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Calibration of Size Exclusion Chromatography in 1-Methyl-2-pyrrolidinone for Coal-Derived Materials Using Standards and Mass Spectrometry M. J. Lazaro, C. A. Islas, A. A. Herod,* and R. Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College (University of London), Prince Consort Road, London SW7 2BY, U.K. Received May 24, 1999
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 probeMS 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.
Introduction Gas chromatography (GC) and GC coupled to mass spectrometry provide powerful tools for the identification and structural characterization of unknown compounds and mixtures. The identification of azaarenes and thiaarenes in coal-derived liquids by these methods has been reviewed;1 Nishioaka2 solvent-extracted coals and compared the pyrolysis product of the extracts with the pyrolysis products from the coal, using GC-MS. Liu and Meuzelaar3 have examined the products of hydropyrolysis of Blind Canyon coal by high-pressure thermogravimetry using GC-MS. However, GC and GC-MS normally show up aromatic compounds up to ∼300 u and aliphatic compounds up to ∼500 u. Alternative methods are required for examining samples suspected of containing material with molecular mass (MM) ranges exceeding these limits. The need for more complete information on MMdistributions of coal-derived liquids extends to areas as * Corresponding author. (1) Herod, A. A. Azaarenes and thiaarenes. Handbook of Environmental Pollution; Neilson, A. H., Ed.; Springer-Verlag: Heidelberg, Germany, 1997; Chapter 7, p 271. (2) Nishioka, M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1989, 34 (3), 685. (3) Liu, K.; Meuzelaar, H. L. C. Fuel Proc. Technol. 1996, 49, 1.
varied as combustion rates in the near burner zones of power station burners, to the refining and upgrading coal liquefaction extracts and to environmental pollution diagnostics. In attempting to examine the full MM-distribution of complex coal-derived mixtures, size exclusion chromatography (SEC) has emerged as one of the more commonly used techniques [e.g., refs 4-10]. Much of the early SEC work was carried out using tetrahydrofuran (THF) as eluent.11,12 In our work on coal pyrolysis tars, (4) Handbook of Size Exclusion Chromatography, Chromatographic Science Series, Vol. 69; Wu, Chi-san, Ed.; Marcel Dekker: New York, 1995. (5) Lafleur, A. L.; Nakagawa, Y. Fuel 1989, 68, 741. (6) Herod, A. A.; Shearman, J.; Lazaro, M.-J.; Johnson, B. R.; Bartle, K. D.; Kandiyoti, R. Energy Fuels 1998, 12, 174. (7) Herod,A. A.; Zhang, S.-F.; Johnson, B. R.; Bartle, K. D.; Kandiyoti, R. Energy Fuels 1996, 10, 743. (8) Domin, M.; Moreea, R.; Lazaro, M. J.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1997, 11, 638. (9) Lazaro, M. J.; Herod, A. A.; Cocksedge, M.; Domin, M.; Kandiyoti, R. Fuel 1997, 76, 1225. (10) Herod, A. A.; Johnson, B. R.; Bartle, K. D.; Carter, D. M.; Cocksedge, M. J.; Domin, M.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1995, 9, 1446. (11) Bartle, K. D.; Taylor, N.; Mulligan, M. J.; Mills, D. G.; Gibson, C. Fuel 1983, 62, 1181. (12) Bartle, K. D.; Mulligan, M. J.; Taylor, N.; Martin, T. G.; Snape, C. E. Fuel 1984, 63, 1556.
10.1021/ef990099z CCC: $18.00 © 1999 American Chemical Society Published on Web 09/14/1999
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the technique showed traces of material with MMs up to between 4000 and 6000 u.13,14 The use of THF as eluent in SEC has, however, given rise to a number of problems.7,15-17 First, retention times were shown to be structure dependent;11,12,17 furthermore, the relatively low solvent power of THF gave rise to adsorptive interactions between sample and packing, resulting in signal from low-MM material being found at retention times greater (longer) than the permeation limit of the column itself. Finally, partial loss of sample has been observed10 to take place by precipitation of the heavier fractions out of solution. Clearly, without knowing molecular structures of coal liquids and with an unpredictable blend of size and surface separation, SEC with THF can give no useable information. These difficulties, including the all-important solubility limitations for coal-derived liquids, have been overcome by Lafleur and Nakagawa,5 through the use of 1-methyl-2-pyrrolidinone (NMP) as eluent. The same researchers attempted to establish an NMP-based MMcalibration for coal-derived liquids. They determined the retention times of sets of model compounds and compared them with values expected of their molecular masses on the basis of a calibration curve for polystyrene standards. They found that the polarity of sample molecules could cause shifts in the retention times of individual compounds (used as standards) to significantly smaller valuessi.e., to greater apparent MMss relative to the values expected on the basis of the polystyrene calibration. These findings were interpreted in terms of a multimode mechanism for elution based on molecular sizemodified (distorted) by earlier-than-expected elution of polar molecules. However, when the same authors attempted to characterize material eluting near the exclusion limit of the column by heated-probe mass spectrometry,5 they found little ion abundance above 250 u; the reported upper limit of the scan was 500 u. Taken together, the two sets of data led to the conclusion that material showing signal at the exclusion limit of the SEC-column (corresponding to the largest apparent MMs) consisted of clusters of small (1.2 min) at ambient temperature compared to operation at 80 °C. These shifts were clearly minor and comparable to those of the polystyrene standards. In previous work,5 acetone eluted 10.7 min earlier, pyridine 6.8 min earlier, carbazole 5.5 min earlier, and fluorene eluted 5.6 min later than predicted by the polystyrene calibration. Thus, the data of Table 4 suggest that column operating temperature alone could not explain the difference in behavior observed between the two studies. Other (37) Herod, A. A.; Lazaro, M.-J.; Moreea, R.; Dubau, C.; Richaud, R.; Card, J.; Jones, A. R.; Domin, M.; Kandiyoti, R. J. Chrom. A, submitted.
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Figure 2. Structures of standard compounds. Table 3. Polar and Oxygenate Compounds MW
te
tcalc
∆tcalc
58 208 258 392
21.5 20.7 20.5 19.4
21.4 19.7 19.4 18.9
+0.1 +1.0 +1.1 +0.5
Carboxylic Acids benzoic acid 122 cyanuric acid 129 salicylic acid 138 3-phenylpropenoic acid 148 3-dimethylaminobenzoic acid 165 hydrocaffeic acid 182 3,5-dimethoxybenzoic acid 182 3,5-dimethoxy-4-hydroxycinnamic acid 182 9-anthracene carboxylic acid 222
19.6 18.7 16.9 19.4 19.5 18.3 19.2 18.3 16.4
20.4 20.4 20.3 20.2 20.0 19.9 19.9 19.9 19.7
-0.8 -1.7 -3.4 -0.8 -0.5 -1.6 -0.7 -1.6 -3.2
Figure 3. Comparison of te values from ref 5 (calculated from elution volumes) and this work for common compounds; values attached to points are molecular masses of compounds.
21.5 20.3 21.4 20.0
+1.2 +1.4
Table 4. Effect of Column Temperature on Elution Times
19.9 18.8 19.6 18.6 18.6 19.4
-0.9 -1.5 -0.5 -1.3 -1.1 0.0
Carbonyls acetone anthraquinone 1,2-benzanthraquinone 3,4,9,10-perylen tetracarboxylic dianhydride (cf. Figure 2)
benzo[b]thiophene dibenzofuran
Furans and Thiophenes 134 168
phenol pyrogallol 3-nitrobenzyl alcohol coniferyl alcohol sinapyl alcohol stearyl alcohol
Hydroxy Compounds 94 126 153 180 210 270
elution time (min) at temperature
20.8 20.4 20.1 19.3 19.7 19.4
factors likely to influence the results include the column packing material. Molecular Structural Aspects. Structural information provided by the combination of SEC with MS-based techniques (albeit often supplemented by UV-fluorescence spectroscopy) has proved to be limited.38 Recently, we have correlated SEC-chromatograms of fractions (38) Meuzelaar, H. L. C. A. C. S. Div. Fuel Chem. 1994, 39 (1), 36.
compound
80 °C
ambient
difference (min)
acetone pyridine carbazole fluorene polystyrene [22000] polystyrene [7000] polystyrene [3250] polystyrene [1700] polystyrene [580]
21.5 21.9 19.9 21.6 13.4 14.7 15.6 18.3 19.6
22.7 22.7 20.6 22.7 14.0 15.4 16.4 19.1 20.6
1.2 0.7 0.7 1.0 0.6 0.7 0.8 0.8 1.0
separated by planar chromatography of the same coal tar pitch (as in ref 20) with their analysis by pyrolysisGC-MS.39 Within this framework, pyrolysis-GC-MS was used to break down and analyze fragments from coal tar pitch fractions, known to have different MMdistributions and polarities, to identify and compare the
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structural features of the fractions. The work has given rise to some unexpected results, showing alkene fragments as the most abundant pyrolysis products in the higher mass, more polar fractions, also shown by SEC to contain greater MM-distributions. The trends have been correlated with 13C NMR and UV-fluorescence spectra of the fractions; they suggest the higher-mass, more polar fractions to be composed of larger aromatic clusters that could not be pyrolyzed into fragments able to pass through the GC columnslinked by aliphatic structures. In this work, the standards used are all of relatively small molecular mass with the largest at 1086 u, alcian blue; the elution times all correspond to those of relatively small molecules and there are no serious differences from the approximation presented by polystyrene standards, and also no overlap into exclusion time. However, the SEC profile of coal-derived samples5-9 in NMP solution all showed material excluded from the porosity of the columns used, indicating that the molecular structures are larger than any of the standards either used here or available in the chemical catalogues. In particular, a coal extract and a set of hydrocracked products from it, only showed7 a sensible sequence (large molecules in the extract progressively breaking down with increasing reaction temperature) using NMP as eluent; SEC in THF eluent failed to show the extract as having larger molecules in the exclusion region than the products. Similarly, SEC of TLC fractions of the pitch used in this work failed to show15 the increasing size range of fractions with increasing immobilty on the TLC plate, as observed later41,42 using SEC with NMP. The evidence, therefore, favors the indication from SEC with NMP that the material excluded from the column porosity is of large size, but there is not yet confirmation that the polystyrene calibration is applicable in this region. 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 polymers 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 work. Further work is necessary to begin to understand the structural complexities of these materials. As an introduction to this further work, we have examined fractions of coal tar pitch recovered from the analytical column, described below. SEC Fractions of a Coal Tar Pitch. Figure 4 shows the SEC profile of the coal tar pitch. Fractions of the effluent from the SEC column were collected at 1-min intervals from 8 to 25 min; the successive effluent fractions from the column were immediately re-injected into the apparatus to determine their behavior as isolated fractions. Had the short retention time (appar(39) Herod, A. A.; Islas, C.; Lazaro, M.-J.; Dubau, C.; Carter, J. F.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1999, 13, 201-210. (40) Johnson, B. R.; Bartle, K. D.; Domin, M.; Herod, A. A.; Kandiyoti, R. Fuel 1998, 77, 933. (41) Herod, A. A.; Kandiyoti, R. J. Planar Chromatography 1996, 9, 16. (42) Herod, A. A.; Zhang, S.-F.; Carter, D. M.; Domin, M.; Cocksedge, M. J.; Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; Johnson, B. R.; Bartle, K. D.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1996, 10, 71.
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Figure 4. SEC profile of the whole pitch at 5 wavelengths: 1, 280 nm; 2, 300 nm; 3, 350 nm; 4, 370 nm; 5, 450 nm.
ently high-MM) fractions corresponded to molecular aggregates, reinjection of a dilute solution would be expected to show some disaggregation and shift of some or all of the material appearing as excluded in the chromatogram to longer elution times. Figure 5a-g shows the SEC profiles of selected fractions obtained following re-injection of successive time-resolved pitch fractions exiting from the column. The first two fractions representing the 8-9 min and the 9-10 min intervals, respectively (Figure 5a,b), gave small secondary peaks corresponding to retained material between 15 and 20 min. Fraction 3 in Figure 5c (1011 min) showed a shift to shorter time (9-10 min) and no secondary peakssin the retained regionssignificantly above noise level. The fractions between 11 and 16 min were too dilute to give appreciable signal above the level of instrument noise and have not been shown. Fractions 10, 11, and 12 in Figure 5d-f corresponding to the bulk of the material separated by the column at elution times 17-20 min showed a small excluded peak as well as the major peak within the time interval of collection. Fraction 13 in Figure 5g showed no signal in the excluded region at all. As may be observed from Figure 1, the elution time of 16 min corresponds to polystyrene of mass about 3400 u. The behavior of these fractions is difficult to understand in the absence of more detailed structural information. It is possible that the relatively small proportion (judged from the areas of Figure 4) of excluded material may overload the capacity of column to resolve this material in the interparticulate space, leading to a severe broadening of the peak near the exclusion limit. Thus, the material of the excluded peak may extend beyond the exclusion limit into the region of the chromatogram where the column porosity would be expected to be the main separation mechanism. More detailed work is required to investigate structural features of material eluting at or near the exclusion limit of the column; an initial attempt at the structural characterization of the heaviest fractions has recently been described elsewhere.39 In previous work where the coal tar pitch was fractionated using a preparative column and THF solvent, the fractions did show excluded material in addition to a large retained peak on injection into an SEC column with NMP solvent;40 similar observations apply to an Athabascan bitumen separated by prepara-
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Figure 5. SEC profiles of fractions 1, 2, 3, 10, 11, 12, and 13 at elution times (A) 8-9 min; (B) 9-10 min; (C) 10-11 min; (D) 17-18 min; (E) 18-19 min; (F) 19-20 min; (G) 20-21 min, respectively, at 5 wavelengths: 1, 280 nm; 2, 300 nm; 3, 350 nm; 4, 370 nm; 5, 450 nm.
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Figure 6. Synchronous UV-fluorescence spectra of the pitch fractions (A) 1, 2, and 3; (B) 4, 5, and 6; (C) 7, 8, and 9; (D) 10, 11, and 12; (E) 13, 14, and 15; (F) fraction 13 and the whole pitch P.
tive SEC in THF and examined by analytical SEC in NMP19 and to a coal extract and hydrocracking products at different temperatures.7 UV-Fluorescence Spectroscopy of Pitch Fractions Separated by SEC. Figure 6a-f presents heightnormalized synchronous UV-fluorescence spectra of the pitch and its fractions 1-15. Fractions 1 (8-9 min) to 9 (16-17 min) show very weak fluorescence, showing very low quantum yields which suggest the presence of very large polynuclear aromatic ring systems. The fluorescence intensity of fractions 1-8 were not significantly greater than the signal from the NMP itself. These pitch fractions contain the material eluting before the major SEC peak between 16 and 25 min in Figure 4. Fraction 10 (17-18 min) to fraction 15 (22-23 min) showed relatively strong fluorescence intensities. The position of the maximum intensity of fluorescence in the fractions varied, with shifts toward 550 nm from fractions 1-9 and then a shift back to about 390 nm for fraction 13 (Figure 6f). These shifts suggest that the material collected in the exclusion region and near the exclusion limit contain aromatic clusters which were larger and more complex than those in later fractionss the material retained by column porosity. Figure 6f shows the UV-fluorescence spectra of the original pitch and of fraction 13 (20-21 min). The UV-fluorescence spectrum of the unfractionated pitch closely resembles
the spectrum of fraction 13, showing that the characteristic spectrum of the pitch corresponds to the low MM fractions only. The work showed that size exclusion chromatography used in preparative mode can be a powerful method for isolating high MM materials in the earlier fractions by separating them from the smaller molecules. Similar considerations apply to planar chromatographic separations which have the added advantage of using smaller volumes of solvent,15,41-43 but would not perhaps achieve as clean a separation as SEC itself. Probe Mass Spectrometry of Pitch Fractions Separated by SEC. Probe mass spectrometry has been used to examine all fractions separated by SEC. One aim of this phase of the work was to investigate whether early eluting fractions in SEC showed any evidence of association or aggregation by small molecules. The results obtained for the first eleven fractions did not show the presence of any compounds considered to be typical of the small polar molecules normally detectable by probe-MS in coal liquids.15 Figure 7a,b shows spectra for fractions 3 and 11, selected from among the full set present. The main peaks observed in the spectra of early fractions corresponded to NMP itself (m/z 99) and N-methyl succinimide (m/z 113), observed to be (43) Herod, A. J.; Gibb, T. C.; Herod, A. A.; Shearman, J.; Dubau, C.; Zhang, S.-F.; Kandiyoti, R. J. Planar Chromatography 1996, 9, 361-367.
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Figure 7. Probe mass spectra of fractions (A) 3 and (B) 11.
present in NMP after oxidation.44 No fragments or molecular ions at masses greater than m/z 113 could be observed in the probe mass spectra of fractions 3 and 11. Very little sample-related material was observed in the spectra and the glass sample holders taken from the mass spectrometer probe tip after the analysis showed that the solid residues remained in the holder with no apparent change of appearance, indicating that the bulk of the sample remained on the probe tip under the conditions of the experiment. The material detected and shown in Figure 7a,b corresponded to the residues of the SEC solvent after evaporation of the collected fraction for the probe experiment. Clearly, aggregates of small molecules would be expected to dis-aggregates at least to a limited extentsunder the conditions encountered in the mass spectrometer. These observations are consistent with interpreting peaks at or near the exclusion limit in SEC-chromatograms as indicating the presence of very high molecular mass materials rather than aggregates of small molecules. The first probe mass spectrum that showed the presence of significant amounts of coal-derived material (other than solvent residues) was that of fraction 12 (Figure 8a), corresponding to an elution time of 19 min in the SEC profile of Figure 4. This spectrum shows a wide distribution of m/z values from 426 to 44, including the NMP and N-methyl succinimide solvent peaks. Peaks from m/z 217 to 426 correspond to pitch components such as benzocarbazole (217), dibenzocarbazole (267), picene isomers (278), dibenzopyrenes (302), rubicene (326), tribenzopyrenes (352), dibenzocoronenes (400), etc. Peaks between m/z 120 and 200 very likely represent doubly charged molecular ions. The mass spectrum of the subsequent fraction 13 is shown in Figure 8b and corresponds to a retention time
Figure 8. Probe mass spectra of fractions (A) 12; (B) 13; (C) 14, and (D) 15.
of 20 min. The maximum peak intensity was observed at m/z 252 corresponding to benzopyrene isomers, with the highest m/z value at 352 corresponding to tribenzopyrenes and the lowest m/z value at 202 correspond(44) White, C. M.; Rohar, P. C.; Veloski, G. A.; Anderson, R. R. Energy Fuels 1997, 11, 1105.
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ing to pyrene and fluoranthene. As could have been expected, the profile of masses in this fraction shifted to lower masses than observed in fraction 12, although with some overlap, because peak shapes of individual components are not sufficiently narrow to allow their resolution between fractions over 1 min. The mass spectrum obtained for the fourteenth fraction, shown in Figure 8c, did not show a significant distribution of molecular ions corresponding to aromatics, with only low-intensity signals for benzopyrene (m/z 252) and indenopyrene (m/z 276). Since the boiling point of NMP is around 200 °C, the lower mass components of the solution are likely to have been lost in the vacuum-drying stage. The probe mass spectrum of fraction 15, shown in Figure 8d, shows that nothing except NMP was detected up to the permeation limit; this fraction gave a UV-fluorescence spectrum and presumably contains molecules amenable to gas chromatography that are more volatile than the solvent, NMP. Our probe-MS results indicate that the mass range decreased steadily toward the permeation limit of the column. In addition, the results for standards confirm that the molecular mass decreases with increasing elution time for the small molecules observed in the pitch. There is no evidence that these small molecules aggregate and elute at times other than characteristic of their molecular mass. Summary and Conclusions In this paper, we have described an attempt to develop a calibration curve relating MMs to retention times for SEC using NMP as eluent; the elution behavior of model compounds was examined in a Mixed-D column with polystyrene-polydivinylbenzene packing operated at both 80 °C and ambient temperature. The degree of overlap of signal caused by the polarity of sample molecules is one of the problems examined within the work presented below. Supporting analytical work has been carried out by heated-probe mass spectrometry and UV-fluorescence spectroscopy. Model compounds covering a wide polarity range were studied to shed light on the separation mechanism and retention times (volumes) were found to be relatively independent of molecular structure. All of the compounds studied, polycyclic aromatic hydrocarbons, azaarenes, and polar compounds, showed predominantly sizedependent elution. These compounds are abundant in coal-derived products. Most compounds of pyridines and pyrroles eluted within 1 min of the expected time indicated by the polystyrene calibration. Amino-substituted compounds and more complex nitrogen compounds eluted close to the elution times expected from their molecular mass as indicated by polystyrene stan-
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dards. Adding one or more nitrogen atoms to an aromatic ring system did not significantly change the elution behavior compared with the parent PAH. Decreasing the column temperature to ambient caused both standard compounds and polystyrenes to elute slightly later than at the high temperature (by up to 1 min), but no dramatic changes in elution behavior were observed. Fractions of coal tar pitch were recovered from the analytical column and reinjected; fractions of excluded material and those near the exclusion limit gave extra peaks of low intensity across the exclusion limit. The reason for this behavior is not clear and requires more work. It may result from overloading of the column capacity near the exclusion limit. The synchronous UVfluorescence spectra of the fractions indicated that the earliest fractions showed very weak fluorescence signaling the presence of very high MM material. The latereluting fractions showed relatively strong fluorescence intensities with the position of the peaks shifting to shorter wavelengths as the elution time increased, suggesting the presence of large polycyclic aromatic ring systems in the earliest fractions. Probe mass spectra show that for the small size fractions the molecular mass decreased with increasing elution time. Only the last few fractions gave any response to the probe-MS method, as implied by the mass ranges indicated by the polystyrene calibration. For the range of aromatic standards used here, the calibration based on polystyrene standards is shown to be a good approximation to molecular mass, a conclusion reinforced by our earlier work using both MALDI-mass spectrometry and planar chromatography as a fractionating medium.7,15 The excluded material observed in SEC is shown to be large molecular mass material as indicated by its early elution time rather than aggregations of small polar molecules. SEC in NMP at high temperatures can be used as a method to determine MM-distributions in coal-derived liquids. The column in the present work operates preferentially by a sizedependent mechanism rather than by a multimode mechanism involving surface sorption effects. Acknowledgment. The authors thank the European Community for a postdoctoral grant for M.J.L. (Marie Curie Research GrantsNon-Nuclear Energy Program). Support by the British Coal Utilization Research Association (BCURA) and the UK Government Department of Trade and Industry (DTI) under project B44 is gratefully acknowledged. The authors thank the University of London Intercollegiate Research Service (ULIRS) for provision of probe mass spectrometry facilities (Kings College). EF990099Z
