Characterization of Heavy Hydrocarbons by Chromatographic and

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Energy & Fuels 2007, 21, 2176-2203

Characterization of Heavy Hydrocarbons by Chromatographic and Mass Spectrometric Methods: An Overview Alan A. Herod,*,† Keith D. Bartle,‡ and Rafael Kandiyoti† Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom, and Department of Chemistry, UniVersity of Leeds, Leeds LS2 9JT, United Kingdom ReceiVed December 15, 2006. ReVised Manuscript ReceiVed March 13, 2007

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.

1. Introduction The analytical chemistry of coal and petroleum-derived materials is relatively well-established, up to molecular masses of ∼400-450 u. The first section of this paper will review some of the more-familiar and several of the more-advanced techniques for analyzing this lower-molecular-mass (MM) range of components in complex, fossil-fuel-derived mixtures. These methods include several types of gas chromatography (GC), gas chromatography coupled with mass spectrometry (GC-MS), and other mass spectrometric methods, including two-dimensional gas chromatography and liquid chromatography mass spectroscopy.1,2 Results from these analyses of complex coal-derived liquids have been compared with data from probe mass spectroscopy (probe-MS) and field-ionization mass spectroscopy (FI-MS).3 There seems to be no obvious way of translating these methods to characterize larger-molecular-mass species. The extrapolation of known small-molecule structures to larger * Author to whom corespondence should be addressed. E-mail address: [email protected]. † Department of Chemical Engineering, Imperial College London. ‡ Department of Chemistry, University of Leeds. (1) Herod, A. A.; Stokes, B. J.; Major, H. J.; Fairbrother, A. Analyst (Cambridge, U.K.) 1988, 113, 797. (2) Herod, A. A.; Ladner, W. R.; Stokes, B. J.; Berry, A. J.; Games, D. E.; Hohn, M. Fuel 1987, 66, 935. (3) Herod, A. A.; Stokes, B. J.; Schulten, H.-R. Fuel 1993, 72, 31.

masses is usually found to give structures with very different atomic C/H ratios than “heavier” (higher-MM) samples.4 Furthermore, most analytical techniques optimized for the analysis of material up to ∼450 u cannot readily be adapted to examine samples that contain higher-MM materials. In chromatography and in several of the mass spectrometric methods involved (e.g., FI-MS), analyses are limited by the volatility of the samples. For example, it is normally possible to detect less than a quarter of a coal tar pitch using GC-MS. The material above the 400-450 u level must be characterized by methods that are much less precise and have been the subject of some debate. The second objective of this review is to summarize recent work from the three authors and their collaborators that is related to the molecular mass characterization of these “heavier” materials. The emphasis will be on the use of size exclusion chromatography (SEC) and comparing the results with those from several mass spectrometric methods. An early comparison of results from SEC and from fast atom bombardment mass spectroscopy (FAB-MS) has been presented.5 The analytical approaches used will be described in some detail (also see ref 6). The particular mix of methods grew out of experiments on (4) Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; Herod, A. A.; Stokes, B. J.; Kandiyoti, R. Fuel 1993, 72, 1381. (5) Herod, A. A.; Stokes, B. J.; Tye, R. E.; Kandiyoti, R.; Gaines, A.; Li, C.-Z. Fuel 1993, 72, 1317.

10.1021/ef060642t CCC: $37.00 © 2007 American Chemical Society Published on Web 06/30/2007

Characterization of HeaVy Hydrocarbons

coal-derived tars, extracts, and pitches,1-3,5,7-25 on kerogens and kerogen extracts,26-28 petroleum-derived vacuum resids and asphaltenes,21-23,29-31 tars from the thermal treatment of several types of biomass24,32-37 and other fossilized materials, such as amber and its extracts.38,39 The work on large-MM materials grew from efforts to examine the upper mass limits in coal and petroleum-derived liquids. However, sharply increasing prices for lighter crudes since mid-2004 and their potentially diminishing availability is generating an impetus for a focus on processing heavier feedstocks. To date, researchers in petroleum-related industries have largely dismissed heavier petroleum streams as consisting of molecular “aggregates”. Acceptably wasteful in times of plenty, hasty judgments on heavy crudes and petroleum residues have not, to date, brought dividends, in terms of facilitating (6) Kandiyoti, R.; Herod, A. A.; Bartle, K. D. Solid Fuels and HeaVy Hydrocarbon Liquids: Thermal Characterization and Analysis; Elsevier Science Publishers: Amsterdam, Oxford, London, New York, 2006; cf. Chapters 7 and 8. (ISBN: 0-08-044486-5.) (7) Islas, C. A.; Suelves, I.; Herod, A. A.; Kandiyoti, R. In Proceedings of the 11th International Conference on Coal Science, September 30October 5, 2001, San Francisco, CA; ICCS-Paper 215. (8) Herod, A. A.; Lazaro, M.-J.; Suelves, I.; Dubau, C.; Richaud, R.; Shearman, J.; Card, J.; Jones, A. R.; Domin, M.; Kandiyoti, R. Energy Fuels 2000, 14, 1009. (9) Apicella, B.; Ciajolo, A.; Suelves, I.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Comb. Sci. Technol. 2002, 174 (11-12), 345. (10) Bartle, K. D.; Taylor, N.; Mulligan, M. J.; Mills, D. G.; Gibson, C. Fuel 1983, 62, 1181. (11) Johnson, B. R.; Bartle, K. D.; Herod, A. A.; Kandiyoti, R. Prepr. Pap.sAm. Chem. Soc., DiV. Fuel Chem. 1995, 40 (3), 457. (12) 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. (13) Herod, A. A.; Kandiyoti, R. J. Planar Chromatogr. 1996, 9, 16. (14) 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, 171. (15) Herod, A. A.; Zhang, S.-F.; Kandiyoti, R.; Johnson, B. R.; Bartle, K. D. Energy Fuels 1996, 10, 743. (16) Lazaro, M. J.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Energy Fuels 1999, 13, 1212. (17) Herod, A. A.; Lazaro, M.-J.; Domin, M.; Islas, C. A.; Kandiyoti, R. Fuel 2000, 79, 323. (18) Karaca, F.; Islas, C. A.; Millan, M.; Behrouzi, M.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2004, 18, 778. (19) Johnson, B. R.; Herod, A. A.; Bartle, K. D.; Domin, M.; Kandiyoti, R. Fuel 1998, 77, 933. (20) Islas, C. A.; Suelves, I.; Millan, M.; Apicella, B.; Herod, A. A.; Kandiyoti, R. J. Sep. Sci. 2003, 26, 1422. (21) Pindoria, R. V.; Megaritis, A.; Chatzakis, I. N.; Vasanthakumar, L. S.; Lazaro, M. J.; Herod, A. A.; Garcia, X. A.; Gordon, A.; Kandiyoti, R. Fuel 1997, 76, 101. (22) Deelchand, J.-P.; Naqvi, Z.; Dubau, C.; Shearman, J.; Lazaro, M.J.; Herod, A. A.; Read, H.; Kandiyoti, R. J. Chromatogr. A 1999, 830, 397. (23) Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. J. Chromatogr. A 2004, 1024, 227. (24) Richaud, R.; Lazaro, M.-J.; Lachas, H.; Miller, B. B.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2000, 14, 317. (25) John, P.; Johnson, C. A. F.; Parker, J. E.; Smith, G. P.; Herod, A. A.; Gaines, A. F.; Li, C.-Z.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1991, 5, 364. (26) Madrali, E. S.; Rahman, M.; Kinghorn, R. R. F.; Wu, F.; Herod, A. A.; Kandiyoti, R. Fuel 1994, 73, 1829. (27) Li, C.-Z.; Herod, A. A.; John, P.; Johnson, C. A. F.; Parker, J. E.; Smith, G. P.; Humphrey, P.; Chapman, J. R.; Rahman, M.; Kinghorn, R. R. F.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1994, 8, 823. (28) Herod, A. A.; Lazaro, M.-J.; Rahman, M.; Domin, M.; Cocksedge, M. J.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1997, 11, 1627. (29) Domin, M.; Herod, A. A.; Kandiyoti, R.; Larsen, J. W.; Lazaro, M.-J.; Li, S.; Rahimi, P. Energy Fuels 1999, 13, 552. (30) Suelves, I.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2001, 15, 429. (31) Suelves, I.; Islas, C. A.; Millan, M.; Galmes, C.; Carter, J. F.; Herod, A. A.; Kandiyoti, R. Fuel 2003, 82, 1.

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their processing in times of greater need. In the past, the industry has mostly disposed of such materials through delayed coking, which is a process that makes some hydrogen and hydrocarbons and much “pet-coke”, the latter of which has often proved difficult to dispose of. Large price excursions in the energy world are thus leading in directions that require new chemical approaches for handling progressively larger molecular-mass materials. In oil refineries and elsewhere, fresh ideas are needed for successfully processing these heavier fractions, to reduce precipitation and fouling in heat exchangers, line blockages, and other forms of deposit formation, as well as carbon laydown and catalyst fouling during upgrading processes. Attaining these goals requires broader perspectives in determining molecular mass distributions and structural features of these heavier materials. It is expected that the analytical approaches described in this paper will be of increasing utility in examining these materials, as more sophisticated processing of progressively heavier feedstocks becomes commercially relevant. We will begin our survey, however, by reviewing methods designed for the analysis of the lower range of molecular masses (to ∼450 u). 2. Chromatography of Lower-Molecular-Mass-Range Materials in Fuel-Derived Samples The analysis of fuel-derived materials with molecular masses below ∼450 u is usually achieved by chromatographic means. The methods are named after the fluid used as the eluent: gas chromatography (GC), supercritical fluid chromatography (SFC), or liquid chromatography (LC). Fuel-derived mixtures are usually highly complex and identification is required in addition to separation. Mass spectrometry (MS) is the method of choice in many such cases (see Section 6), although, as we will see, element (viz. sulfur, nitrogen)-specific detection also finds application. We will also briefly review how resolution may be multiplied by the coupling of chromatographic methods. 2.1. Gas Chromatography of Fuel-Derived Lower-RangeMolecular-Mass Samples. 2.1.1. Capillary Gas Chromatography. Capillary-column GC can provide unparalleled resolution for analytes ranging from permanent gases to hydrocarbons and their derivatives with masses up to ∼400 u.40,41 It is the method of choice, when sample materials are sufficiently volatile below temperatures where analyte or column-coating degradation occurs. The columns are usually made of surface-deactivated fused silica and are 10-25 m long, with an internal diameter of 0.2-0.3 mm (but see below). For relatively nonpolar applications, the column may be internally coated with a methyl (32) Pindoria, R. V.; Lim, J.-Y.; Hawkes, J. E.; Lazaro, M.-J.; Herod, A. A.; Kandiyoti, R. Fuel 1997, 76, 1013. (33) Pindoria, R. V.; Megaritis, A.; Herod, A. A.; Kandiyoti, R. Fuel 1998, 77, 1715. (34) Pindoria, R. V.; Chatzakis, I. N.; Lim, J.-Y.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Fuel 1999, 78, 55. (35) Lazaro, M. J.; Domin, M.; Herod, A. A.; Kandiyoti, R. J. Chromatogr. A 1999, 840, 107. (36) Purevsuren, B.; Herod, A. A.; Kandiyoti, R.; Morgan, T. J.; Avid, B.; Davaajav, Ya. Eur. J. Mass Spectrom. 2004, 10, 101. (37) Purevsuren, B.; Herod, A. A.; Kandiyoti, R.; Morgan, T. J.; Avid, B.; Gerelmaa, T.; Davaajav, Ya. Fuel 2004, 83, 799. (38) Pipatmanomai, S.; Islas, C. A.; Suelves, I.; Herod, A. A.; Kandiyoti, R. J. Anal. Appl. Pyrolysis 2001, 58, 299. (39) Islas, C. A.; Suelves, I.; Carter, J. F.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2001, 15, 845. (40) Lee, M. L.; Yang, F. J.; Bartle, K. D. Open Tubular Column Gas Chromatography; Wiley: New York, 1984. (41) Bartle, K. D. In Handbook of Polycyclic Aromatic Hydrocarbons, Volume 2; Bjorseth, A., Ramdahl, T., Eds.; Marcel Dekker: New York, 1985; Chapter 6.

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Figure 1. Gas chromatography (GC) chromatogram of coal tar on a 20-m-long column coated with a mesogenic stationary phase. Insets show portions of the chromatogram on an SE-52 methylphenyl stationary phase. Peak assignments: 5, triphenylene; 6, benz[a]anthracene; 7, chrysene; 8, benzo[j]fluoranthene; 9, benzo[b]fluoranthene; 10, benzo[k]fluoranthene; 11, benzo[e]pyrene; 12, perylene; 13, benzo[a]pyrene and 14, indeno [1,2,3-cd] pyrene. (Reproduced with permission from ref 40; copyright Wiley, 1984.)

siloxane (or similar) stationary phase, which sometimes contains a small proportion of phenyl groups and is cross-linked to increase thermal stability. For specific separations, more polar phases may be used. Liquid-crystalline poly(mesogenmethyl)siloxanes with a wide temperature range (70-300 °C) exhibit high selectivities for polycyclic aromatic compounds (PACs). This has proved especially useful in the separation of a number of coal-derived PAC isomer groups, with m/z 228 and 252 (see Figure 1). Figure 1 also illustrates the general limitations of gas chromatography. The actual mass range of the tar sample almost certainly extends considerably beyond 300 u. In the chromatogram, however, peak intensities rapidly fall toward zero after the benzopyrene isomers (MM 252 u). GC columns not optimized for higher-temperature operation have a tendency to reach a maximum aromatic mass at ∼302 u (dibenzopyrene isomers). The chromatograms distinctly show diminishing peak intensities after about benzo[ghi]perylene, with a mass of 276, which indicates the onset of a loss of aromatics of greater mass in the injector-column system. In practice, the maximum aromatic size likely to pass through a polysiloxane-coated GC column has a mass of 352, equivalent to tribenzopyrene isomers. Higher-mass components are either not sufficiently volatile to pass through the GC column or are pyrolyzed to char in the injector. PACs that contain eight or nine aromatic rings and have masses up to 456 u have been determined by high-temperature gas chromatography (HTGC)42 and high-temperature gas chromatography coupled with mass spectroscopy (HTGC-MS). The molecular mass range of GC on capillary columns can be extended if silylarene siloxane polymer stationary phases are (42) Romanowski, T.; Funcke, U.; Grossman, I.; Konig, J.; Balfanz, E. Anal. Chem. 1983, 55, 1030.

used;43 however, eventual thermal degradation of the phase layer is inevitable. In any case, the polyamide protective outer coating of fused silica columns cannot repeatedly withstand temperatures of >430 °C, becoming brittle after extended use. Although these problems have been partially resolved by the use of aluminumcoated fused silica or stainless-steel capillaries,44 there remains the possibility of thermally induced reactions of the analytes at high column temperatures. For example, it is likely that molecules such as hydroaromatics would begin to degrade before temperatures of 380-400 °C were reached. Indeed, HTGC-MS45 shows markedly lower concentrations of PACs with masses of >278 u, when compared with probe-MS.46 Simulated Distillation (SIMDIST). Distillation data are of obvious importance in the characterization of petroleum products, as well as coal-derived and other complex liquids. Simulated distillation (SIMDIST) via GC with calibration from the retention of n-alkanes is a common approach. HTGC on thermally stable capillary columns has extended the more standard ASTM packed-column GC methods for SIMDIST of petroleum products. Despite the limitations previously discussed, an upper temperature limit equiValent to an atmospheric-pressure boiling temperature of 847 °C could be achieved47 using a column temperature up to 425 °C, eluting n-alkanes to at least C92. Element-Specific Detection. The often-high concentrations of nitrogen- and sulfur-containing compounds in the heavier coal (43) Bemgard, A.; Colmsjo, A.; Lundmark, B. O. J. Chromatogr. A 1993, 630, 287. (44) Takagama, Y.; Takeichi, T.; Kawai, S. J. High Res. Chromatogr. 1988, 11, 732. (45) Al-Muhareb, E. M.; Karaca, F.; Morgan, T. J.; Herod, A. A.; Bull, I. D.; Kandiyoti, R. Energy Fuels 2006, 20, 1165. (46) Herod, A. A.; Kandiyoti, R. J. Chromatogr. A 1995, 708, 143. (47) Reddy, K. M.; Wei, B.; Song, C. Catal. Today 1998, 43, 187.


and petroleum-derived fractions have led to the development of element-selective detection, as a complement to the universal flame ionization detection (FID). The thermionic detector (for nitrogen compounds) and the flame-photometric detector (for sulfur compounds) have been useful in the analysis of coalderived fractions.40 However, the ultimate in element selectivity is achieved with atomic emission detection (AED), in which the GC column effluent is passed into a microwave discharge and the lines in the resulting atomic emission spectra are resolved and their intensities recorded to provide elemental chromatograms.48 In, principle, good selectivity may be achieved using AED for any element but helium, and up to four elements with emission wavelengths within a 20-25 nm interval (e.g., carbon, 193 nm, providing a “universal” detector chromatogram, and sulfur, 181 nm) may be monitored with high sensitivity and little breakthrough. Element responses are essentially linear, although there may be some compound dependence at high concentration. The resulting unparalleled selectivity has led to fruitful applications to both basic and nonbasic nitrogen compounds in coal liquids; among the nitrogen compounds identified in pyrolysis tars49 from Polish coals were quinolines, acridines, and carbazoles and their benzologues and alkyl derivatives.50 The inherent low response of the 174-nm atomic nitrogen emission has led to a search for alternatives.51 However, most rewarding has been the use of AED as a sulfur-selective detector; this approach allowed the analysis of the above pyrolysis tars50 and petroleum distillates52 for numerous polycyclic aromatic sulfur heterocycles (PASHs)sin particular, alkylated benzo[b]thiophens, dibenzothiophens, and naphthobenzothiophensswhereas reactions that occur during the pyrolysis of coal tar pitch were identified from GC-AED analyses for the low concentrations of phenyl-substituted PASHs.53 The identification and quantitation of oxygen compounds in coal products is another promising application,54 whereas HTGC with AED showed the presence and distribution of organic nickel compounds in higher-MM petroleum fractions.55 Fast GC. Recent advances in GC technology have placed an emphasis on reducing the analysis times through the use of small-diameter columns and rapid temperature programming56the so-called “fast GC”. Column lengths and retention times may also be reduced using selective stationary phases to reduce the number of theoretical plates necessary for given separations. In comparison with analysis on more-conventional 0.25-mminner-diameter (0.25-mm-id) capillaries, separations are more rapid and efficient57 on 0.1-mm-id columns (see Table 1). Optimization of injector volumes is necessary, however, and the column oven must be capable of temperature programming at rates up to 20 °C/min. Even faster analysis may be achieved if hydrogen is used as a mobile phase. (48) Sullivan, J. J.; Quimby, B. D. In Element-Specific Chromatographic Detection by Atomic Emission Spectroscopy; Uden, P. C., Ed.; American Chemical Society: Washington, DC, 1992; Chapter 4. (49) Holden, K. M. L. Ph.D. Thesis, University of Leeds, U.K., 1997, p. 163. (50) Holden, K. M. L.; Bartle, K. D. Unpublished measurements, 1997. (51) Gonzalez, A. M.; Uden, P. C. J. Chromatogr. A 2000, 898, 201. (52) Schmid, B.; Andersson, J. T. Anal. Chem. 1997, 69, 3476. (53) Meyer zu Reckendorf, R. Chromatographia 1997, 45, 173. (54) Sumbogo Murti, S. D.; Sakanishi, K.; Mochida, I. Prepr. Pap.s Am. Chem. Soc., DiV. Fuel Chem. 2000, 45 (4), 860. (55) Quimby, B. D.; Dryden, P. C.; Sullivan, J. J. J. High Resol. Chromatogr. 1991, 14, 110. (56) Cramers, C. A.; Janssen, H.-G.; van Deursen, M. M.; LeClerq, P. A. J. Chromatogr. A 1999, 856, 315. (57) Boden, A. R.; Ladwig, G. E.; Reiner, E. J. Polycyclic Aromat. Compd. 2002, 22, 301.

Energy & Fuels, Vol. 21, No. 4, 2007 2179 Table 1. Comparison of Properties of Capillary GC Columns in PAC Analysisa parameter

Fast GC

Column Standard GC

Fast GC

column length (m) internal diameter (µm) film thickness (µm) theoretical plates per m total theoretical plates relative column efficiency relative analysis timeb

10 100 0.1 8600 86000 0.93 0.38

30 250 0.25 3300 99000 1 1

20 100 0.1 8600 172000 1.32 0.55

a

From ref 57. b For benzo[ghi]perylene.

GC remains a powerful technique for the analysis of coalderived materials that have been thermally or chemically degraded, as well as for samples from biomass, petroleum, and other fossil fuels. When, however, coal tars and extracts are produced under conditions minimizing secondary reactions (cf. ref 6), products contain few lower-MM-range materials amenable to GC analysis and other methods must be used. 2.1.2. Two-Dimensional Gas Chromatography. We have already explained that the complexity of mixtures from the processing of fossil fuel requires analytical methods that are capable of very high resolution. To this end, numerous procedures have been developed for coupling chromatographic techniques together.58 Truly comprehensive hyphenation (all mixture components separated in the first chromatographic dimension are passed into a second chromatographic dimension or column) involves the coupling of two columns in series, and then frequent transfer from the first to the second, in which very rapid analysis occurs.59 Using a different chromatographic separation principle (e.g., first volatility, then polarizability), components that have been separated in the first column are not allowed to recombine. In orthogonal two-dimensional GC60 (the so-called “GC×GC” procedure), modulated transfer by either a thermal method, in which fractions condensed at the end of the first column are then introduced into the second column by rapid heating, or via multiport valving for fraction transfer, allows the introduction of fractions from a nonpolar capillary column into a short secondary more-polar column. This allows for rapid analysis based on polarity or, for hydrocarbon mixtures, polarizability. The resulting two-dimensional chromatogram shows not only greatly enhanced peak capacity, but a considerable increase in sensitivity caused by peak amplitude enhancement61 from compression of the chromatographic zone during transfer (the collection period of, for example, 8 s is released into the second column in 300 000). However, collection and transmission into the ion trap of highmass components from polydisperse mixtures (coal liquids and petroleum asphaltenes) seems to be incomplete. In the case of the coal tar pitch examined by ESI-MS, elemental analysis indicates that each molecule above a mass of 1250 u is likely to contain at least one N atom. Instead, the practical upper mass limit of FT-MS instruments in ESI mode seems to be m/z ∼1200 for singly charged ions. The other mass spectrometric methods described thus far (GC-MS, probeMS, and FI-MS) are primarily limited by the volatility of sample components. Therefore, we must turn to other techniques to characterize samples with components that contain higher molecular masses. The methods available are considerably less precise than those described previously, but allow examination of much-wider MM ranges. 4. Size-Exclusion Chromatography (SEC) 4.1.1. General Considerations. Above the ∼450-500 u range, no single method is unambiguously capable of indicating MM distributions or chemical structural features in complex fuel-derived mixtures. We usually do not have precise information on the upper mass limits of such samples. Advances in this field require the assembly and comparison of relevant evidence from several independent analytical methods. Comparison of samples often improves our understanding of these (94) Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Energy Fuels 2001, 15, 492. (95) Schaub, T. M.; Hendrickson, C. L.; Quinn, J. P.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2005, 77, 1317. (96) Schaub, T. M.; Rodgers, R. P.; Marshall, A. G.; Qian, K.; Green, L. A.; Olmstead, W. N. Energy Fuels 2005, 19, 1566.


complicated materials. Structural studies using fractionation and bulk characterization methods generally assist in coming to grips with this difficult set of problems. Much of the work indicating mass limits of ∼1000-1500 u for coal tars, pitches, and petroleum asphaltenes has been explained in terms of the limitations of the particular analytical techniques (e.g., cf. ref 97). SEC is one of the analytical methods with a broader range of MM values, which can be used in this type of sample characterization. The formulations of SEC described below have allowed estimation of the MM values of coal-derived liquids,18 up to ∼3000 u and to within a factor of 2-2.5 at up to 15 000 u. SEC is not limited by volatility considerations but rather by the solubility of the sample in the chosen solvent (eluent). In principle, a “universal” calibration can be applied to SEC in the form of a plot of log10 ([η´ ]Mn) against elution time or volume, where [η´ ] is the intrinsic viscosity and Mn is the number-average MM value of the polymer sample. This calibration would account for the elution behavior of all polymers in a given solvent. However, universal calibration was considered to be an experimentally unreliable procedure when it was determined that solutions of coal extract subfractions in THF (tetrahydrofuran) have intrinsic viscosities very similar to that of the solvent.98 4.1.2. Detector Configurations. Detection of material eluted from the SEC column is often achieved by UV absorbance. In the work described below, an evaporative light scattering (ELS) detector has also been used in series with the UV-absorption detectors (a variable-wavelength single-channel detector usually set at 450 nm and a diode-array detector set at 280, 300, 350, and 370 nm). The ELS detector is capable of indicating the presence of molecules with no UV absorbance, but it is limited to significantly larger molecular species, compared to the solvent used as an eluent. The temperatures of operation with 1-methyl2-pyrrolidinone (NMP) as an eluent were 150 °C at the nebulizer and 210 °C at the evaporator, with a nitrogen flow of 0.8 L/min. The ELS detector is significantly more sensitive than refractive index detectors and has similar sensitivity to UV-absorbance devices. When the more-sensitive UV-fluorescence detector is used in series with the UV-absorbance detector,99 the system must be operated at low solute concentrations, to avoid overloading the fluorescence detector. 4.1.3. Sample Fractionation. Determinations of MM distributions are often affected by sample polydispersity. Most available characterization techniques have a tendency to provide information on the properties of the more-abundant materials/ fractions of complex mixtures. Often, the properties of lessabundant fractions are masked. Therefore, fractionation of such mixtures is a necessary first step in attempting to obtain a morecomplete picture, covering all “fractions” of the sample. In the work described below, several different methods have been used to fractionate samples to reduce polydispersity, prior to examination by SEC and allied spectrometric methods. These separation methods include planar chromatography,13,22,23,35,46 column chromatography on silica,100-103 preparative SEC,104,105 and separation by solvent solubility.30,31,106 A detailed comparison of these methods has been presented in ref 6. Each of these fractionation methods has its own advantages and problems. Planar chromatography is a relatively fast and (97) Herod, A. A.; Bartle, K. D.; Kandiyoti, R. Fuel 2006, 85, 1950. (98) Bartle, K. D.; Mulligan, M. J.; Taylor, N.; Martin, T. G.; Snape, C. E. Fuel 1984, 63, 1556. (99) Morgan, T. J.; Millan, M.; Behrouzi, M.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2005, 19, 164.

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low-cost method capable of using any combination or sequence of solvents. The heaviest (immobile) fraction remains at the origin of the plate and may be examined in situ or recovered for further examination by any desired analytical technique. On the deficit side, only small quantities of sample can be fractionated and material recovery is not quantitative, leaving the heaviest material (∼10% of the sample) stuck on the plate. Column chromatography allows quantitative recovery of fractions from relatively large sample quantities, but the recovery of the heaviest fractions is again incompletesas in the case of planar chromatographysbecause of sample adherence to the silica. Preparative SEC can, in principle, give fractions of narrow polydispersity. However, the sample is recovered as a dilute solution in thermally sensitive NMP. It is not possible to counter the dilution by increasing the amount of sample, because the excluded region overloads quickly. This has the effect of smearing out excluded material across later-eluting fractions. Solvent solubility separation provides fractions with greater overlap in SEC and in terms of polarity, compared to fractions from planar or column chromatography.107 However, there is no loss of material on chromatographic surfaces or any loss of trace element-bearing components, as found for planar and column chromatography. Furthermore, NMP need not be used, avoiding the marked difficulty of removing this particular solvent from the heaviest fractions. 4.2. Calibration of SEC with NMP as an Eluent. 4.2.1. OVerView. A large amount of SEC work has been done in the past, using THF as an eluent.10,46,98,108 Since then, it has become clear that THF does not dissolve the largest molecules in coal liquids and it does not have sufficient solvent power to block surface interactions between solute molecules and packing particles within SEC columns.109 In one application, fractions of a coal tar pitch separated by planar chromatography were examined by SEC using THF as an eluent; as shown in Figure 4, larger-molecular-size materials were smeared over the entire chromatogram46 of the pitch sample, with no regular sequence of elution time becoming earlier as the molecular size and immobility in TLC increased. As discussed below, when the work was repeated23 using SEC in the NMP solvent, the elution of fractions did shift to earlier times with increasing molecular size and immobility in TLC. In contrast to THF, NMP completely dissolves most coalderived liquids. Examination of the guard column used during SEC with THF revealed deposited material that could be washed clean with NMP. THF had to be abandoned as a valid eluent for the samples on hand, because precipitation from solution had indicated a partial loss of sample. Following the lead by Lafleur and Nakagawa,110 THF was replaced with NMP in (100) Islas, C. A.; Suelves, I.; Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Fuel 2003, 82, 1813. (101) Herod, A. A.; George, A.; Islas, C. A.; Suelves, I.; Kandiyoti, R. Energy Fuels 2003, 17, 862. (102) Karaca, F.; Behrouzi, M.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2005, 19, 187. (103) Karaca, F.; Behrouzi, M.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Fuel 2005, 84, 1805. (104) Islas, C. A.; Suelves, I.; Carter, J. F.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2002, 16, 481. (105) Islas, C. A. Ph.D. Thesis, London University, London, U.K., 2001. (106) Al-Muhareb, E.; Morgan, T. J.; Herod A. A.; Kandiyoti, R. Pet. Sci. Technol. 2007, 25, 81-91. (107) Lazaro, M.-J.; Herod, A. A.; Kandiyoti, R. Fuel 1999, 78, 795. (108) Bartle, K. D.; Mills, D. G.; Mulligan, M. J.; Amaechina, I. O.; Taylor, N. Anal. Chem. 1986, 58, 2403. (109) Johnson, B. R.; Bartle, K. D.; Herod, A. A.; Kandiyoti, R. J. Chromatogr. A 1997, 758, 65. (110) Lafleur, A.; Nakagawa, Y. Fuel 1989, 68, 741.



Figure 4. Size exclusion chromatography (SEC) in tetrahydrofuran (THF) solution of selected fractions of coal tar pitch from thin-layer chromatography (TLC), compared with the unfractionated pitch. Fractions numbered from 1 (for the most mobile) to 15 (for the least mobile), with the whole pitch marked P. (Constructed from the data of ref 46, previously not shown in this form.)

subsequent work. NMP also dissolves part of the aromatic component of petroleum vacuum residues, as well as biomass tars, with their normally higher proportion of oxygenated species, compared to coal or petroleum liquids.11-15 On the other hand, NMP is unable to dissolve aliphatic material. In one recent case, it was found to dissolve only ∼50% of a petroleum asphaltene.106,111 Therefore, the more-aliphatic fractions of petroleum residues cannot be characterized by SEC when using pure NMP as an eluent. 4.2.2. Calibration of SEC Columns. The estimation of MM ranges of hydrocarbon mixtures by SEC relies on the assumption112 that the sizes (hydrodynamic volumes) of molecules or polymers used as MM standards correlate with the sizes of sample molecules of similar molecular mass. This assumption allows the calculation of number- and weight-average molecular masses (Mn and Mw, respectively) of samples. The ratio of these two parameters (Mw/Mn) is defined as the polydispersity of the mixture. For a fraction of narrow MM range, it can be assumed that the peak mass (Mp) in SEC approximates to both Mn and Mw. Polymer standards of narrow polydispersity (Mw/Mn < 1.1) have a tendency to show linear relations between the logarithm of molecular mass (log10(MM)) and elution time (or elution volume) over wide ranges of masses.18 The SEC columns used in our work are from Polymer Laboratories (Church Stretton, U.K.) and are known as Mixed-E, Mixed-D, and Mixed-A. They are of the mixed-bed type, with a range of porosities suitable for eluting polystyrenes of particular mass ranges in linear relations between log10(MM) and elution times, as defined in solution in toluene as Mixed-E (100-20 000 u), Mixed-D (100200 000 u), and Mixed-A (100-40 000 000 u). The ranges of porosities remain confidential to the manufacturer. The column packings are composed of a polystyrene-polydivinylbenzene copolymer and have diameters of 3 µm (Mixed-E), 5 µm (Mixed-D), and 20 µm (Mixed-A). (111) Ascanius, B. E.; Merino-Garcia, D.; Andersen, S. I. Energy Fuels 2004, 18, 1827. (112) Malawer, E. G. In Handbook of Size Exclusion Chromatography; Wu, C.-S., Ed.; Chromatographic Science Series, Vol. 69; Marcel Dekker: New York, 1995; Chapter 1.

We have explained earlier that not enough is known about the structures and molecular masses of samples above the 450500 u level. Therefore, it is not possible to construct a calibration line that would correspond directly to the heavier fractions of actual coal or petroleum-derived samples. The next-best objective would be to construct an SEC system and accompanying calibration line that would be, as much as possible, independent of the structural features of eluting molecules. In this sense, pure NMP has turned out to be the best eluent for coal-deriVed materials. In the work described below, several methods have been pursued simultaneously. We intended to arrive at a set of calibrations that would indicate possible levels of error, as well as provide estimates of the ranges of molecular masses of the samples. The resulting body of data is extensive. Details of the work have been described in ref 16, ref 18, and Chapter 8 of ref 6. Briefly, the first step was to use commercially available polystyrene (PS) and poly(methyl methacrylate) (PMMA) molecular mass standards to draw a basic calibration line. This procedure allowed us to compare the elution volumes of two sets of structurally distinct samples. Up to masses of ∼1 500 000 u, the elution times of these two sets of standards were observed to be, statistically, almost indistinguishable. In view of the structural differences between PS and PMMA, these findings were encouraging. The resulting calibration line will be referred to below as the “PS-PMMA line”. Subsequently, polysaccharide (PSAC) standards also were used. Figure 5 shows the calibration with PS, PMMA, and PSAC for the Mixed-A column. Next, the elution times of a wide range of standard (pure) compounds, of variable heteroatom content and polarity and with molecular masses up to 1086 u (alcian blue) were determined.16-18 Elution times of almost all compounds were mostly within (1 min (occasionally (2 min) away from the PS-PMMA line; a 1 min shift in elution time is equivalent to a factor of 2.3 (Mixed-D) or 3.5 (Mixed-A), whereas 2 min is equivalent to a factor of 5.4 (Mixed-D) or 12.8 (Mixed-A) in the molecular mass of polystyrene standards. Only a fullerene mixture (C60 and C70, with MM ) 720 and 840 u, respectively)


Figure 5. Calibration graph of a Mixed-A column using NMP as an eluent and polystyrene, poly(methylmethacrylate) (PMMA), and polysaccharide standard polymers as calibrants. (Reproduced with permission from ref 18; copyright American Chemical Society, 2004.)

eluted at much-shorter elution times than would have been expected of their molecular masses. The result, which appeared far above the 1 million u mark, grossly overestimated the known molecular masses of fullerenes. We will discuss the possible significance of this finding somewhat later in this paper. A calibration test was then attempted, by collecting narrow polydispersity SEC effluent fractions of coal tar pitch with known (successive) elution times. The average molecular masses of these narrow cuts were determined using MALDI-MS and LD-MS.7,19,20 The masses thus determined by mass spectrometry were compared with masses obtained from the PS-PMMA calibration curve, which correspond to the elution times of the coal tar pitch fractions determined using the SEC column. Clear agreement was obtained with the PS-PMMA line for MM values up to slightly more than 3000 u. Figure 6 shows the agreement between the PS calibration and the mass values derived from MS. Next, the PS-PMMA line was plotted alongside elution times of several polymeric MM standards with entirely distinct structural features. These standards included the following: 1, poly(vinyl acetate) (Mp ) 170 000 g/mol); 2, poly(N-vinylcarbazole) (Mp ) 90 000 g/mol); 3, poly(ethylene oxide) (Mp ) 58 400 g/mol); 4, poly(vinyl pyrrolidinone) (Mp ) 58 000 g/mol); 5, polyethylene adipate (Mp ) 10 000 g/mol); 6, poly(ethylene glycol) (Mp ) 4120 g/mol); and 7, poly(vinyl pyrrolidinone) (Mp ) 3500 g/mol).

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The greatest departure from the PS-PMMA line for any member of this disparate set of samples did not much exceed (1 min. It was noted that oxygenated compounds had a tendency, preferentially, to elute earlier than would be indicated by their MMs, according to the PS-PMMA calibration. To probe the widening discrepancy with oxygenated species, a set of PSAC standards (up to 788 000 u) have been eluted in NMP. The results confirmed the trend for oxygenates showing greater discrepancy and shorter elution times, compared to the PSPMMA line. The discrepancy between the two calibration lines clearly increased with molecular mass. Agreement was observed up to 3000 u, between two entirely independent techniques (MALDI-MS and SEC (the PS-PMMA line)) for coal-derived and similar materials. For coal-derived samples, elution times in NMP for masses up to 3000 u may thus be taken at face value. On the other hand, the PSAC calibration line clearly showed the greatest departure from the PS-PMMA line observed in this entire body of work. For highly polar samples, it seems that molecular masses may still be estimated from elution times, as lying somewhere between the PSAC and the PS-PMMA calibration lines. This estimate is subject to growing error with increasing molecular mass. Assuming the PSAC line to be the worst-possible departure from the PS-PMMA line (according to all work to date), the MM values of PSACssor any other sample with MM < 15 000 us may be estimated6,18,113 to within an error factor of ∼2-2.5. The single most-significant departure from these calibration lines was shown by the manifestly three-dimensional fullerenes samples, to which we will return presently. 4.3. Two Peaks Observed in SEC When Using NMP as an Eluent. A quick examination of Figures 7 and 8, which show fractions of pitch by solvent solubility and column chromatography, respectively, shows that size-exclusion chromatograms of samples eluted in NMP present two successive peaks. The leading edge of the later-eluting, smaller-mass (second) peak shows signal from material that was retained and resolved by the column porosity. Depending on the nature of the sample and the particular SEC column, the leading edge of this peak appears between ∼3000 u and 9000 u. As already explained, the MM values of material appearing under this peak may be taken at face value, up to ∼3000 u and to within a factor of (at worst) 2-2.5 of molecular masses up to ∼15 000 u. The latter figure would amply cover the earliest eluting material under the (resolved) second peak. However, interpretation of the

Figure 6. Polystyrene calibration graph of Mixed-D column, showing average masses (log Mp) of pitch fractions collected from SEC, determined by matrix-assisted laser desorption/ionization (MALDI-MS) or laser desorption-mass spectroscopy (LD-MS) and plotted against the elution times determined using the Mixed-D column. (Reproduced with permission from ref 18; copyright American Chemical Society, 2004.)



Figure 7. Coal tar pitch fractions from solvent solubility in acetone (curve 1), pyridine (curve 2), and insoluble in pyridine (curve 3), by SEC in an NMP eluent, Mixed-D column. (Reproduced with permission from ref 101; copyright American Chemical Society, 2003.)

Figure 8. SEC on Mixed-A column in NMP eluent, of low-temperature coal tar after fractionation by column chromatography using (1) pentane (50 mL), (2) pentane (50 mL), (3) toluene, (4) acetonitrile, (5) pyridine, (6) NMP and (7) water. (Previously given as Supporting Information for ref 45.)

apparently much-larger masses shown by the calibration for the early eluting (excluded) peak in this type of chromatogram is uncertain. This early eluting peak shows signal from material largely excluded from column porosity. Evaluated against the PSPMMA calibration line, the elution times correspond to MM values exceeding several hundred thousand universal atomic units, and, at times, indicating molecular masses in the region of several million universal atomic units. As observed in Figures 7 and 8, there is always a valley of approximately zero intensity between the excluded peak and the retained peak observed for almost all samples and fractions. However, nothing we know about these samples would lead us to believe that a bimodal MM distribution corresponds to reality. One possible solution to the problem emerges if there is a transition from one type of molecular conformation to another and it occurs at MM values that correspond to those values between the two peaks.

An early result from the SEC of fullerenes has provided a useful insight in this respect. The sample was a mixture of C70 and C60 (∼90% C60 and ∼10% C70). The estimated solution volume of these molecules is large (∼0.66 nm3, corresponding to a diameter of 1.1 nm).114 In the Mixed-D column (Polymer Labs, U.K.), the mixtures of fullerenes eluted at 9.0 min. The elution time, based on its MM value and the polystyrene calibration, would have been ∼18 min.16 Fullerenes were the only species among compounds tested, which showed as great a discrepancy with the elution times as that expected from the PS-PMMA calibration. In estimating molecular masses, SEC explicitly relies on the assumption of a correlation between molecular mass and molecular size. In this sense, fullerenes (which have spherical shapes) provide an obvious exception, greatly exaggerating molecular size, in relation to molecular mass. In SEC, its threedimensional structure would make it behave (and appear) as if it were a molecule with a much-larger mass.


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Figure 9. Calibration of Mixed-A column using three-dimensional standards in an NMP eluent. (Revised from ref 18 using a fresh fullerite sample.)

Similarly, it was earlier reported that soot samples could be fractionated by filtering out material caught by a 20-nm filter. When the material retained on the filter was redissolved in NMP, the earlier chromatograms that showed signal in the excluded zone could be reproduced.8,9 Clearly, these are materials with diameters of sizes in the range of tens of nanometers. As in the case of fullerenes, elution times well inside the excluded zone suggested a relationship between short elution times and the sizes of these more fully three-dimensional objects in SEC columns. To follow up this line of thinking, a set of colloidal silica samples (Nissan Chemical Industries, Ltd., Houston, TX) with diameters of 9, 12, and 22 nm have been used to test the relationship between actual particle diameters and elution time. Figure 9 shows the relation between the logarithm of the diameter (in nanometers) and the elution time for the threedimensional standards on the Mixed-A column.18 The data in the Mixed-A and Mixed-D columns, respectively, show plots that conform to linear behavior. The data point pairs on the two diagrams belonging to the mixture of C60 and C70 fullerenes, eluting at 13.5 min in the Mixed-A and 10.5 min in the Mixed-D columns, seem to follow the same relationship as the colloidal silica particles. This result clearly indicates that the sample of mixed fullerenes had eluted according to their molecular sizes rather than their molecular masses. We note that a previous elution time for fullerene for the Mixed-D column was 9 min,16 and the difference is believed to result from the prolonged use of the column and the gradual blocking of the interparticular spaces by debris, because the elution times of polystyrenes able to penetrate the particle porosity did not change. Existing data still leave unclear the relationship in SEC between diameter and molecular shape. The materials selected for the experiments just described were clearly spherical. It is interesting to contemplate whether rodlike molecules of similar diameters would elute according to diameter or length. However, the consistency of the fullerenes data with the linear relationship of silica particles suggests, at least to a first approximation, that, for samples with well-defined three-dimensional conformations, the effect of particle density on elution times may be negligible. The observed internal consistency of these data also addresses (113) Podzimek, S. Int. J. Polym. Anal. Charact. 2005, 9, 305. (114) Ruelle, P.; Farina-Cuendet, A.; Kesselring, U. W. J. J. Chem. Soc. Chem. Commun. 1995, 1161.

concerns that the fullerenes might have been eluting in the form of aggregated clusters. The results just described suggest, meanwhile, that it might be possible to explain the bimodal distributions observed in SEC chromatograms in terms of sample molecules above a certain molecular threshold adopting more three-dimensional conformations. It is likely that a change of conformation of molecules above a certain molecular mass/size (∼5000-8000 u) from planar (and able to penetrate the porosity of the SEC column) to a three-dimensional structure would impart a larger hydrodynamic volume. The change of conformation would thus allow the already quite large molecules to adopt sizes that would exclude them from column porosity. They would thus appear at shorter elution times than their real molecular masses would warrant, showing larger molecular masses in the PS-PMMA calibration. These results show the necessity for caution in the levels of confidence attached to precise estimates of molecular masses of structurally less-well-defined samples, when using the PSPMMA calibration line. Clearly, these low levels of precision of mass measurement bear no relation to the mass measurement achieved by working with GC-MS and other high-precision methods of analysis. However, many materials lie outside the reach of these well-known techniques. SEC, in conjunction with LD-MS techniques (to be discussed below) has served well in attempting to assess the ranges of MM values of these heavier samples. Taken together, within the described limits to the PS-PMMA calibration, SEC with NMP as an eluent may be considered to have advanced our knowledge of materials above the 450-500 u MM range. Samples known to contain small molecules, such as essential oils (tea tree oil, lavender oils)115 and creosote oil and anthracene oil (both distillate fractions) all eluted as expected from SEC in NMP, with no suggestion of aggregation to form a peak in the excluded region. Furthermore, fractions of a pitch and an atmospheric petroleum residue, prepared by planar chromatography, showed the expected shifts to larger molecules and an increasing proportion of the excluded material, as the polarity of the development solvent (in planar chromatography) increased.23 The main drawback to the method has (115) Morgan, T. J.; Morden, W. E.; Al-Muhareb, E.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2006, 20, 734.


been the inability of NMP to dissolve petroleum asphaltenes completely.106 In particular, the method has been an effective and verifiable tool in comparing the molecular masses of complex samples. 4.4. UV Fluorescence and the Detection of Large Molecular Masses in SEC. Many studies in the literature have evaluated the performance of UV fluorescence in detecting highmass materials. The consensus that has emerged between (at least) four laboratories is that UV fluorescence is unable to detect masses greater than ∼3000 u. As will be discussed, in one case, this inability to detect high-mass materials has been interpreted in terms of the absence of higher-mass material in the samples examined. In an early study, a UV-fluorescence (UV-F) spectrometer equipped with a flow cell was used as detector in SEC,116 connected in series with a conventional UV-absorption (UVA) detector. The measurements were made as part of a coal liquefaction experiment, where the samples of the product stream exiting from a semicontinuous reactor were sequentially examined. The two spectrometric methods showed broadly similar signals for the lighter fractions extracted at lower temperatures. However, as the extraction deepened and the extents of coal conversion increased, the UV-absorption detector showed signal at progressively shorter elution times, whereas the time of initial signal from the UV-F detector remained fixed. In other words, the signal from UV-F detection did not change significantly with changing (heavier) sample properties. For the heaviest coal extract fraction isolated in these experiments, UV-A showed sample eluting from the column as early as 10 min. Meanwhile, no UV-F signal was detected at times earlier than 14 min in any of the experiments. In terms of the polystyrene calibration of the particular column used in that study, 14 min corresponded to a value of log10(MM) ) 3.5, which is slightly less than 3200 u. Clearly, this was the upper mass limit of sample that could be detected by the UV-F spectrometer. It was reported at the time116 that UV-F was unable to observe some of the apparently higher-mass material that was easily observed by the UV-A detector during the same experiment. These observations were made during SEC experiments with THF as an eluent. Following the adoption of NMP as the preferred eluent,15,110 a new attempt was made to use the UV-F spectrometer as a detector during SEC. These measurements were conducted on the pentane-insoluble fraction of a coal extract and hydrocracking products of this fraction, which were treated catalytically between 320 °C and 460 °C.15 Other improvements were also introduced. The earlier data116 had been acquired at a single UV-F excitation wavelength (at 254 nm) and a single emission wavelength (at 420 nm). The second study using NMP as the eluent was somewhat more detailed. A wider selection of excitation/emission frequency combinations was used: 300/325, 325/350, 350/400, 375/425, 400/450, 425/475, and 450/500 (all values given in nanometers). Using the same SEC column (Mixed-E; Polymer Labs., U.K.), but with NMP as the eluent, no signal could be observed from the UV-F detector at elution times earlier than 14 min, as observed previously. In contrast, when using UV-A detection, the higher-solvent power of NMP allowed the detection of largesized material eluting at the exclusion limit of the chromatographic column, which was observable from 6.5 min onward.15 It was clear that the UV-F spectrometer lacked detection power for material eluting at elution times shorter than ∼14 min. (116) Li, C.-Z.; Wu, F.; Xu, B.; Kandiyoti, R. Fuel 1995, 74, 37.


Explanation for these observations may be found within the fundamental limitations of UV-F spectrometry. Fluorescence intensity is known to decrease with increasing molecular size and with increasing sizes of PAC units embedded within large molecules. Intramolecular energy transfer within large molecules (red-shifting the fluorescence) and diminishing quantum yields of larger polynuclear aromatic units are thought to extinguish the intensity of fluorescent light eventually.117,118 This earlier attempt to use a UV-F spectrometer as an SEC detector had thus served as a method for estimating the upper mass-detection limit of UV-F for coal-derived samples. More recently, the sensitivity of UV-A and UV-F detectors in series were compared, using two heavier coal-derived fractions and three petroleum asphaltenes.99 Once again, the UV-F detector showed no sensitivity to material excluded from column porosity and some of the early eluting material in the resolved peak. In the case of the lightest fraction (the acetonesolubles of the coal tar pitch), there was agreement between the chromatograms from the two detectors. One of the petroleumderived samples showed trace intensity of fluorescence for material eluting at the exclusion limit of the column. Thus, the UV-F detector showed very little sensitivity to rather important changes in the sample properties. It was also noted that not all the undetected material had appeared under the excluded peak. It was concluded that UV-F spectroscopy is not capable of detecting the full breadth of the MM distribution in these complex samples. Several other laboratories have been working on MM distributions and problems around the lack of sensitivity of UV-F spectrometry for higher-mass materials. In one case, where UV-F depolarization experiments failed to detect material much larger than the 1000 u average mass in samples of petroleum asphaltenes, the conclusion was drawn that the samples did not contain larger-molecular-mass material.119,120 Strausz and co-workers have detailed reasons for the fall in sensitivity of UV-F based on measurements using samples with greater molecular masses.121 Many structures in petroleum asphaltenes do not absorb UV light and not all absorbing species fluoresce; this explanation goes a long way to explain the weakening signal with increasing molecular mass, particularly for petroleum-derived samples. In an independent study, Ascanius and co-workers111 have reported on the NMP solubility of a range of asphaltenes of various origins. They observed that substantial fractions (between 9% and 59%) of the set of petroleum asphaltenes examined did not dissolve in NMP. Significantly, in all cases, the NMP-insoluble fraction of these asphaltenes gave no UV-F signal in solution in toluene. 4.5. Aggregates in the Excluded Peak! Aggregates in the Resolved Peak! We have seen in Figures 7 and 8 that the latereluting, resolved (i.e., second) peaks can show a leading edge at MM values between ∼3000 u and 9000 u, depending on the nature of the sample. We have also described near-total agreement between MALDI-MS and SEC, at MM values up to 3000 u (see Figure 6). These findings have not gone unchallenged. The early 1990s saw a series of reports that estimated upper limits of ∼1000 u in coal-derived liquids, based on several (117) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York and London, 1983. (118) Li, C.-Z.; Wu, F.; Cai, H.-Y.; Kandiyoti, R. Energy Fuels 1994, 8, 1039. (119) Groenzin, H.; Mullins O. C. Energy Fuels 2000, 14, 677. (120) Badre, S.; Goncalves, C. C.; Norinaga, K.; Gustavson, G.; Mullins, O. C. Fuel 2006, 85, 1. (121) Strausz, O. P.; Peng, P.; Murgich, J. Energy Fuels 2002, 16, 809.


different mass spectrometric techniques.122-124 These workers, as well as researchers performing UV-F depolarization experiments (for an average mass of 1000 u) have suggested119,120,125 that material identified with molecular masses much above the 1000 u ceiling would be required to be composed of smaller, aggregated molecules. This point of view is similar to long and firmly held opinion in the petroleum industry. Heavy “cuts” have routinely been pyrolyzed for their meager hydrogen contents and for any “lights” that might be recovered. Depending on the crude that is being processed, anywhere from 5% to 25% of a barrel of oil may be discarded in this way or sold, or utilized in situ, as inexpensive fuel. Clearly, there is every reason for concentrated or “dried” heavy fractions to contain molecular aggregates. However, the challenge has always been to demonstrate that these heavier cuts were composed of aggregates of smaller (some would claim “more polar”) molecules, at solute concentrations encountered during determinations involving SEC. As an aside, by assuming the absence of larger molecules (for example, 3000-10000 u) in heavy petroleum fractions has manifestly not produced dividends in helping to make something more useful from these materials. Clearly, an improved understanding of MM distributions and chemical structures provides no a priori key to unlock the economic potential of heavy petroleum cuts. However, it seems reasonable to think that moredetailed knowledge is a necessary element that would contribute to whatever informed economic evaluation might follow, regarding these heavier materials in the present high-oil-price environment. 4.5.1. Adding Salts to the Eluent. One determined attempt to demonstrate the aggregation of the solute (i.e., sample) during SEC involved the dissolution of various ionic materials in the NMP to be used as an eluent in SEC. The experimental observation of wholesale shifts in SEC chromatograms toward longer elution times (smaller apparent molecular masses) was reported as evidence that the salt was disaggregating the aggregated polar sample molecule clusters. The first of these salts to be announced was LiBr.126-129 However, it seems that the shift to lower apparent masses caused by LiBr could also be observed, among others, with nonpolar samples such as a mixture of fullerenes and a naphthalene pitch. Furthermore, when eluted in LiBr-doped NMP, part of the naphthalene pitch sample was observed to elute after the permeation limit of the column. This provided evidence that the role of LiBr was to affect the solvent properties of NMP and to promote surface effects between the solute and the column packing.130 Slightly more investigation showed contradictory results. The addition of LiBr to the sample solution prior to injection into the SEC column (instead of doping the eluent) did not have the effect of shifting chromatograms to longer elution times. It (122) Winans, R. E.; McBeth, R. L.; Hunt, J. E.; Melnikov, P. E. In Proceedings of the International Conference on Coal Science; Newcastleupon Tyne, U.K., September 1991; Butterworths-Heinemann: Oxford, 1991; p 44. (123) Hunt, J. E.; Lykke, K. R.; Winans, R. E. Prepr. Pap.sAm. Chem. Soc., DiV. Fuel Chem. 1991, 36 (3), 1325. (124) Winans, R. E. J. Anal. Appl. Pyrolysis 1991, 20, 1. (125) Mullins, O. C. Fuel 2007, 86, 309. (126) Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1994, 8, 395. (127) Masuda, K.; Okuma, O.; Knaji, M.; Matsumara, T. Fuel 1996, 75, 1065. (128) Mori, S. Anal. Chem. 1983, 55, 2414. (129) Chen, C.; Iino, M. Fuel 2001, 80, 929. (130) Herod, A. A.; Shearman, J.; Lazaro, M.-J.; Johnson, B. R.; Bartle, K. D.; Kandiyoti, R. Energy Fuels 1998, 12, 174.

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seems difficult to envisage a scenario where the sample (in the bottle, prior to injection) could be considered to be dissociated by the salt but, upon injection into the NMP of the SEC column, would reaggregate to form exactly the same chromatographic peaks that could be obtained without LiBr addition.15 Following the withdrawal of LiBr as a valid disaggregating agent, many other salts have been proposed as performing a similar function.129 In subsequent work, these propositions were put to the test.102,103 The action of salt could be distinguished from the SEC mechanism by performing the fractionation of pitch by planar chromatography, using solvents with and without salt additions (LiBr, tetrabutylammonium acetate (TBAA), trichloroacetic acid (TCAA)). The collected bands of material, mobile in the acetonitrile and pyridine (with and without salt addition), were compared by SEC. Parallel fractionations were performed in experiments using column chromatography rather than planar chromatography. In all cases tested, the “salty” solvent was observed to mobilize a heavier sample, in addition to that mobilized by the pure solvent alone. In all cases, the extra sample mobilized by the “salty” solvent was of larger molecular size than that mobilized by the solvent alone. There was no evidence of disaggregation of sample molecules. In planar and column chromatography, the addition of salt was thus observed to displace more (and heavier) material from the polar sites of the silica than the pure solvent. In terms of action on silica surfaces, the addition of salt appears to enhance solvent polarity compared to the solvent alone. Once again, when samples in “salty” solution were analyzed by SEC in (pure) NMP as an eluent, the samples showed no evidence of the desired “disaggregation” but simply of carrying heavier components. The small amount of salt carried by the injected sample solution does not seem to have affected the SEC mechanism or to have promoted surface interactions between sample and packing. It seems important in such experiments to distinguish between desired effects and what other unexpected effects the newly introduced parameter might have caused. It seems clear that a few control experiments with salt plus known standards would have shown that what was observed was not disaggregation. 4.5.2. Effect of Concentration on Sample Aggregation. It is readilysand probably generallysaccepted that increasing the concentration of complex hydrocarbon samples would lead to aggregation. The opposite proposition, that dilution may disaggregate aggregated samples, unfortunately cannot be demonstrated directly. Sheu and co-workers have measured surface tension as a function of asphaltene concentrations in nitrobenzene and in pyridine.131 In both solutions at 25 °C, discontinuities were observed in the surface tension as solute concentrations increased above ∼0.05 wt %. By analogy with the behavior of surfactant solutions, these authors considered the observed discontinuity as the “critical concentration” above which micelles would aggregate. While analogy does not constitute proof, the argument seems reasonable. This critical micelle concentration can readily be compared with the estimated concentration of sample detected under the excluded peak during SEC in the stronger solvent, NMP. This (sample) concentration has been calculated to be at least 1 order of magnitude lower than the critical micelle concentrations18 of ∼0.05%. Using the much-stronger solvent, NMP (which is (131) Sheu, E. Y.; De Tar, M. M.; Storm, D. A. In Asphaltene Particles in Fossil Fuel Exploration, RecoVery, Refining and Production Processes; Sharma, M. K., Yen, T. F., Eds.; Plenum Press: New York and London, 1994; p 118.


capable of dissolving the last 15% of a coal tar pitch that could not be dissolved in pyridine), may be considered as an additional factor that impedes sample aggregation under these conditions. Moreover, when fractions eluting at the exclusion limit of the Mixed-D column were re-injected, the much-diluted sample eluted at the same elution time as before.16 If dilution is a relevant parameter, any aggregates would have been expected to show some disaggregation and shift to longer elution times. Parallel work in other laboratories has suggested that molecular aggregates do not form in NMP under the dilute conditions used in SEC, but that they can and do form in other less-powerful solvents.132,133 4.5.3. OVerloading the Excluded Signal in SEC. As already indicated, SEC using NMP as an eluent has required careful attention to the concentration of the injected sample. It was observed, furthermore, that, at higher sample concentrations, the signal from the excluded region had a tendency to spread into the valley between the “excluded” and “retained” regions. The effect was identified as overloading of the excluded region. The sample capacity of the excluded region seems to be lower than that of the retained region. After the sample loading was adjusted to obtain a clean signal for the excluded peak, further sample dilution produced no further shifts in the relative sizes, shapes, or intensities of the two peaks. The material showing up under the early eluting peak of a coal tar pitch was fractionated using analytical SEC.16 Three sequential fractions were collected corresponding to the excluded material, each over a 1-min interval. On re-injection, the first two fractions eluted over the same time periods (1 min) of collection from the original elution; however, the third fraction eluted earlier than its time of collection, corresponding to the collection time of the second fraction. This observation suggests that (far from dissociating on dilution), in the original elution, the material in fraction 3 had been slightly delayed by the presence of other samples, somewhat overloading the excluded region of the column. None of these fractions resulted in a lateeluting peak that might have been expected if the “aggregating” material had been diluted and disaggregated. One example of aggregate formation in SEC has been given132 where a polymer (poly(bisphenol A carbonate)) was analyzed by SEC, using THF as an eluent and monitored by MALDIMS. The MALDI spectra were expected, by these authors, to show narrow mass distributions. However, wider-than-expected distributions were found and interpreted by the authors as being caused by hydrogen bonding through terminal hydroxyl groups. The self-association of the polymer disappeared as the sample loading onto the column was reduced and as a more-polar solvent was added (ethanol). Such effects have not been observed in SEC work, using NMP as an eluent. On the other hand, an examination of fractions of Athabasca bitumen collected from preparative SEC in THF showed (upon analysis by SEC in NMP) that the material under the excluded peak in NMP had been spread throughout the elution range of the sample in THF.29 This failure of SEC in THF was considered to be the result of interactions between the column packing surface and the bitumen materials rather than self-association. Figure 10 shows the SEC chromatograms of the samples using THF and NMP as an eluent, respectively. To date, none of the polycyclic aromatic hydrocarbon standards tested, up to rubrene and rubicene (and other known (132) Montaudo, G.; Samperi, F.; Montaudo, M. S.; Carroccio, S.; Puglisi, C. Eur. J. Mass Spectrom. 2005, 11, 1. (133) Thiyagarajan, P.; Cody, G. D. Prepr. Pap.sAm. Chem. Soc., DiV. Fuel Chem. 1997, 42 (1) 253.


compounds, as well as distillate samples), used in the calibrations have shown a tendency to form an early eluting peak. π-π bonding has often been mentioned as a possible cause for such molecules to aggregate. Of the range of standard materials used to test the SEC calibrations, none resulted in an early eluting peak, excepting the (nonpolar) fullerene mixture, as discussed previously. Detailed structures of coal tar and petroleum asphaltene components, which lie outside the range of GCMS, remain unclear and discussion of the propensity for the π-π bonding of these materials during SEC does not seem to be based on detailed experimental evidence. 4.6. Characterization of Mixtures of Aliphatic Compounds by SEC. Because aliphatic compounds are relatively insoluble in NMP, they cannot be examined by SEC using NMP as an eluent. An alternative system has been developed using heptane as an eluent, with detection using an evaporative light-scattering detector.45 The method was able to examine alkanes up to C50 without needing to isolate the aliphatics from aromatics. N-heptane seems to be capable of eluting aliphatics by a sizeexclusion mechanism; however, aromatics are eluted in heptane with apparently much surface interaction, delaying their elution until well after the permeation limit. The method provides a mirror image to the elution of aromatics in NMP, where the solvent increases sample-solvent interactions at the expense of sample-column packing interactions and elutes by size exclusion but is unable to dissolve the aliphatics. The use of an eluent capable of dissolving both aromatics and aliphatics might be seen as desirable, but the problem then is to distinguish aliphatics coeluting with aromatics. THF is just such a solvent but has been observed not to avoid surface interactions between aromatic species and column packing. 5. Mass Spectrometry of Higher-Mass Materials The mass spectrometric methods described in Section 3 were all restricted to work with samples that could be volatilized prior to ionization. Among these techniques (GC-MS, probe MS, FI-MS, and ESI-MS), the largest molecular masses in coalderived materials have been observed by FI-MS3,88-91 at 12001500 u. To extend the range of detection beyond the restricted span typical of these techniques requires ionizing the sample in the condensed phase. We will show data from three methods that make this possible: fast atom bombardment (FAB-MS), laser desorption (LD-MS), and matrix-assisted laser desorption ionization (MALDI-MS). A comparison of results with plasma desorption (PD-MS) will also be discussed. 5.1. Fast Atom Bombardment-Mass Spectroscopy. The possibilities of FAB-MS for detecting high-mass material were investigated, using the pentane-insoluble (PI) fractions of several liquefaction extracts and fractions of a hydropyrolysis tar. The FAB-MS spectrum of a liptinite-concentrate liquefactionextract PI fraction mounted in a thiodiethanol matrix gave signal to above 2000 u, with trace signal up to MM values of ∼4000 u. These seem to be the highest reported MM values identified by FAB in a coal-derived material.5 In the same study, other liquefaction extract PI fractions showed narrower MM distributions. Fractions of a hydropyrolysis tar showed signal up to 1500-2000 u. Further work seems to be required before FAB-MS may be considered a full-fledged method for analyzing complex hydrocarbon liquids. Detailed mechanisms of sample vaporization, ionization, and detection must be clarified, particularly for the case of high-molecular-mass materials. Furthermore, the selection of experimental parameters useful for extending the range of MM values amenable to detection remains a largely empirical


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Figure 10. Preparative, repeated SEC chromatograms of Athabasca bitumen (A) showing five approximately equal time-zone fractions (THF eluent) and (B) their chromatograms in NMP on an analytical Mixed-D column; “0” denotes the whole sample. (Reproduced from ref 29 with permission; copyright American Chemical Society, 1999.)

procedure.5 The latter include, crucially, the choice of composition (and polarity) of matrices to be used and methods for their preparation for sample mounting. 5.2. Laser Ionization Mass Analysis. The range of MM values identified in coal-derived liquids was considerably extended through the use of LD-MS. The work was initially undertaken in an attempt to provide independent confirmation for the identification of molecular masses of up to 4000-6000 u found by SEC.134,135 During this preliminary stage of the study, signal up to the limit of a laser ionization mass analysis (LIMA) instrument of m/z 12 000 were detected in liquefaction extracts and a coal tar pitch.4,25,136,137 Experiments were conducted at an ion extraction voltage of 20 kV, with a defocused beam to prevent breaking up sample molecules under the power of the laser. However, lacking a spectral addition facility, the LIMA instrument produced single-laser-shot spectra, which looked rather jagged. The LIMA instrument was equipped with a Nd: YAG laser. When the laser was defocused, the spectrometer was useful in showing ions up to its upper mass limit4,25,136 of m/z 12 000. However, if used in full focus, this laser was (134) Li, C.-Z.; Bartle, K. D.; Kandiyoti, R. Fuel 1993, 72, 3. (135) Li, C.-Z.; Bartle, K. D.; Kandiyoti, R. Fuel 1993, 72, 1459. (136) Herod, A. A.; Kandiyoti, R.; Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; Li, C.-Z. J. Chem. Soc., Chem. Commun. 1993, (9), 767. (137) Herod, A. A.; Kandiyoti, R.; Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; Li, C.-Z. Rapid Commun. Mass Spectrom. 1993, 7, 360.

sufficiently energetic to destroy molecular ions and produce sets of carbon cluster ions.137 5.3. Matrix-Assisted Laser Desorption Ionization-Timeof-Flight Mass Spectroscopy (MALDI-TOF MS). Extension of the work using MALDI-TOF instruments enabled the range of MM values detected to be extended, with identification of masses between m/z 20 000 and 30 000 in pyrolysis tars and liquefaction extracts138 and up to m/z 270 000, when using solid coal particles as the target.139,140 This particular instrument (Kratos Kompact MALDI III) allowed the co-addition of spectra and could be used in linear mode. These findings, once again, probably reflected the detection limit of the particular instrument, but clearly went far beyond the MM range previously identified by LIMA. The results also far surpassed the range of masses identified by SEC, where THF had been used as an eluent. The next stage of the work required the choice of an eluent with greater solvent power than THF and led to the use of NMP as an eluent in SEC. (138) Herod, A. A.; Li, C.-Z.; Xu, B.; Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; Humphrey, P.; Chapman, J. R.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1994, 8, 815. (139) Herod, A. A.; Li, C.-Z.; Parker, J. E.; John, P.; Johnson, C. A. F.; Smith, G. P.; Humphrey, P.; Chapman, J. R.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1994, 8, 808. (140) John, P.; Johnson, C. A. F.; Parker, J. E.; Smith, G. P.; Herod, A. A.; Li, C.-Z.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1993, 7, 795.


Variations in MM distributions as a function of changes in the ion accelerating voltage in MALDI-TOF MS were investigated using the pyridine insoluble fraction of a coal tar pitch.141 With increasing ion extraction voltage, the spectra showed increased intensities, particularly at the higher masses. These observations were important in verifying that the recorded signal actually corresponded to the presence of material with the indicated MM values. This is because increasing the ion extraction voltage simply serves to accelerate species already ionized, without otherwise disturbing the sample. Therefore, the spectra obtained at high extraction voltages may be viewed as providing a more complete inventory of already ionized species, by significantly enhancing the kinetic energy imparted to largerMM materials. 5.4. Nature of the Matrix. The possible effect of sample and matrix composition on the quality of MALDI spectra was investigated.28 The samples used in the study were three geologically “younger” and three geologically “older” kerogens of similar type. Experiments were conducted using three distinct matrix formulations: sinapinic acid, 2,5-dihydroxybenzoic acid, and R-cyano-3-hydroxy cinnamic acid. Wider MM distributions were observed in the presence of all three matrices, from the three geologically “younger” kerogens, compared to spectra of extracts from the corresponding “older” kerogens. The result was contrary to trends observed in the absence of a matrix, where extracts from older kerogens gave broader MM distributions. Results from SEC and UV-F agreed with trends observed in MALDI spectra acquired in the presence of a matrix; the differences were clearer in the case of sinapinic acid. By contrast, it was observed that high-mass signal in the MALDI spectra of the pyridine-insoluble fraction of a coal tar pitch, obtained in the absence of a matrix, closely matched signal obtained in the presence of a sinapinic acid matrix. Compared to the highly condensed aromatic structure of the coal tar pitch, the kerogen extracts (of considerably higher oxygen content) were thus less able to readily absorb incident laser radiation (337 nm), in the absence of an externally added matrix. The argument was consistent with apparently more-efficient absorption of incident laser radiation by the geologically older samples, when no matrix was used. Structural characteristics suggest that the geologically older samples would be better able to act as a “self-matrix”. Thus, differences in structure and composition of the samples seem to go some way toward explaining differences between trends observed in the presence/absence of a matrix. Dependence of the quality of MALDI spectra on molecular structure has also been found for two sets of biomassderived samples. 5.5. Effect of Sample Polydispersity. The effect of sample polydispersity on MALDI-TOF spectra of mixtures of polystyrene MM standards was investigated. The polystyrene MM standards, with a peak mass of 100022000, were run singly, in pairs, in groups of three, and as a mixture of the seven distinct samples.142 The individual standard samples all had polydispersities of almost unity. When run singly or in pairs, the spectra of all the standards could be observed clearly and showed approximately equal areas. Mixtures of multiple standards have been run at variable ion extraction voltages. When the extraction voltage was reduced to 1.2. With increasing ion extraction voltage, the MALDI spectra of a coal tar pitch and its THF-insoluble fraction showed increasing intensities of higher-mass ranges. It was concluded that, because of the wide polydispersity of coal-derived samples, MALDI spectra would be expected to severely underestimate the proportion of high-mass materials present. For complex mixtures, extensive fractionation seems to be desirable to reduce sample polydispersity. 5.6. Comparison with PD-MS. In an allied study, the responses of 252Cf PD-MS and MALDI-MS to identical samples29,144,145 were compared. The two pairs of samples selected for the comparison were known from previous work to differ significantly in their high-mass contents. MALDIMS showed large differences in the MM distributions within both pairs of samples. The PD-MS data showed a degree of similarity between one pair of samples (pyridine-soluble/ insoluble fractions of a coal tar pitch), whereas, for the second pair (a coal extract and its hydrocracked product), trends from the two MS techniques agreed closely. The MM range observed by PD-MS was narrower, extending to 3000-5000 u. Significant differences within pairs of samples were observed by SEC and by UV-F spectroscopy, providing somewhat closer agreement with the MALDI spectra. The two MS instruments differed in two important respects: the ionization system (i.e., plasma desorption versus laser desorption) and the maximum available ion extraction voltage (30 kV for the MALDI-MS instrument and 15 kV for the PDMS instrument). Therefore, the comparison of PD-MS versus LD-MS could not happen at high ion extraction voltages. Work at up to 30 kV in the MALDI instrument indicated better sensitivity to high-mass materials at higher ion extraction voltages. Nevertheless, the qualitative similarity of results from the two MS-techniques was apparent. Furthermore, the range of MM values observed in PD-MS as well as in MALDI-MS were much larger than those reported elsewhere.122-124 5.6.1. OVerView of Results by MALDI-MS. Thus, the effect of changes in laser power, ion-extraction voltage, method of sample loading, and composition of matrix used to enhance the ionization process on MALDI-MS spectra have all been examined.12,14,28,138,139,141,142,146,147 (143) Klee, J. E. Eur. J. Mass Spectrom. 2005, 11, 591. (144) Domin, M.; Li, S.; Lazaro, M.-J.; Herod, A. A.; Larsen, J. W.; Kandiyoti, R. Energy Fuels 1998, 12, 485. (145) Johnson, B. R.; Bartle, K. D.; Ross, A. B.; Herod, A. A.; Kandiyoti, R.; Larsen, J. W. Fuel 1999, 78, 1659. (146) John, P.; Johnson, C. A. F.; Parker, J. E.; Smith, G. P.; Herod, A. A.; Li, C.-Z.; Kandiyoti, R.; Humphrey, P.; Chapman, J. R. Fuel 1994, 73, 1606.


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Figure 11. MALDI-MS spectrum of polystyrene of Mp ) 126 500 u. Bruker Daltonics Reflex IV MALDI-TOF MS in linear mode, with Ag and dithranol matrix (previously not shown).

Problems associated with the detection of large-MM ions in MALDI-MS have been discussed by several other research groups, in terms that are broadly consistent with findings previously outlined. The observation of higher-mass ions has been reported to require higher laser power levels,148 whereas several instrumental variables have been identified as discriminating against observing high-mass components present in complex mixtures.149,150 It was deemed necessary to use different sample-to-matrix ratios, to generate molecular ions from highmass polystyrenes compared with low-mass polystyrenes; a greater excess of matrix was required for high-mass samples.151 Higher-mass ions were also thought to produce relatively less signal at the detector, compared to smaller ions, because the impact velocities of larger ions are usually less than those of smaller ions.152 In other work, the proportion of the kinetic energy of large ions (m/z > 20 000) impacting on the ion collector by collision has been observed to diminish quite significantly as the mass of sample ions increases. As little as one-half of the kinetic (147) Lazaro, M-J.; Herod, A. A.; Cocksedge, M. J.; Domin, M.; Kandiyoti, R. Fuel 1997, 76, 1225. (148) Martin, K.; Spickermann, J.; Ra¨der, H. J.; Mu¨llen, K. Rapid Commun. Mass Spectrom. 1996, 10, 1471. (149) Schriemer, D. C.; Li, L. Anal. Chem. 1997, 69, 4169. (150) Schriemer, D. C.; Li, L. Anal. Chem. 1997, 69, 4176. (151) Schriemer, D. C.; Li, L. Anal. Chem. 1996, 68, 2721. (152) Lloyd, P. M.; Suddaby, K. G.; Varney, J. E.; Scrivener, E.; Derrick, P. J.; Haddleton, D. M. Eur. Mass Spectrom. 1995, 1, 293.

energy of a large protein was transferred to the ion collector, compared to approximately three-quarters of the kinetic energy of a (small-MM) matrix molecular ion (sinapinic acid).153 Processes that can explain such losses of energy include fragmentation on impact and ejection of molecular fragments. A detector based on cryogenic microcalorimetry was proposed, to avoid the mass discrimination associated with microchannel plate detectors.153 5.7. Safe Upper Limit of the Molecular Mass Range Detected in MALDI-TOF MS. As a first test of data quality, the effect of the co-addition of increasing numbers of singleshot spectra was examined.154 Spectra composed of increasingly larger numbers of scans (10, 30, 50, and 100 scans) were obtained and compared. A decrease in relative noise levels with increasing numbers of scans would imply that the spectra could be considered as stable. The data showed visually observable reductions in noise levels, which was consistent with robust and statistically meaningful signal. Standard deviations at the detection limit of the instrument were calculated. Assuming all recorded signal at the detection limit of the instrument to be random noise, an average signal intensity at the detection limit was calculated and considered as the zero-offset of the spectrum. Three separate types of post(153) Hilton, G. C.; Martinis, J. M.; Wollman, D. A.; Irwin, K. D.; Dulcie, L. L.; Gerber, D.; Gillevet, P. M.; Twerenbold, D. Nature 1998, 391, 672. (154) Lazaro, M. J.; Herod, A. A.; Domin, M.; Zhuo, Y.; Islas, C. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1999, 13, 1401.


acquisition calculation were used to identify safe high-mass limits of the spectra: (i) The method of Montaudo et al.155 This is based on the computation of (number and weight) average molecular masses over MM intervals with progressively increasing upper limits. The high-mass limit of the spectrum is considered to have been reached when the calculated (number and weight) average MM values no longer increase with the increasing range (upper limit) of MM values included in the calculation. Using polymer samples of known mass, these authors have previously shown good agreement between MALDI-TOF mass spectra and determinations by SEC. (ii) Comparison of instrument signal with multiples of the standard deViation. The safe high mass limit of the spectrum was taken to be the largest MM values for which recorded signal was greater than multiples (1, 2, 3, and 5) of the standard deviation (σ). For this purpose, the standard deviation σ was calculated from data at the detection limit of the instrument. Using greater multiples of σ gave increasingly conservative estimates of the safe upper-mass limit of the spectra. (iii) Calculation of the slope of the spectrum. The method relied on identifying “lift-off” of the smoothed spectrum from baseline at the high mass limit, by matching the angle between the smoothed spectrum and the baseline to a predetermined value (e.g., 0.5° and 1°). A greater number of data pointssgreater than that observed in the aforementioned two methodsswere required in the smoothed file to define the slope over a relatively small mass range. When a high-mass limit could be established for a spectrum (by any of the three methods), the number- and weight-average molecular masses (Mn and Mw, respectively) could be calculated. However, the arbitrary specification of the key parameter (in each case) did not allow the establishment of any of the methods on a firm basis. The choice of a criterion for estimating safe high-mass limits of MALDI spectra thus remains a semiquantitative procedure. On balance, the comparison of signal with five times the standard deviation (at the high-mass limit) may be considered as a reasonably conservative method for estimating the highmass limit of a spectrum. However, evaluation of the sizeexclusion chromatograms of the present samples by polystyrene standards suggests that MM distributions of pitch samples determined by MALDI-MS might be, at least in part, determined by the limitations of the available instruments. 5.8. High-Mass Peak in MALDI-TOF MS Spectra. The work previously outlined has a tendency to suggest that high extraction voltages, high detector sensitivities, and high laser fluences all contribute to improving signal quality and intensity at high mass. However, the three-way combination of these parameters has a tendency to saturate the spectrometer detector. The detection of high-mass material in coal tar pitch and petroleum asphaltenes by MALDI-TOF MS was always a matter of balancing the effects of several parameters to get the clearest possible result. A combination of parameters that was developed relatively recently has achieved results that seem to reinforce previous observations of material well beyond the 10 000 u range in the heavier coal- and petroleum-derived fractions.156 The combination of parameters was selected to reduce the ion intensities of the low-mass range of molecular ions (3000 u do not fluoresce significantly. 6.2. Infrared Spectroscopy. Infrared spectra have been obtained for series of fractions from column chromatography (pitch, a coal digest, a low-temperature tar) and fractions from solvent solubility (pitch) using both the KBr disc method and attenuated total reflection (ATR) using a diamond cell. The results from both methods agree in showing similarities between spectra of the “whole” samples and the lighter solubility fractions (acetonitrile- or acetone-solubles). The main change in the spectra was evident in the NMP-solubles (from column chromatography) and the pyridine-insolubles, where the absence of out-of-plane bending vibrations of substituted benzenoids at frequencies from 900 cm-1 to 700 cm-1 was obvious. The loss of these frequencies points to the complexity of the structures of the largest molecules: presumably, there are very few such sites left in the molecules. Taken together with the presence of the “excluded” peaks in the SEC chromatograms, the data suggest the presence of large aromatic structures embedded in molecules with three-dimensional conformations. Figure 16 shows the ATR spectra of a coal extract and its fractions separated by column chromatography.105 6.3. X-ray Diffraction Studies. XRD spectra have been obtained161 for dried samples of pyridine-soluble and pyridineinsoluble fractions of a coal tar pitch, after the acetone-soluble material had been separated. The spectra were compared with equivalent spectra for polystyrene and PMMA standards with a molecular mass of MM ≈ 1 000 000. The dried pyridineinsoluble fraction showed some order at low-angle diffraction. Evidence of ordered structures was less pronounced in the spectrum of the pyridine-solubles fraction and was entirely absent in the spectra of the polymer samples (Figure 17).


Herod et al.

Figure 15. SECs with detection by UV absorbance (UV-A, curve 1) and UV fluorescence (UV-F, curve 2) (in series) of the pyridine-insoluble fraction of a coal tar pitch. (Reproduced with permission from ref 99; copyright American Chemical Society, 2005.)

Figure 16. Fourier transform infrared (FT-ir) spectra by attenuated total reflectance (ATR) of a coal tar pitch and fractions from column chromatography soluble in acetonitrile, pyridine, and NMP. (Reproduced from ref 105.)

Because of the fact that these spectra were obtained from the dried (solid) samples, aggregation of sample molecules would be expected to precede the ordering of these structures. 6.4. Pyrolysis and Hydropyrolysis, Hydrocracking Reactions of Large Molecules. 6.4.1. Pyrolysis in a Wire-Mesh Pyrolysis Reactor. The wire-mesh pyrolysis reactor is a variable heating rate device that is designed to rapidly remove volatiles from the heated zone by a stream of sweep gas. It is optimized to suppress the secondary reactions of evolved volatiles. Tar formed during the pyrolysis process is collected in a cold trap, while gases are allowed to escape and the char residue remains on the wire mesh. A fuller description of the instrument may be found in Chapter 3 of ref 6. When the pyridine-insoluble fraction of the coal tar pitch was pyrolyzed161 in the atmosphericpressure wire-mesh reactor, ∼70% of the sample reverted to

char, whereas 15% was collected as tar and ∼15% of the sample formed light gases. As evidence that these are molecular masses that are too large to pass through a GC column, none of the tar components could be detected by GC analysis. Figure 18 shows SEC chromatograms of the pyridine-insoluble fraction of the tar from the wire-mesh pyrolysis experiments and the acetonesoluble fraction of the coal tar pitch. The excluded peak of the pyridine-insolubles does not appear in the chromatogram of the pyrolysis tar. However, the sizes of tar molecules were not reduced to the level of the acetone-soluble fraction and showed no signal in probe MS (i.e., below ∼500-600 u). 6.4.2. Pyrolysis-GC-MS of the NMP-Solubles. Similarly, the pyrolysis-GC-MS of the NMP-solubles (heaviest ∼15% fraction) of the pitch failed to produce molecular components that were sufficiently volatile to pass through the GC column


Figure 17. X-ray diffraction (XRD) spectra of pitch pyridine-solubles (curve 3) and pyridine-insolubles (curve 4), polystyrene with a mass of 1 950 000 u (curve 1) and poly(methylmethacrylate) (PMMA) with a mass of 1 500 000 u (curve 2). (Constructed from the data of ref 161.)

Figure 18. SEC chromatograms of the pitch pyridine-insoluble fraction (curve 1), the wire-mesh pyrolysis tar from the pyridine-insolubles (curve 2) and the acetone-soluble fraction of pitch (curve 3). (Constructed from the data of ref 161.)

to the mass spectrometer in detectable amounts.104,107,163 Thus, the attempted breakdown of these heavy pitch fractions failed to produce molecules small enough (or sufficiently volatile) to pass through the GC column, despite the relatively severe pyrolysis temperature of 700 °C in both the wire-mesh and the pyrolysis-GC-MS instruments. Similarly, the pyrolysis-GCMS of the pyridine-immobile fraction from planar chromatography produced no fragments of significance, showing residual solvent and some alkyl products.163 It may be expected that the breaking of single bonds would lead to smaller molecular fragments relatively easily. Evidence from coal pyrolysis and liquefaction experiments (cf. Chapter 6 in ref 6) suggests that primary extracts and tars are released only after the rupture of a multiplicity of covalent bonds. Taken together with data from infrared spectroscopy (previously cited, showing few aromatic substituents), all evidence from heavy pitch fractions point to largely graphitized and cross-linked structures, where temperatures sufficient for pyrolyzing solid coals are not able to produce tractably small molecular fragments. Examination of fractions of pitch collected from preparative SEC104 also showed that the fractions containing the largest molecules produced few small fragments on pyrolysis. The two heaviest (pyridine-soluble and NMP-soluble) fractions of a low(163) Herod, A. A.; Islas, C.; Lazaro, M.-J.; Dubau, C.; Carter, J. F.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1999, 13, 201.


temperature tar164 and a coal liquefaction extract,165,166 fractionated by column chromatography, also gave few pyrolysis fragments. These studies indicated that the materials detected by pyrolysis-GC-MS of the corresponding “whole” samples were essentially those from the lighter (acetonitrile-soluble or acetone-soluble) fractions and that larger molecules did not break down into fragments small enough to pass through the GC column. Figure 19 compares pyrolysis chromatograms of pitch fractions. 6.4.3. Catalytic Hydrocracking of a Pitch Pyridine-Insolubles Fraction. Catalytic hydrocracking of a pitch pyridine-insolubles fraction167 produced rapid destruction of the material shown under the “excluded” peak in SEC. However, the process did not produce material of masses small enough to pass through a GC column. Catalytic hydrocracking of extracts of Point of Ayr coal in tetralin168-175 showed that the excluded material of these coal-derived liquids broke down to smaller molecules quite rapidly. Some of the larger-MM material collected on catalyst particles and was lost from solution. Nevertheless, the overall shift of molecular size was toward smaller molecules. Figure 20 shows SEC chromatograms of pitch pyridine-insolubles before and after catalytic hydrocracking in tetralin solution. In tetralin solution, any free radicals produced by thermal pyrolysis of the pitch molecules will be stabilized by hydrogen donation from tetralin to produce smaller molecules, rather than undergoing aromatization to larger molecules. The process has been described in Chapter 5 of ref 6. 6.4.4. Petroleum Asphaltenes. Petroleum asphaltenes (heptane-insoluble fractions) contained large aromatic molecules and material excluded from SEC; however, in pyrolysis-GC-MS, these compounds gave mainly alkanes and alkenes with aromatic fragments no larger than naphthalene. The fragments were thought to be from side-chains that could be removed by singlebond scission, while the main aromatic systems in molecules up to m/z 15 000 formed char during the pyrolysis step of pyrolysis-GC-MS.31 6.4.5. Pyrolyzed Baltic Amber. Baltic amber pyrolyzed in the same wire-mesh reactor did not leave a significant char residue ( coal digest > pitch It might be expected that the concentration of carboxyl groups in the samples would decrease with increasing thermal treatment. Thus, if the oxygen contents were correlated with the traceelement content, their concentrations would be expected to be greatest in the low-temperature tar. The data101,105 showed that this was not the case. Instead, it is likely that the trace elements were associated with the larger aromatic-ring systems. The data of Table 2, where an approximate mass balance was achieved in fractions from the solvent separation of coal tar pitch101 (acetone-solubles and acetone-insolubles but also pyridinesolubles and pyridine-insolubles) indicate quite definitely that the molecules that contain the largest aromatic systems (pyridine-insolubles) are associated with the highest concentration of trace elements. (182) Herod, A. J.; Gibb, T. C.; Herod, A. A.; Zhang, S. F.; Xu, B.; Kandiyoti, R. Fuel 1995, 75, 437. (183) Herod, A. J.; Gibb, T. C.; Herod, A. A.; Shearman, J.; Dubau, C.; Zhang, S.-F.; Kandiyoti, R. J. Planar Chromatogr. 1996, 9, 361. (184) Richaud, R.; Lachas, H.; Lazaro, M.-J.; Clarke, L. J.; Jarvis, K. E.; Herod, A. A.; Gibb, T. C.; Kandiyoti, R. Fuel 2000, 79, 57. (185) Lachas, H.; Richaud, R.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2000, 14, 335.


Herod et al.

Table 2. Trace-Element Concentrations in the Pitch and the Solvent Solubility Fractions Concentrationa LODb

LOQc

element/isotope

(ppm)

(ppm)

pitch (ppm)

acetone- soluble (mg/kg)

pyridine-soluble (mg/kg)

pyridine-insoluble (mg/kg)

normalized sum of fractions (ppm)

24Mg

3.9 9.1 39 0.76 1.1 0.02 0.47 0.12 14.7 0.017 0.61 1.45 0.18 0.03 0.12 0.04 0.12 0.006 0.019 0.058 0.20 0.013 0.002 0.27 0.024 0.019 0.0025 0.68 0.003 0.002 0.0008

13.0 30.3 132 2.5 3.6 0.06 1.57 0.42 49 0.058 2.03 4.85 0.59 0.09 0.41 0.14 0.39 0.019 0.063 0.19 0.65 0.044 0.006 0.91 0.081 0.064 0.0085 2.27 0.01 0.006 0.0025

7.4 18.0 59 0.4 0 0.2 1.0 7.37 254 0.24 1.26 1.2 8.64 16.6 2.57 0.31 0.69 0.03 0.15 2.23 1.90 0.30 0.039 0.85 0.45 0.05 0.033 197 8.69 0.004 0.010

0 0 78 1.1 0 0.05 3.6 0.35 50 0.06 0 0 1.83 4.0 0.97 0.24 0.25 0 0.62 0.34 0 0.08 0.04 0.51 0.06 0 0.12 5.4 0.16 0 0.004

11.4 12.4 209 1.1 0 0.43 1.6 2.72 181 0.19 0.82 0 3.49 10.0 1.31 0.21 0.98 0.004 1.82 1.72 0 0.09 0.04 1.68 0.09 0.01 0.73 2.8 0.05 0.002 0.003

37.3 198 133 0.8 7.8 0.63 1.5 19.7 633 0.75 3.18 0.97 20.9 40.8 4.70 0.51 2.03 0.13 0.69 7.23 6.95 1.23 0.15 5.52 0.29 0.25 0.13 547 20 0.025 0.018

16.1 69 143 1.1 2.5 0.37 2.3 7.5 286 0.33 1.32 0.32 8.68 18.2 2.34 0.32 1.08 0.04 1.07 3.07 2.27 0.46 0.078 2.56 0.15 0.085 0.33 182 6.63 0.009 0.008

27Al 44Ca 45Sc 47Ti 51V 52Cr 55Mn 56Fe 59Co 60Ni 63Cu 70Zn 75As 77Se 85Rb 88Sr 89Y 95Mo 111Cd 118Sn 121Sb 133Cs 137Ba 139La 140Ce 182W 204Pb 205Tl 232Th 238U

sum a

596

148

446

1695

758

An entry of “0” indicates a value less than the limit of detection. b Limit of detection. c Limit of quantitation.

7. What Do We Really Know about Pitches and Asphaltenes? Where Do We Go from Here? Analytical methods developed for characterizing complex liquid mixtures derived from fossil fuels have been reviewed. The analysis of fractions with masses up to ∼400-450 u has been described in terms of gas and liquid chromatography coupled to mass spectrometry (GC-MS and LC-MS, respectively). Molecules within this low-mass region are well-known. Apart from their intrinsic value as powerful analytical tools, an understanding of methods for analyzing samples with masses up to ∼400-450 u (and their limitations) is necessary to evaluate the methods involved to characterize higher-molecularmass (higher-MM) materials. Many of the techniques optimized for the analysis of material up to ∼450 u cannot readily be adapted for examining samples that contain higher-MM materials. It has been necessary to develop methods to overcome the limitations imposed by lack of volatility or solubility in common solvents. Upper mass limits indicated by these methods in coal liquids and asphaltenes have been the subject of some debate. It remains difficult to define upper limits for many common samples, such as petroleum asphaltenes. Much of the work indicating upper mass limits of ∼1000-1500 u for coal tars, pitches, and petroleum asphaltenes can be explained in terms of the 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. In times of plenty, the heavier fractions of fossil fuels have been dismissed as “aggregates” and largely wasted. With sharply increased fossil fuel prices, there is now pressure to get more out of a barrel of crude oil into the marketplace as refined fuel.

Attaining these goals requires new, broader perspectives in determining MM distributions and structural features of these heavier materials. Above the ∼450-500 u range, no single method is unambiguously capable of indicating MM distributions or chemical structural features in complex fuel-derived mixtures. Advances in this field require assembling and comparing relevant evidence from several independent analytical methods. This review has primarily focused on results from size exclusion chromatography (SEC), laser desorption-mass spectroscopy (LD-MS), and matrix-assisted laser desorption/ionization-mass spectroscopy (MALDI-MS). The particular combination of techniques has grown out of experience in examining coal, kerogen, and petroleum-derived samples and several types of biomass and amber and their extracts. Probably the most important finding is that MM values estimated by SEC for material under the “retained” peak (and not amenable to gas chromatography-mass spectroscopy (GCMS) and probe MS) are in broad agreement with direct measurements using MALDI-MS. SEC using 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 sample molecules 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 1-methyl-2-pyrrolidinone (NMP) as an eluent. It remains necessary to prove that the material excluded in SEC is composed of large, three-dimensional molecules. The upper mass limits of coal liquids have not been defined by mass spectrometry, despite extending the measured upper


mass to beyond m/z 100 000 by MALDI-MS. Mass spectrometric methods cannot easily define the upper mass limits of complex fractions, because an absence of signal does not necessarily mean an absence of material. Instead, it may simply indicate that the limit of ionization with increasing MM under the particular conditions may have been reached. Further investigation of the high-mass range of the MALDI mass spectra is required to establish the method. The evidence presented indicates that the MM values and structures of small, medium, and large molecules of coal liquids, petroleum asphaltenes, amber, and biomass tars change in ways that have not been hitherto generally discussed. Taken together, the evidence suggests that existing models of coal and of petroleum asphaltene structures may need to be revised, to accommodate the existence of large polynuclear aromatic groups embedded in larger molecules, possibly displaying threedimensional conformations. These largest-mass materials have been observed to carry the bulk of the trace metals associated with coal liquids. There is evidence suggesting that the adherence of trace metals to these larger-MM fractions occurs via some mechanism that is not associated with polar oxygen groups such as carboxylic acids. Determinations of MM distributions are often affected by sample polydispersity. Most available characterization tech-


niques have a tendency to provide information on the properties of the more-abundant materials/fractions of complex mixtures. Often, the properties of less-abundant fractions are masked. Therefore, fractionation of such mixtures is a necessary first step in attempting to obtain a more-complete picture, covering all “fractions” of the sample. Several powerful analytical techniques (including infrared, nuclear magnetic resonance (NMR), and ultraviolet (UV)fluorescence spectroscopies) may be used in modified form with the intractable fractions, to provide structural information to supplement this emerging picture. All the techniques used to date clearly indicate that materials in the heavier fractions are structurally different from material identified in the lighter fractions. Acknowledgment. Much of the work described has been supported by successive BCURA/DTI, EPSRC and European Union research grants and contracts. We are grateful to Professor J. Harrison, Professor J. S. Higgins, Dr. D. J. A. McCaffrey, and Mr. Trevor Roberts for their support and encouragement. Our thanks also go to the post-graduate students and post-doctoral researchers, technical staff, and collaborating academics associated with the work, cited as coauthors in the various papers referenced below. EF060642T

Characterization of Heavy Hydrocarbons by Chromatographic and

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