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 reviews analytical methods that have been developed for characterizing complex liquid mixtures derived from fossil fuels. The analysis of fractions with masses up to ∼400−450 u normally involves gas and liquid chromatography, coupled with mass spectrometry (GC−MS and LC−MS, respectively). However, these techniques cannot readily be adapted to examine samples that contain higher-molecular-mass materials. Chromatographic and mass spectrometric methods are often limited by the volatility of the samples, while liquid chromatographic methods may be limited by solubility in the solvents used. Materials of higher mass are characterized using methods that have been developed to overcome the limitations imposed by volatility and solubility in common solvents. As outlined in the text, they have been the subject of some debate. Much of the work that indicates upper mass limits of ∼1000−1500 u for coal tars, pitches, and petroleum asphaltenes can be explained in terms of limitations of the particular analytical techniques. The new emphasis on characterizing increasingly heavier materials grows out of a need in oil refineries and elsewhere, for fresh ideas about processing higher-mass feedstocks. Currently, above the ∼450−500 u range, no single method is unambiguously capable of indicating molecular mass distributions or chemical structural features in complex fuel-derived mixtures. Advances in this field require a comparison of evidence from several independent analytical methods. This review is mainly focused on the results from size exclusion chromatography (SEC), laser-desorption mass spectroscopy (LD−MS) and matrix-assisted laser desorption/ionization mass spectroscopy (MALDI−MS). SEC, using 1-methyl-2-pyrrolidinone (NMP) as an eluent, has shown agreement with LD-MS and MALDI-MS up to ∼3000 u and to within a factor of 2−2.5 at up to 15 000 u. Suggestions that the samples formed aggregates have been investigated. There is no confirmable experimental evidence, either from our work or in the literature, showing that aggregation occurs under the dilute conditions prevailing during SEC, using NMP as an eluent.

Laser-desorption mass spectrometry (LD-MS) method development was undertaken to improve estimates of mass ranges for complex hydrocarbon mixtures. A creosote oil, an anthracene oil, and a mixture of known polynuclear aromatic hydrocarbon (PAH) compounds were examined. The data on the mixture of the four PAHs made it possible to define LD-MS conditions necessary to generate artifacts such as cluster ions by the combination of high laser power and high-mass accelerator voltage. The formation of cluster ions was possible without overloading the detector system. These multimer ions overlapped with higher-mass ion signals from the sample. However, careful balancing of sample concentration, laser power, total ion current, and delayed ion extraction appears to show high-mass materials without generating high-mass multimer (artifact) ions. It is possible to suppress the formation of cluster ions by keeping low target concentrations and, consequently, low gas phase concentrations formed by the laser pulse. The principal method used in this work was the fractionation of samples by planar chromatography followed by successive LD-MS analysis of the separated fractions directly from the chromatographic plates. This method separated the more abundant small molecules from the less abundant large molecules to permit the generation of their mass spectra independently, as well as reducing the concentration of sample by spreading over the PC-plate. The technique demonstrably suppressed multimer formation and greatly improved the reproducibility of the spectra. Results showed the presence of molecule ions in the ranges m/z 1000−2000 for the anthracene oil sample and m/z 600−1500 for the creosote oil sample, tailing off to m/z ∼5,000. The creosote oil contained significantly less of this high-mass material than the anthracene oil sample, and in both cases, high-mass material was only present in low quantities. Ion mass range estimates were in close agreement with molecular mass ranges from size exclusion chromatography, and findings were consistent with changes observed in the UV-fluorescence spectra. The method outlined in the paper appears directly applicable to the characterization of heavier coal and petroleum derived fractions.

3897 3.4. High Mass Limits to UV-Fluorescence 3899 4. Examining High Mass Fractions by Size Exclusion Chromatography (SEC) 3900 4.1. Limitations of Using Tetrahydrofuran As Eluent in SEC 3901 4.2. Limitations of Using NMP as Eluent in SEC 3902 4.3. How To Explain the "Excluded Peak"? 3902 4.4. The Use of NMP−Chloroform Mixtures As Eluent 3903 5. Examining High Mass Fractions by Mass Spectrometry 3904 5.1. Gas-Chromatography − Mass Spectrometry 3904 5.2. Pyrolysis-GC-Mass Spectrometry (Py-GC-MS) 3904 5.3. Heated Probe-Mass Spectrometry 3906 5.4. Field Ionization Mass Spectrometry 3906 5.5. Laser Induced Acoustic Desorption (LIAD) 3907 5.6. Complex, Polydisperse Samples by FT-ICR-MS and Different Ionization Methods 3907 5.6.1. Electrospray Ionization Mass Spectrometry (ESIMS) 3907 5.6.2. Field Desorption and Atmospheric Pressure Photoionization Methods 3908 5.7. Analysis of Complex, Polydisperse Samples by Laser Desorption/Ionization Mass Spectrometry (LDTOFMS) 3908 5.8. Upper Mass Detection Limits of LD-MS Systems 3909 6. Examining Higher Mass Fractions by Solution State 13 C NMR 3910 7. Comparing Calculated Parameters from Three Distinct Samples 3912 7.1. Coal Tar Pitch Fractions 3912 7.2. Fractions of Maya (Mexican) Heavy Crude 3913 7.3. Examining Fractions of Synthetic Crude Prepared from the Athabasca Tar Sands 3914 7.4. Common Features of Results from the Three Sets of Samples 3917 8. Summary and Conclusions 3918 8.1. Aims of the Review 3918 8.2. The Need for Fractionation 3919 8.3. Limitations of Individual Analytical Techniques 3919 8.4. Several Novel Approaches of the Work 3919 8.5. Closing Emphatic Remarks 3919 Author Information 3920 Corresponding Author 3920 Present Address 3920 Notes 3920 Biographies 3920 Acronyms 3921 References 3921

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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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Fractions of Maya (Mexico) crude oil were examined by quantitative liquid-state 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy. The NMR data was combined with molecular-mass estimates determined in a previous work to calculate average structural parameters. The approach clearly showed structural differences between the fractions and allowed for the inference of detailed information on the chemical structures involved. The sample of crude oil was first fractionated into maltenes and asphaltenes, and the asphaltenes were further sub-fractioned into 1-methyl-2-pyrrolidinone (NMP)-soluble (labeled as "MNS") and NMP-insoluble (labeled as "MNI") fractions. The aromatic rings per average molecule value derived were 2-5 for the MM sample, 8-10 for the MA sample, ∼5 for the MNS sample, and 11-38 for the MNI sample. Thus, while some differences between maltenes and asphaltenes could be expected from prior general knowledge, some results were unexpected. The MNS fraction turned out to be more aliphatic than the MNI fraction. On average, the MNS sample appeared to contain a greater number of smaller aromatic cores, linked by means of biphenyl-like aromatic-aromatic single bonds and aliphatic/naphthenic bridges, and shorter aliphatic side chains. If the aromatic groups are in external positions, this would account for the solubility of these species in NMP (a more open structure). Meanwhile, results for the MNI sample strongly suggest fewer, larger aromatic cores per average molecule, linked by long alkyl side chains and naphthenic structures. The alkyl substituents would appear to form a barrier between the aromatic core and the solvent. These findings have implications regarding how different structural sub-groups within an asphaltene will respond to ultraviolet (UV) laser desorption-mass spectrometry (LD-MS) and UV-fluorescence spectroscopy.