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 similar to2-2.5. 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 masses-as 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.

Two petroleum residues from European crudes have been fractionated using solvent (heptane) separation and column chromatography. The residues and the separated fractions have been characterised by size exclusion chromatography (SEC) and by UV-fluorescence spectroscopy (UV-F). Matrix assisted laser desorption/ionisation-mass spectrometry of the whole residues and the heptane insoluble fractions indicated that the bulk of the residues covered the mass range m/z 300-2000, while the heptane insolubles (1-2% of the whole) contained material in the mass range from about m/z 300 to 10 000. The upper mass ranges indicated by SEC using polystyrene standards were higher; the earliest eluting material from both distillation residues eluted at times corresponding to polystyrene standards of MMs above 1.85 million u. Possible reasons for the different observations are given. Data from UV-F suggests that the heptane solubility separation method was the most successful for the separation of the largest molecular mass and also probably the most polar materials in these residues. However, all three fractionation methods produced similar trends, showing greater polarity of the fractions to correlate with increasing molecular mass. The shift of maximum intensity of fluorescence towards longer wavelengths (in UV-fluorescence) with increasing molecular size, as indicated by SEC, strongly suggests that the fluorescing molecules are large rather than aggregates of small molecules. Differences in comparison with American petroleum residues can be observed. (C) 2002 Elsevier Science Ltd. All rights reserved.

A coal tar pitch, a coal liquefaction extract, and a low-temperature coal tar have been fractionated by molecular mass, using column chromatography, and the fractions have been analyzed for trace-element content. The solvents used for sequential extraction were acetonitrile, pyridine, and 1-methyl-2-pyrrolidinone (NMP). Trace elements were determined by inductively coupled plasma-mass spectroscopy (ICP-MS) after digestion of the liquids and fractions in a microwave bomb, using nitric acid and hydrogen peroxide. The mercury content was determined using a Leco model AMA254 analyzer. The larger portion of the trace elements analyzed have been found to associate preferentially with fractions that have been shown by size exclusion chromatography to contain the largest molecules. Some of the largest-molecular-mass material adhered to the silica that was used for fractionations. Trace-element mass balances for fractions separated by column chromatography were very poor, because of higher concentrations of trace elements in the largest organic molecules that were held onto the silica. One of the samples, the coal tar pitch, was fractionated by solvent solubility, without contact with filtration media. The method led to somewhat less-sharp molecular-mass separations; however, trace-element analyses of these fractions gave much-improved mass balances. Structural data from this work and previous characterizations suggest that, within larger molecules, increasingly large polycyclic aromatic (PCA) ring systems are being held together by a variety of aliphatic and alicyclic bridging structures. In the absence of mineral matter or other solids, it is thought that the high trace-element concentrations represented organic associations with these complex molecules.

This short paper highlights the unusual properties of the high-mass material of coal liquids isolated by their insolubility in pyridine and solubility in NMP. The separation has been achieved by a column chromatography method. One gram quantity have been processed and near quantitative recovery of the sample as fractions has been achieved. This fractionation permitted recourse to a broad range of analytical methods, including some (e.g. 13C NMR), which require large sample sizes. Multiple macro analyses have been undertaken, using elemental analysis, TGA proximate analysis, NMR and FT-ir in addition to the micro-analytical methods used previously—pyrolysis-gc-ms, SEC, UV–fluorescence, probe-ms and MALDI-ms. The fractions show increasing concentrations of large molecular mass material with increasing polarity of successive eluents used in the fractionation. Evidence from solid-state 13C NMR and UV–fluorescence spectroscopy show progressive structural changes with increasing apparent molecular mass.

Near-burner-zone combustion rates in pulverized-fuel (PF) combustors depend primarily on the amount and the structure/composition of volatiles released from the fuel. Mathematical simulations of PF flames often assume that volatiles combustion takes place at rates similar to those of light hydrocarbons. However, tars constitute 60-70% of volatiles released by most power station coals; tar molecules are far larger and oxidize more slowly. The problem is usually ignored. This article describes the characterization of structures and compositions of molecules in a low-temperature coal tar, typical of material combusted in the near-burner zone. Several techniques have been used in combination. The procedure relies on the estimation of molecular mass distributions by size exclusion chromatography coupled with bulk structural characterization by several established techniques (e.g., C-13-NMR, UV-fluorescence spectrometry). Sample fractionation by polarity and molecular mass have been found necessary for enhancing the resolution of the analytical tools, as well as for allowing meaningful correlations to be established between changing molecular mass ranges of successive fractions of the tar and the structural features of each fraction. The approach, described in some detail, extends the range of molecular masses amenable to examination to levels far above ceilings imposed by limitations of conventional GC, GC-MS, and probe-MS. The distribution of structures and molecular masses found in the tar suggests reaction pathways for the formation of soot during PF combustion.

A coal tar pitch has been fractionated by preparative and analytical size exclusion chromatography (SEC) to provide harrow time-elution fractions, expected to have relatively low molecular dispersities. The fractions were characterised by analytical SEC, by UV-fluorescence spectroscopy, and by laser-desorption MS and by MALDI-MS using sinapinic acid as matrix. The MALDI spectra were evaluated by measuring the peak-intensity mass, M-p, and by calculating number (M-n) and weight (M-w) average masses. The high mass limits of the spectra were determined by three methods based on (i) truncating the spectrum when the signal fell below 5 times the standard deviation of signal at the instrument limit, (ii) taking the upper mass as defined by a minimum slope of the spectrum from baseline, and (iii) taking all of the high mass signal as from sample. The laser desorption spectra were evaluated by M-p values only; the results below m/z 1,000 were in excellent agreement with the polymer calibration. The MALDI mass spectra gave a good fit between M-p values below m/z 3,000 and the calibration of SEC by polymers. Working with M-p gave better results than the M-n and M-w values from Method (i). Results from Methods (ii) and (iii) were significantly worse. Reasons for the lack of agreement between results above m/z 3,000 by MALDI and above m/z 1,000 by LD-MS have been discussed.

Samples of a coal extract and its hydrocracked products have been examined by solution-state and solid-state NMR spectroscopy to establish if these methods could follow the reaction process that is expected to convert bridgehead C atoms into hydrogen-substituted C atoms. The changes have also been followed by size-exclusion chromatography (SEC), elemental analysis, basic -OH titration, and UV-fluorescence spectroscopy. NMR spectra in CDCl3 indicated that the products contained more bridgehead C atoms than the extract. Spectra obtained using a better solvent, 1-methyl-2-pyrrolidinone (NMP), did indicate that the extract contained more bridgehead C atoms than the products. The basic -OH measurement indicated rapid removal of the group by hydrocracking; however, this was a relatively minor correction to the overall quaternary carbon concentration. The evidence from SEC and LTV-fluorescence suggested that the large molecules that were indicated to be present in the coal extract and the products did not give good NMR spectra, because the solvents did not dissolve them. Therefore, results from solution-state NMR of such coal liquids must be considered as only partial results and not covering the entire sample. Solid-state C-13 NMR methods showed that the process solvent could not dissolve the most aromatic components of the coal, with a significant loss of aromaticity in the digestion stage. Nonquaternary suppression (NQS) spectra of the microbomb product indicated the formation of methyl groups by the formation of methyl-substituted five-membered rings from tetralin. Solid-state H-1 spectra of the coal extract and products showed a decrease of aromatic to aliphatic hydrogen on reaction, as expected.

A sample of high-temperature coal tar pitch has been fractionated by preparative scale size exclusion chromatography (SEC). Twenty-four fractions corresponding to narrow molecular mass distributions were collected at sequential elution times of 3 minutes. The preparative scale column is thought to have filtered out most of the very large molecular mass material and the analysis focused on smaller mass components. The recovered fractions were characterised by analytical SEC. Pyrolysis-gas chromatography/mass spectrometry (pyrolysis-GC/MS) of the fractions was used to evaluate changes in main structural features with changing molecular size. As expected, analytical SEC of the fractions showed a systematic shift to smaller molecular sizes with increasing elution time. UV-fluorescence spectra of the samples showed parallel structural shifts to shorter wavelengths and gains in intensity with increasing elution times. Pyrolysis-GC/MS showed parent polycyclic aromatic components considered typical of pitch. The major fragments detected were pyrenes and fluoranthenes (m/z 202) and chrysenes (m/z 228). Benzopyrenes (m/z 252) and larger aromatics were not prominent amongst the pyrolysis products. Taken together, molecular structures were found to change significantly from fraction to fraction, but no systematic pattern emerged with changing elution time. Some components were identified as having formed during the analytical pyrolysis procedure itself. As aromatic products larger than dibenzopyrene (m/z 302) would not elute from the column, detected fragments are thought to have detached from larger pitch molecules. The pyrolysis of the largest species gave only alkanes and alkenes, with the aromatic fragments thought to be too large to elute through the GC column. The work provides information regarding the aromatic groups that are readily lost from larger molecules. Larger aromatic structures are thought to condense (to char) during the analytical pyrolysis step; the method is limited in not being able to provide information about their structural features. Copyright © 2002 John Wiley & Sons, Ltd.

A low-temperature coal tar has been fractionated by column chromatography into acetonitrile, pyridine and 1-methyl-2-pyrrolidinone- (NMP) solubles. The tar and its fractions have been examined by pyrolysis-gas chromatography/mass spectrometry (GC/MS). Fractionation by planar chromatography was also carried out for purposes of comparison. Molecular masses of the fractions were estimated by size-exclusion chromatography (SEC), and bulk structural characterisation was carried out by (13)C-NMR and UV-fluorescence spectrometry. SEC showed that the fractions shifted to progressively shorter elution times (higher apparent masses) with diminishing solubility, i.e. from acetonitrile to NMP solubles. UV-fluorescence spectra showed parallel shifts to longer wavelengths and lower fluorescence quantum yields, indicating increasing sizes of aromatic ring systems and increasingly complex molecules. GC/MS analysis of the tar showed alkanes from C10 to C32 and extensive series of alkylated aromatics, phenols, indenes, naphthalenes, phenanthrenes and fluoranthenes. Pyrolysis-GC/MS results for the acetonitrile solubles closely resembled the data for the tar sample, with extensive series of alkylated benzenes, phenols and naphthalenes as well as alkanes from C16 to C28. The pyridine-soluble fraction showed no significant aromatic pyrolysis products and only relatively weak signals for alkanes between C16 and C27. The NMP-soluble fraction showed even less overall signal, with no significant aromatic components and weak signals for alkanes between C21 and C25, even though (13)C-NMR analyses showed that approximately half of the carbon detected was aromatic. The aliphatics are assumed to provide bridging structures between polycyclic aromatic (PCA) ring systems.

Two petroleum residues have been fractionated using solvent (heptane) separation, planar and column chromatography. The residues and the separated fractions have been characterized by size exclusion chromatography (SEC), MALDI (matrix-assisted laser desorption/ionization) mass spectrometry, and by W-fluorescence spectroscopy (UV-F), MALDI mass spectrometry has indicated both residues to contain material with molecular mass ranges up to 15 000 u. The upper mass ranges indicated by size exclusion chromatography using polystyrene standards were higher; the earliest eluting material from both distillation residues eluted at times corresponding to polystyrene standards of MMs above 1.85 million u. Data from UV-F suggests that the heptane solubility separation method was the most successful for the separation of the largest molecular mass-and also probably the most polar-materials in these residues, However, all three fractionation methods produced similar trends, showing greater polarity of the fractions to correlate with increasing molecular mass. The shift of maximum intensity of fluorescence toward longer wavelengths (in UV-fluorescence) with increasing molecular size, as indicated,by SEC, strongly suggests that the fluorescing molecules are large rather than aggregates of small molecules.

A sample of Baltic amber ( approximately 40 million yrs old) has been extracted using pentane, toluene and 1-methyl-2-pyrrolidinone (NMP). The relationship between solubility characteristics of the extracts in relation to molecular mass and chemical makeup has been investigated. The extracts were first characterised by (13)C-NMR spectrometry, size exclusion chromatography (SEC) and UV-fluorescence spectroscopy. The fractions differed less in terms of chemical structural features than they did in terms of molecular mass. This contrasts markedly with data on fractions of coal-derived liquids, but parallels results from petroleum-derived vacuum residues. In SEC, the toluene soluble/pentane insoluble fraction gave a peak for high mass material at about 67 000 u. Material excluded from the column porosity in this fraction and in NMP solubles eluted between 8 and 11 min, corresponding to polystyrene masses between 200 000 and several million u. A column with a larger pore size distribution was calibrated using polystyrene and polymethylmethacrylate standards with detection by a light-scattering evaporative analyser. The largest polystyrene standard (15.4 million u) eluted at 13.4 min, similar to that of the earliest eluting amber-derived material in the NMP solubles fraction. Results from probe-MS and pyrolysis-GC/MS have been used to confirm the similarity of chemical structures of the three solubility fractions. Broadly, low mass ions appear to correspond to the various monomeric units of structures present in the amber, the higher mass ions to dimer units and the molecular ions to the different combinations of three or more monomeric units. The main monomer groups have been identified in detail, showing a situation very different from that of coal-derived materials, where the sizes of aromatic ring systems increase with molecular size.

A sample of Baltic amber believed to be about 40 million years old, has been pyrolysed in a wire-mesh reactor. Nearly all (99%) of the sample was found to volatilise; the condensable tar yield was about 82%. These tars and extracts in 1-methyl-2-pyrrolidinone (NMP), ∼20% of the original amber sample, have been characterised by size exclusion chromatography (SEC) and UV-fluorescence spectroscopy (UV-F). Size exclusion chromatograms of the soluble fraction of the amber gave relatively low intensity signal, indicating the lack of strongly absorbing chromophores. The profiles of the pyrolysis tar were more intense, suggesting that the pyrolytic process promoted aromatisation of the original structures. SEC of the amber extracts showed the presence of apparently large molecular mass material, possibly up to several million units. The SEC of the tar showed lower molecular mass material than that of the extract, with less absorbance at longer wavelengths, suggesting the presence of smaller polynuclear aromatic groups. Synchronous UV-fluorescence spectra suggest that the tar consisted mainly of fragmentation products of larger molecular mass material, deriving mostly from the insoluble, probably partly cross-linked part of the original sample.

Calibration data for size exclusion chromatography and results from the analysis of MALDI–TOF mass-spectra of coal derived liquids have been presented. The work provides the means for obtaining much more quantitative information from these two techniques than has hitherto been possible. The polystyrene based calibration has been matched against elution times of a wide range of model compounds (PAH, azaarenes, other nitrogen bearing compounds, several dyes, polars and numerous oxygenated compounds), covering a molecular mass range up to 1086 u. Compounds in all groups appeared to elute with a predominantly size dependent mechanism. With the exception of the somewhat atypical fullerene mixture, none of the model compounds eluted at times so early as to be detected in the excluded region of the chromatogram (i.e. near 10.5 min). Several semi-quantitative methods have been used to establish the upper mass limit of corresponding MALDI–TOF mass-spectra, which can be safely considered as representing signal. Spectra of a coal tar pitch and its pyridine insoluble fraction were analysed. The highest estimates of the high mass limits based on the calculation of number and weight average parameters was >300,000 u. A conservative estimate obtained with a new method based on subtracting multiples of the standard deviation from the signal gave 42,000 and 95,600 u, for the two samples, respectively. Fractions of the same coal tar pitch (separated by planar chromatography) have been characterised, to test for changes in structural features with changing MM-distributions. SEC and MALDI–TOF–MS showed decreasing MM-distributions with increasing mobility in planar chromatography. However, 13C NMR and pyrolysis–GC–MS showed that the immobile fraction contained greater proportions of aliphatic material compared to more mobile fractions. The mobile material consists of the aromatic systems normally associated with coal tar. The relatively immobile (larger-MM) fractions consisted of aromatic systems too large to elute through the chromatographic column (in pyrolysis–GC–MS) and linked together by aliphatic chains, which were released on pyrolysis and detected as major pyrolysis products.