility behavior. In addition, the quantitative results of the SEC are considerably different from the reports from other techniques, e.g., LD, MALDI, FT-ICR MS, and molecular diffusivity methods. Thus, SEC results cannot be used quantitatively to determine mass distributions of asphaltenes.

The size-exclusion chromatography (SEC) of acetone-soluble, pyridine-soluble and pyridine-insoluble fractions of a coal tar pitch indicates a bimodal distribution in each fraction. The proportion of high-mass material excluded from the SEC column porosity increases with solvent polarity. The polymer calibration of SEC shows the mass range of the small molecules to be from approximately 100 u to approximately 6000 u, with the mass range of the large excluded molecules above 200 000 u and up to several million u. In contrast, matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS) shows a similar low-mass range of ion abundances (< m/z 6000), but with a smaller range of high-mass ion abundances, from approximately m/z 10 000 to 100 000. The large molecules may have three-dimensional structures to allow molecules of relatively low mass to behave as if they are of large size in SEC. Laser desorption mass spectrometry of the acetone- and pyridine-soluble fractions produced molecular ions of polycyclic aromatics that can be related to the known compositions from gas chromatography (GC) mass spectrometry. The experimental conditions used to generate the bimodal distribution by MALDI-MS involve reducing the ion signal intensities to avoid overload of the detector and enable detection of the high-mass ions, by reducing the high-mass detector voltage (i.e. sensitivity) and increasing the laser power.