Characterization of Molecular Mass Ranges of Two Coal Tar Distillate

Energy & Fuels 2008, 22, 3275–3292

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Characterization of Molecular Mass Ranges of Two Coal Tar Distillate Fractions (Creosote and Anthracene Oils) and Aromatic Standards by LD-MS, GC-MS, Probe-MS and Size-exclusion Chromatography ´ lvarez, M. Millan, A. A. Herod,* and R. Kandiyoti T. J. Morgan, A. George, P. A Department of Chemical Engineering, Imperial College London, London SW7 2AZ, U.K. ReceiVed May 11, 2008. ReVised Manuscript ReceiVed July 14, 2008

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

There will be increased need to process heavy crudes as the availability of light crude oils diminishes.1 Similarly, there is need to process coal-derived byproduct tars and pitches. Looking beyond delayed coking and similar processes toward the development of more efficient routes for utilizing relatively heavy hydrocarbon liquids, it is necessary to first arrive at accurate molecular mass estimates and evaluations of structural features of these materials. Previous work in this laboratory has aimed to improve definitions of molecular mass ranges and structural characterizations of complex materials such as coal tars, pitches, and petroleum asphaltenes.2,3 Much of this work has focused on the heavier fractions of these materials, which mostly contain compounds above the mass range amenable to analysis by gas chromatography-mass spectrometry (GC-MS). No single tech-

nique has, to date, been able to provide complete answers to questions regarding the molecular mass distributions or, indeed, the structural features of such samples. Solution state nuclear magnetic resonance (NMR) methods4 have shown that the heavier fractions of pitch are unlike the well-known low-mass components detected by GC-MS. Confirmation of findings is usually sought by deploying combinations of analytical methods. Clearly, much work remains to be done. Size exclusion chromatography (SEC) has been extensively used in efforts to characterize molecular mass ranges of heavy hydrocarbon liquids.2,3 Several mass spectrometric techniques have been used by us to supplement information gained by SEC. These include GC-MS, probe-MS, laser-desorption mass spectrometry (LD-MS), matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS), and, less frequently, pyrolysis-GC-MS.5-23 Each of these techniques presents particular

* To whom correspondence should be addressed. E-mail: a.herod@ imperial.ac.uk. (1) Wiehe, I. A. Process chemistry of petroleum macromolecules. Chemical Industries, Series 121. CRC Press, Boca Raton, FL, USA, 2008. (2) Kandiyoti, R.; Herod, A. A.; Bartle, K. D. Solid Fuels and HeaVy Hydrocarbon Liquids, Thermal Characterization and Analysis; Elsevier: Amsterdam, The Netherlands, 2006. (3) Herod, A. A.; Bartle, K. D.; Kandiyoti, R. Energy Fuels 2007, 21, 2176.

(4) Morgan, T J.; George, A.; Davis, D. B.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2008, DOI: 10.1021/ef00715w. (5) Karaca, F.; Islas, C. A.; Millan, M.; Behrouzi, M.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2004, 18, 778. (6) Lazaro, M. J.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Energy Fuels 1999, 13, 1212. (7) Millan, M.; Morgan, T. J.; Behrouzi, M.; Karaca, F.; Galmes, C.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2005, 19, 1867.

Introduction

10.1021/ef800333v CCC: $40.75  2008 American Chemical Society Published on Web 08/20/2008

3276 Energy & Fuels, Vol. 22, No. 5, 2008

limitations that must be evaluated within the context of individual sample properties and the type of data that needs to be acquired. With 1-methyl-2-pyrrolidinone (NMP) used as eluent, size exclusion chromatograms of the heavier fractions of coal liquids and petroleum residues consistently show bimodal distributions.2,5-7,23 The earlier eluting peak shows signal at elution times corresponding to material excluded from column porosity. The later eluting peak shows a signal apparently resolved by column porosity and corresponding to lower-mass components of the sample compared to material eluting as the excluded peak. The valley between the two peaks nearly always approaches zero intensity, provided the excluded region of the column is not overloaded with sample. The molecular mass ranges represented by the excluded peak cannot be defined with any degree of certainty, because the peak lies at and above the upper limit of the column calibration. These calibrations are obtained using a range of polymeric molecular mass standards as well as available standard (i.e., known) compounds. In our recent work, the solvents used as eluent in SEC have included NMP 2,3,5,14-22 and, for heavy petroleum-derived samples, mixtures of NMP and chloroform.24,25 It is likely that the apparently very large molecular masses corresponding to the excluded peak in SEC chromatograms relate to material eluting according to solution volume or molecular size but not directly related to molecular mass. One such example is provided by fullerenes. The elution times of these species are far shorter (and apparent molecular masses are far larger) than would be expected from fullerene molecular masses on the basis of the polystyrene-based calibration. Further experiments with three-dimensional standards such as soot and colloidal silicas 2,3,5,25 have tended to support this suggestion. It is thus possible that the early elution (in the excluded zone) of some heavy hydrocarbon material may be related to three(8) Karaca, F.; Millan-Agorio, M.; Morgan, T. J.; Bull, I. D.; Herod, A. A.; Kandiyoti, R. Oil Gas Sci. Technol. 2008, 63, 129. (9) Herod, A. A.; Kandiyoti, R. J. Chromatogr. A 1995, 708, 143. (10) 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. (11) Domin, M.; Moreea, R.; Lazaro, M.-J.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1997, 11, 638. (12) Domin, M.; Moreea, R.; Lazaro, M.-J.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1997, 11, 1845. (13) Domin, M.; Li, S.; Lazaro, M.-J.; Herod, A. A.; Larsen, J. W.; Kandiyoti, R. Energy Fuels 1998, 12, 485. (14) Herod, A. A.; Islas, C.; Lazaro, M.-J.; Dubau, C.; Carter, J. F.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1999, 13, 201. (15) Lazaro, M. J.; Herod, A. A.; Domin, M.; Zhuo, Y.; Islas, C. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1999, 13, 1401. (16) Islas, C. A.; Suelves, I.; Carter, J. F.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2000, 14, 1766. (17) Islas, C. A.; Suelves, I.; Carter, J. F.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2002, 16, 481. (18) Islas, C. A.; Suelves, I.; Carter, J. F.; Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2002, 16, 774. (19) Islas, C. A.; Suelves, I.; Li, W.; Morgan, T. J.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. ACS Fuel Chem. DiV. Preprints 2002, 47 (2), 647. (20) Islas, C. A.; Suelves, I.; Millan, M.; Apicella, B.; Herod, A. A.; Kandiyoti, R. J. Sep. Sci. 2003, 26, 1422. (21) Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. J. Chromatogr A 2004, 1024, 227. (22) Al-Muhareb, E.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Pet. Sci. Technol. 2007, 25, 81. (23) Millan, M.; Behrouzi, M.; Karaca, F.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Catal. Today 2005, 109, 154. (24) Paul-Dauphin, S.; Karaca, F.; Morgan, T. J.; Millan, M.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2007, 21, 3484. (25) Berrueco, C.; Venditti, S.; Morgan, T. J.; Alvarez, P.; Millan, M.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2008, 22, 3265.

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dimensional conformations adopted above some critical mass, giving signal at elution times related to size and no longer directly related to mass. However, such changes in conformation have not been established beyond reasonable doubt, and we remain uncertain about the mass ranges of material excluded from column porosity. 1.1. Mass Spectrometry-based Estimations. GC-MS and probe-MS are powerful and well-understood techniques. However, GC-MS is limited by sample volatility in the GC column and that of probe-MS by sample volatility in the vacuum of the mass spectrometer ion source. The practical upper mass limit of aromatic species that can be analyzed by GC-MS is around 300-350 u. The symbol u is the IUPAC unified atomic mass unit,26sometimes also called the dalton (Da). For probe-MS, the upper limit of analyses extends to ions of about m/z 600.9 Some of the heavier coal tar pitch fractions examined in the past have shown no signal in either the GC-MS or the probe-MS ranges, apart from signals from trace amounts of solvent.14,16-18 LD-MS and MALDI-MS are not limited by volatility, although the mechanisms of ionization and desorption processes taking place in polydisperse samples are not well-understood. A series of reviews 27-30 has considered the ionization process and the production of aggregate ions in MALDI-MS and LDMS, but in relation to the ionization of biomolecules rather than the polydisperse samples used here. The role of extensive secondary ion processes in the dense plume is becoming accepted.31,32 Ionization by LD-MS presumably occurs at or near the target plate and in the condensed phase as well as in the dense plume. Mass discrimination problems have been encountered,23 where ionization tends to favor the observation of more abundant smaller molecules in complex mixtures. Meanwhile, the larger molecules either remain unionized or form ions of relatively low intensity. Effectively, this means they may remain undetected. In recent attempts to study polydisperse samples by LD-MS methods, relatively high laser power levels were used while preventing the maximum ion current of each mass spectrum (per laser shot) from rising to high values.7,22,23 Low ion currents were achieved by reducing the high-mass accelerator (HMA) voltage in linear time-of-flight (TOF) mode. 1.2. Gas-phase Sample Concentration near the Ion Extraction Zone. The quantity of sample desorbed from the target by each laser shot appears to be directly related to the thickness of the sample on the target up to a certain thickness; the laser may penetrate32 a thick layer to a depth on the order of 100 nm. When using high laser powers and thick sample layers, the HMA must be reduced to avoid overloading the detector system. These conditions, however, do not affect the gas phase concentration of sample in/near the ion-extraction zone. It appears possible to generate and to observe cluster ions if/when high gas-phase sample concentrations are encountered in/near the ion extraction zone, even when operating conditions and/or

(26) Mills, I.; Cvitasˇ, T.; Homann, K.; Kallay, N.; Kuchitsu, K. Quantities, Units and Symbols in Physical Chemistry. IUPAC Phys. Chem. DiV., 2nd ed.; Blackwell Scientific Publications: Oxford, UK, 1993. (27) Knockenmuss, R.; Zenobi, R. Chem. ReV. 2003, 103 (2), 441. (28) Karas, M.; Kruger, R. Chem. ReV. 2003, 103 (2), 427. (29) Dreisewerd, K. Chem. ReV. 2003, 103 (2), 395. (30) Karas, M.; Kru¨ger, R. Chem. ReV. 2003, 103 (2), 427. (31) Hoteling, A. J.; Nichols, W. F.; Giesen, D. J.; Lenhard, J. R.; Knockenmuss, R. Eur. J. Mass Spectrom. 2006, 12, 345. (32) Knochenmuss, R. Analyst. 2006, 131, 966.

Molecular Mass Ranges of Two Coal Tar Distillates

the detector is set for receiving low ion intensities.7,22,23,33,34 A thick layer of sample of wide polydispersity may, therefore, generate molecular ions as well as cluster ions formed from the smaller molecules. It is also possible that the latter may obscure the presence of ions from larger molecules. Our work has involved only positive ions, although negative ions are likely to form as well; because the mass resolution of the spectrometer is low, we assume they are all molecular ions and cannot distinguish between molecular ions and protonated molecules in polydisperse samples. Similarly, the use of high laser power is thought to enhance desorption and ionization of high-mass species within the sample.35 Meanwhile, possible simultaneous fragmentation to smaller molecules/ions coupled to the formation of multimer ions would tend to confuse the results. One of the aims of this study is to try to distinguish between larger molecular ions and cluster ions formed by the combination of smaller molecules. Instrumental parameters may help navigate through some of the difficulties. When a suitably configured MS-instrument is used, it is possible to work with the aid of a delayed ion extraction (DIE) pulse. This facility first allows smaller ions with relatively high kinetic energies to escape from the ionization region of the instrument. This method favors the detection of higher-mass ions (with lower velocities) by reducing the quantity of low-mass ions remaining in the ionization region. Lower kinetic energy ions are then removed from the ionization zone into the (field-free) time-of-flight stage of the MS instrument. 1.3. Molecular Masses of Heavy Hydrocarbon Liquids. The brief review presented above suggests that heavy hydrocarbons contain large molecular mass material (∼1000 to 10 000 u and perhaps greater) but that some techniques may also show high-mass artifacts and that distinguishing between them is not always straightforward. In fact, some authors have suggested that materials above m/z 1000 observed in the spectra of coaland petroleum-derived samples are solely due to the formation of cluster ions in LD-MS.33,34 Therefore, there is a suggestion that the apparently high-mass ions (>1000 u) were artifacts resulting from the use of high laser powers. In trying to improve the quality of molecular mass estimates of these heavy fractions, data from MALDI-MS and size exclusion chromatography have been compared, using coal tar pitch fractions as a coherent set of fractions. Agreement between these two entirely independent techniques has been reported up to about 3000 u. 2,3,5,36 More recent MALDI-mass spectra have presented bimodal spectra with overlapping peaks,2,7,22,23 not unlike SEC profiles, which usually show bimodal behavior, with a near-zero intensity valley between the two peaks. The two high-mass ranges observed in SEC (>200 000 u as defined by the calibration) and in LD-MS (m/z 10 000-80 000) appeared qualitatively comparable. Meanwhile, the data provided justification for the suggestion that material appearing under the excluded peak in SEC was perhaps of a lower mass range than values indicated by the polystyrene calibration. As explained above, the excluded material is thought likely to consist of molecules of different shape, probably approaching three-dimensional conformations.2,3 (33) Hortal, A. R.; Martinez-Haya, B.; Lobato, M. D.; Pedrosa, J. M.; Lago, S. J. Mass Spectrom. 2006, 41, 960. (34) Hortal, A. R.; Hurtado, P.; Martı´nez-Haya, B.; Mullins, O. C. Energy Fuels 2007, 21, 2863. (35) Tanaka, R.; Sato, S.; Takanohashi, T.; Hunt, J. E.; Winans, R. E. Energy Fuels 2004, 18, 1405. (36) Islas, C. A.; Suelves, I.; Herod, A. A.; Kandiyoti, R. Proc. 11th Intl Conf. Coal Sci., Sep. 30 to Oct. 5, 2001, San Francisco, CA, USA; ICCS-Paper 215, 2001.

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These materials would elute at shorter times than was warranted by their molecular masses. Bimodal distributions of ion intensity were also observed by Tanaka et al. in an examination of asphaltene fractions by LDMS.35 The data were considered to reflect the presence of two main groups of components, one with high aromaticity and the other with low aromaticity. These workers found that relatively high laser power levels were needed to ionize and detect larger molecules, compared to smaller ones. However, no attempt was made to limit the total ion current as laser power was increased. Meanwhile, dimers and trimers of standard polynuclear aromatic hydrocarbon (PAH) molecules were observed to form as the laser power and ion current were both increased.35 An examination by Acevedo et al.37 of asphaltenes and derivatized asphaltene samples by LD-MS and other techniques has led to the conclusion that LD-MS gave reasonable estimates of average molecular weights (m/z) of 2000-3000, tailing off toward m/z 10 000. Taking these results together, there appears to be evidence of reliable mass estimates up to about 3000 u and strong indications that some heavy coal- and petroleum-derived samples contain high-mass material, perhaps as high as m/z 10 000. Above this range, results from two independent techniques (LDMS and SEC) provide indications that need to be evaluated with an element of caution and where more work is required. In view of accumulating evidence, assertions33,34,38 that not much material exists in these samples above about 1000 u must be considered as rather conservative; an alternative view is available.39-41 In summary, introducing a measure of clarity to molecular mass estimates, particularly in the higher mass ranges, requires the distinction between signal- from sample-derived high-mass ions and possible artifacts of the technique, such as formation of multimer ions. However, we have no suitable high-mass standard molecules that would help resolve these questions.5,22,42 Instead, further progress in LD-MS requires improved understanding of how laser power, sample concentration, and other operating parameters interact and influence the spectra acquired for particular samples. This paper describes LD-MS method development work undertaken to improve estimates of mass ranges of complex hydrocarbon mixtures. Two relatively light distillate fractions derived from coal tar (creosote oil and anthracene oil) were examined. A mixture of known PAH compounds was studied under comparable operating conditions. The principal method of characterization used was the fractionation of the samples by planar chromatography (PC) followed by successive LD-MS analysis of the separated fractions directly from the silica coated surfaces of the chromatographic plates. This method has the advantage of producing relatively low gas phase sample concentrations at the ion extraction stage. Supporting analytical methods used to evaluate the samples included SEC, GC-MS, probe-MS, and LD-MS. (37) Acevedo, S.; Gutierrez, L. B.; Negrin, G.; Pereira, J. C.; Mendez, B.; Delolme, F.; Dessalces, G.; Broseta, D. Energy Fuels 2005, 19, 1548. (38) Mullins, O. C. Fuel 2007, 86, 309. (39) Herod, A. A.; Stokes, B. J.; Hancock, P.; Kandiyoti, R.; Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P. J. Chem. Soc. Perkin 2 1994, 499. (40) Bermejo, J.; Menendez, R.; Fernandez, A. L.; Granda, M.; Suelves, I.; Herod, A. A.; Kandiyoti, R. Fuel 2001, 80, 2155. (41) Herod, A. A.; Kandiyoti, R.; Bartle, K. D. Fuel 2006, 85, 1950. (42) Morgan, T. J.; Millan, M.; Behrouzi, M.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2005, 19, 164.


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Table 1. GC-MS Data for the Creosote Oil retention time (min:s) 4:40 5:07 5:20 5:36 5:45 5:50 6:10 6:25 6:45 6:55 7:35 7:50 8:20 8:35 8:45 8:50 9:00 9:05 9:20-10:05 9:58 10:20 10:45 11:10 11:20 11:30 11:40 13:00 13:20 13:25 13:40 13:50-14:35 14:50 15:15 15:20 15:35 15:50 16:00 16:20-17:00 17:20 17:40 17:50 18:10 18:30 19:40 19:50 21:20 21:40 23:30

molecular ion (m/z)

name or type

94 120 118 116 108 134 108 134 132 156 122 132 122 132 122 128 134 168 170 136 136 146 129 146 150 142 184 142 160 154 156 196 198 156 172 154 168 168 212 168 170 166 224 226 182 182 180 240 178 254 268

phenol C2-alkyl benzene indan indene methyl phenol C3-alkyl benzene methyl phenol C3-alkyl benzene methyl indan n-C11 alkane C2-alkyl phenol C2-alkyl indan C2-alkyl phenol C2-alkyl indan C2-alkyl phenol naphthalene C3-alkyl benzene c-alkane C12 n-C12 alkane C3-alkyl phenol C3-alkyl phenol C3-alkyl indan pyridine C3-alkyl indan C4-alkyl phenol methyl naphthalene n-C13 alkane methyl naphthalene C4-alkyl indan diphenyl C2-alkyl naphthalene c-alkane C14 n-C14 alkane C2-alkyl naphthalene C4-alkyl indene acenaphthene methyl diphenyl methyl diphenyl n-C15 alkane Dibenzofuran C3-alkyl naphthalene fluorene c-alkane C16 n-C16 alkane C2-alkyl diphenyl C2-alkyl diphenyl Methyl fluorene n-C17 alkane phenanthrene n-C18 alkane n-C19 alkane

UV-fluorescence spectroscopy was used to compare the relative magnitudes of aromatic ring systems. 2. Experimental Section 2.1. Samples. The commercially sourced creosote oil (CO) had a nominal boiling range between 100 and 300 °C, containing a range of components from naphthalene to C19 n-alkane by GC-MS (Table 1). The anthracene oil40 (AO) was obtained from a different commercial source and had a nominal boiling range of 250-370 °C (Table 2). An approximately equal weight mixture (1:1:1:1) in chloroform of the following PAHs was studied by LD-MS: pyrene (molecular mass [MM] 202), benzo(a)pyrene (MM 252), coronene (MM 300), and rubrene (MM 532). The samples were obtained from Aldrich and were used without further purification. This PAH solution was added to the LD-MS target as a 1 µL droplet. A saturated solution (∼3% w/v) was studied, as were subsequent dilutions of the saturated solution, dilutions by factors of 5, 10, and 100 times. 2.2. Planar Chromatography (PC). Polyester- or aluminumbacked 20 cm2 PC plates with silica gel thickness of 250 µm

Table 2. GC-MS Results for Anthracene Oil time (min:s) 8:45 11:10 11:30 13:00 13:10 13:30 13:45 14:00 14:25 14:40 15:20 15:25 15:35 15:55 16:00 16:25 16:40 16:45 17:00 17:25 17:35 17:40 17:50 17:55 18:10 18:30 18:35 19:10 19:30 19:40 19:45 19:55 20:10 20:20 20:25 20:35 20:45 21:10 21:30 21:40 22:05 22:20 22:35 22:50 23:12 23:20 23:35 23:45 24:12 24:20 24:30 24:35 24:50 25:10 25:15 25:25 25:35 25:50 26:10 26:25 26:35 26:45 27:00 27:10 27:20 27:35 27:50 28:00 28:05 28:20 28:30 28:45 28:55 29:05

m/z

atomic composition

128 142 142 158 154 156 156 156 156 156 154 168 168 168 168 182 182 182 182 166 168 168 180 168 182 182 182 180 180 180 196 180 182 196 196 182 184 178 178 180 179 167 204 198 192 192 190 192 192 181 192 208 204 206 206 206 206 206 204 202 203 208 203 202 202 218 218 218 216 216 216 216 216 216 216

C10H8 C11H10 C11H10 C12H14 C12H10 C12H12 C12H12 C12H12 C12H12 C12H12 C12H10 C13H12 C13H12 C13H12 C12H8O C14H14 C14H14 C14H14 C14H14 C13H10 C13H12 C13H12 C14H12 C13H12 C14H14 C14H14 C14H14 C14H12 C14H12 C14H12 C15H16 C14H12 C14H14 C15H16 C15H16 C14H14 C12H8S C14H10 C14H10 C14H12 C13H9N C12H9N C16H12 C13H10S C15H12 C15H12 C15H10 C15H12 C15H12 C13H11N C15H12 C16H16 C16H12 C16H14 C16H14 C16H14 C16H14 C16H14 C16H12 C16H10 C15H11N C16H16 C15H11N C16H10 C16H10 C12H10O C12H10O C12H10O C17H12 C17H12 C17H12 C17H12 C17H12 C17H12 C17H12

name or type naphthalene methylnaphthalene methylnaphthalene alkyl indene diphenyl dimethylnaphthalene dimethylnaphthalene dimethylnaphthalene dimethylnaphthalene dimethylnaphthalene acenaphthene methyl diphenyl methyl diphenyl methyl diphenyl dibenzofuran dimethyldiphenyl dimethyldiphenyl dimethyldiphenyl dimethyldiphenyl fluorene methyl acenaphthene methylacenaphthene methyl fluorene methylacenaphthene dimethylacenaphthene dimethylacenaphthene dimethylacenaphthene dihydrophenanthrene dihydroanthracene methylfluorene trimethyldiphenyl methylfluorene dimethylacenaphthene trimethyldiphenyl trimethyldiphenyl tetrahydrophenanthrene dibenzothiophene phenanthrene anthracene methylfluorene azaphenanthrene carbazole dihydrofluoranthene isomer methyldibenzothiophene methylphenanthrene methylphenanthrene cyclopentenophenanthrene methylphenanthrene methylphenanthrene methylcarbazole methyl anthracene hexahydropyrene isomer dihydrofluoranthene dimethylphenanthrene dimethylphenanthrene dimethylphenanthrene dimethylphenanthrene dimethylphenanthrene dihydropyrene fluoranthene azafluoranthene hexahydropyrene isomer azafluoranthene pyrene pyrene isomer benzonaphthofuran isomera benzonaphthofuran isomera benzonaphthofuran isomera methylpyrene isomerb methylpyrene isomerb methylpyrene isomerb methylpyrene isomer methylpyrene isomer methylpyrene isomer methylpyrene isomer

Molecular Mass Ranges of Two Coal Tar Distillates Table 2. Continued time (min:s) 29:10 29:30 29:55 30:10 30:30 30:45 30:50 31:00 31:15 31:35 31:50 32:00 32:10 32:20 32:40 32:45 32:50 33:00 33:15 33:40 33:45 33:55 34:00 34:10 34:35 34:45 35:30 36:00 36:25 36:50 36:55 37:05 37:25 37:40 38:05 38:50 40:30 40:50 41:30

m/z

atomic composition

216 244 230 230 230 232 234 226 228 234 229 234 244 228 228 228 248 242 217 242 217 242 242 242 240 242 242 254 241 253 252 252 258 252 252 252 266 266 264 278 276 276

C17H12 C19H16 C18H14 C18H14 C18H14 C18H16 C16H10S C18H10 C18H12 C16H10S C17H11N C16H10S C19H16 C18H12 C18H12 C18H12 C17H12S C19H14 C16H11N C19H14 C16H11N C19H14 C19H14 C19H14 C19H12 C19H14 C19H14 C20H14 C18H11N C19H11N C20H12 C20H12 C18H10S C20H12 C20H12 C20H12 C21H14 C21H14 C21H12 C22H14 C22H12 C22H12

name or type methylpyrene isomer C3-alkylfluoranthene isomer dimethylpyrene isomer dimethylpyrene isomer dimethylpyrene isomer methylphenylnaphthalene benzonaphthothiophene benzo[mno]phenanthrene benzo[c]phenanthrene benzonaphthothiophene azachrysene isomer benzonaphthothiophene C3-alkylfluoranthene isomer chrysene isomer chrysene isomer chrysene isomer methylbenzonaphthothiophene methylchrysene isomer benzocarbazole methylchrysene isomer benzocarbazole methylchrysene isomer methylchrysene isomer methylchrysene isomer benzocyclopenteno [def]phenanthrene methylchrysene isomer methylchrysene isomer binaphthyl pyrenopyrrole azabenzopyrene isomer benzofluoranthene benzofluoranthene benzophenanthro[def]thiophene benzo[a]pyrene benzo[b]pyrene perylene methylbenzofluoranthene methylbenzofluoranthene cyclopentenobenzopyrene pentacene indeno pyrene benzo[ghi]perylene

a Alternative: C H 17 14 phenylnaphthalene or dihydrofluoranthene isomer. b Earliest components could be benzofluorenes.

(Whatman, UK) were used. The plates were washed with acetone and then chloroform before use. The AO and CO were spotted onto the PC plate as a solution in acetone; multiple spots were added to build up the amount of sample. Both samples were fully soluble in acetone at the concentrations used. The PC plates were developed with acetone, dried, and then developed with chloroform; they were removed from the development tank and dried once the solvent front had reached a height of approximately 10 cm, for both solvents; that is, the material mobile in acetone was not separated from the species mobile in chloroform, as this was not the aim of the study. The aim was to isolate the immobile material by removing as many of the smaller molecules as possible. For the AO sample, a significant amount of sample was immobile in both eluents. Very little material from the CO sample was immobile in the two solvents, Figures 1a-d show images of the plates under daylight and UV light. The material immobile in both eluents was denoted as PC fraction 1. Subsequent fractions of material with increasing mobility (higher up the PC plate) were taken at regular intervals; the material at the furthest solvent front was labeled fraction 5. Photographs of the AO and CO PC plates showing the different fractions used in this work are shown in Figures 1a-d. The different fractions on the PC plate were cut out and directly stuck on to the LD-MS target using double-sided sticky tape. For SEC and UV-F analysis, the samples were recovered by scraping the silica from the plates and extraction in NMP followed by filtration at 1 µm to remove silica particles.

Energy & Fuels, Vol. 22, No. 5, 2008 3279 2.3. Vacuum Distillation. The high vacuum system of the LDMS system (10k

22.4 538 20.0 5000 n/a n/a n/a

22.8 393 20.8 1895 n/a n/a n/a

23.7 200 22.0 700 200 500 10k

23.7 200 22.0 700 200 1600 10k

24.3 100 22.0 700 250 - 280 450 - 1100 1000- 1200

22.9 363 20.0 5000 500 1000 2500

23.2 287 20.0 5000 n/a n/a n/a

23.3 265 20.5 2682 n/a n/a n/a

24.3 100 22.0 700 150 400 3000

24.4 100 22.0 700 160 500 2000

a Upper limit for SEC refers to the forward edge of the retained peak; the time used is taken as +30 s from where the signal clearly deviates from the baseline. b Upper limit for LD-MS refers to where there is clear ion intensity, assuming there are no multimer ions. c Tail for LD-MS refers to a noisy signal that tails off to high mass; it is unclear if this is a real signal or on artifact.

since it was observed that if a relatively light sample was present in the sample chamber while examining a heavier sample, then significant ion current was detected from volatile low mass components of the lighter sample. These lighter components were not observed when the higher-mass samples (less-mobile PC fractions) were analyzed in isolation. The PAH mixture was added to the target as a saturated solution (∼3% w/v) and after various dilutions (factors of 5, 10, and 100) in chloroform. All sample solutions were spotted as a 1 µL droplet onto the 4 mm diameter target and left to air-dry.

3. Results and Discussion 3.1. Characterization by SEC. Size exclusion chromatograms of the two original distillate samples are shown in Figure 2a. Although there was considerable overlap between the two chromatograms, parts of the (higher-boiling) anthracene oil eluted earlier than the creosote oil. Elution times of the maxima

of the resolved peaks were 23.7 min for anthracene oil and 24.3 min for creosote oil. These times correspond to molecular masses of about 200 u and 100 u, respectively, as evaluated by the column calibration based on PS and standard PAH samples. However, both samples were observed to contain traces of material excluded from column porosity. This was not readily apparent in the presence of the whole sample. Figure 2b shows the area between 14 and 16 min from the SEC chromatogram of the samples (CO at 280 nm, AO at 350 nm) on an expanded scale to highlight the presence of this excluded material. For both samples, the high mass edges of the retained peaks indicated masses of about 700 u (Table 3) with the AO sample showing stronger signal intensity in this region. Chromatograms acquired at higher wavelengths (not shown) provided clear indications that the AO sample contained larger and more condensed aromatic species than the CO sample. In any case,


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Figure 3. Height-normalized synchronous UV-fluorescence spectra of (a) the creosote (curve 1) and anthracene oils (curve 2), (b) the creosote oil PC fractions where fraction 1 was immobile and fraction 5 was the most mobile, and (c) the anthracene oil PC fractions where fraction 1 was immobile and fraction 5 was the most mobile.

the AO sample absorbed at all wavelengths studied (280-370 nm), whereas the (lower boiling) CO sample showed no absorbance at 350 and 370 nm, suggesting the absence of comparably large chromophores. SEC chromatograms of fractions of the CO and AO samples separated by PC are shown in Figure 2, panels c and d, respectively The chromatograms of the CO fractions are clearly shifted to shorter elution times (higher apparent masses) with decreasing mobility on the plate. All except the most mobile fraction showed a significant excluded peak (F1-F3; cf. Figure

1a for fraction labeling). In more concentrated form, these PC fractions clearly reveal the small quantities of excluded material, which could be just observed in the “whole” sample (Figure 2, panels a and b). The SEC chromatogram of the whole CO sample was almost identical to those from the most mobile PC fractions (fractions 4 and 5). This shows that if the sample is not fractionated before analysis, the smallest molecules dominate the chromatogram and the higher mass species are not detected. Similar results were observed for the AO PC fractions (see Figure 2d and Table 3), the higher mass material was present


Figure 4. GC-MS chromatogram of creosote oil.

in significantly greater quantities compared to the CO sample, (cf. Figures 1a-d). The elution times for the peak maxima of the fractions are listed in Table 3 along with the upper mass limits of the retained peaks. It was not possible to obtain a mass balance for the PC fractionation; however, it is clear that the high-mass material corresponds to a minor portion of the whole samples. 3.2. Characterization by UV-F. The synchronous UVfluorescence (UV-F) spectra of the whole samples (Figure 3a) show that the creosote oil has a less-intense signal at the longer wavelengths compared to the anthracene oil. Nevertheless, the creosote oil showed lower intensity fluorescence in the same range as the anthracene oil, between wavelengths 325 and 400 nm. This is usually interpreted43,44 in terms of smaller polynuclear aromatic systems compared to the anthracene oil. The UV-fluorescence spectra of the PC fractions were found to be consistent with these data, suggesting that the CO sample contained smaller polynuclear aromatic systems than the AO sample. The overlap is more clearly apparent in these data and extends to 450 nm in the UV-F spectra of the PC fractions (Figures 3b-c). The least-mobile fraction (F1) of the AO sample showed a series of peaks up to 650 nm, which were not apparent when the whole sample was studied. Previous work has indicated that material with masses approaching 3000 u (or larger) of coal liquids displays negligible fluorescence.42 The overlap of aromatic systems detected in the CO and AO probably reflects the lack of a sharp cutoff during the industrial distillations of the samples. It is also noted that the two samples were derived from different coals and process units. 3.3. Characterization by GC-MS and Probe-MS. The GCMS analysis of the creosote oil sample (Figure 4 and Table 1) gave a molecular mass range from phenol, m/z 94, to C19 alkane, m/z 268. Figure 5 shows the analogous probe-mass spectra, indicating the presence of low-intensity components up to around m/z 350. The analysis by GC-MS of the anthracene oil (Figure 6 and Table 2), showed a range of compounds from naphthalene (m/z 128) to indenopyrene and benzo(g,h,i)perylene (m/z 276) with phenanthrene as the most concentrated component. Probe-MS of the anthracene oil sample showed ions from m/z 128 naphthalene up to m/z 350 and contained the ions for all the


well-known polycyclic aromatics of coal tar pitch. Probe-MS of the least-volatile fraction of the oil, shown in Figure 7 indicates low-intensity ions up to about m/z 380. The two samples covered broadly similar mass ranges, although the creosote oil was found to contain more phenols than the anthracene oil. The difference in the upper mass ranges between GC-MS and probe-MS arise from the lack of sensitivity of the GC-MS to the higher-mass components of these oils. However, the range of the anthracene oil chromatogram, Figure 6, extends in time beyond the range detected for the major components of creosote oil, Figure 4, 45 min compared to 25 min. Both techniques proved useful in broadly identifying the types of more-volatile molecular species present in the two oil samples. However the mass ranges within which the two techniques are capable of identifying sample fall well-short of the higher-mass species detected by SEC. The two mass spectrometric techniques were not capable of detecting the material that appeared (i) either in the upper mass part of the resolved SEC peak, with apparent masses greater than 400 u or (ii) before the exclusion limit of the SEC-column. In looking to corroborate results from SEC, this is the principal reason for resorting to the somewhat less-conventional LD-MS technique. It has a far wider range of detection but, as we shall see, interpretation of data from this technique still requires some careful work. 3.4. Characterization by LD-MS. 3.4.1. Mixture of PAH Standards. Figure 8, panels a-c, presents LD-MS spectra of a saturated solution in chloroform of a mixture of pyrene (202 u), benzo(a)pyrene (252 u), coronene (300 u), and rubrene (532 u). At low laser power (LP 10%) and maximum HMA voltage (10 kV), the LD-mass spectra in Figure 8a show peaks at about m/z 202, 252, 300, and 532 and an unknown peak at 458. The low ion intensity peak at m/z 266 is thought to be the methyl derivative of benzopyrene. As laser power was increased to 20 and 30%, with the HMA still set to 10 kV, higher-mass ions were observed. These are likely to be either impurities or dimer ions. At LP 20 and 30%, cluster ions between m/z 680 and 1400 were observed at low intensity. Finally, the detector was deliberately overloaded with LP 40% and HMA 10 kV (Figure 8a). Ions were then observed between m/z 3000-4000 and 30 000-40 000. These data highlight the possibility of cluster ion formation from four PAHs of mass 202, 252, 300, and 532 u, depending on spectrometer conditions. Figure 8b presents LD-MS spectra for the same saturated solution, as a function of increasing laser power, obtained with reduced HMA voltages, in order to avoid overloading the linear mode detector. At LP 50% and HMA 6.0 kV, cluster ions were clearly observed between m/z 3500 and 35 000 (Figure 8b). It was noted that the detector was far from overloaded and was in what were previously thought to be safe operating conditions suggested by Bruker (i.e., less than 100 ion counts per laser shot). This shows that it is possible to observe cluster ions of significant intensity in relation to the molecular ions without overloading the detector system. Figure 8c shows the effect of increasing the HMA voltage from 6 to 10 kV while keeping the laser power level constant at 40%. The low HMA voltage had the effect of largely suppressing the detection of cluster ions, although traces of multimer ions could be detected. At an HMA value of 7 kV, multimer ions, up to hexamer ions, were observed at low intensity. At 10 kV, the combination of high laser power and high HMA voltage led to both the appearance of artifact peaks and the overloading of the detector.


Figure 5. Probe-MS of creosote oil sample showing the least-volatile components.

Figure 6. GC-MS chromatogram of anthracene oil.

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been defocused to reduce laser power. The work had indicated that use of relatively low power was better in producing molecular ions, compared to high power. Mass spectra at low power showed that hydrogenated ions (addition of 2-4 mass units to the molecular mass) could be formed. At the other extreme, carbon clusters were observed at high laser power, and molecular ions reduced in intensity. The carbon clusters formed at masses less than the molecular ion in each case. They were of the type CnHm, where n was less than the number of carbons in the molecule and m could be 1, 2, or 3. The compositions of these carbon clusters were not similar to the molecular clusters encountered in the present work, probably because the sample loadings were from solution and of relatively low concentration.

Figure 7. Probe-MS of anthracene oil showing the least-volatile components.

In the three sets of spectra reviewed, molecular ions from the saturated PAH solution were the highest intensity ions observed. In reflector mode, aggregates are more difficult to observe because of the lower sensitivity of the detector to highmass material compared with the linear TOF mode. Results obtained in linear mode have allowed two types of aggregation to be identified (Figure 8a-c): (i) formation of multimer ionssdimers and trimers, up to hexamerss(Figure 8b-c), and (ii) at higher laser powers, two broad groups of cluster ions, centered at m/z ∼3500 and 35 000 (Figures 8b-c). Figure 8d presents LD-MS spectra of the same PAH mixture, obtained using a lower sample solution concentration (∼0.6% w/v compared to ∼3% w/v for the saturated solution). The spectra in Figure 8d were obtained using both a high laser power level (50%) and HMA voltages from 7 to 10 kV. Comparing Figure 8d with Figure 8, panels a-c, shows that far fewer artifacts were observed when the more dilute solution was analyzed. Lower concentrations of sample on the target plate would be expected to give rise to lower gas-phase concentrations of sample after the laser pulse has hit the target. In Figure 8d, lower levels of aggregation were observed when the sample was diluted, even though the spectrometer operated under what would otherwise appear as rather extreme conditions, that is, high laser power and high values of the HMA voltage. Thus, in addition to high laser power and HMA values, the formation of cluster ions in Figure 8, panels a-c, was observed to take place due to relatively high initial gas-phase sample concentrations in the chamber following the laser pulse. These findings are consistent with earlier findings by Hortal et al.33,34 The consequences of the high gas-phase sample concentration for a particular laser power level can be gauged by observing the results obtained using the maximum HMA voltage (10 kV), as in Figure 8a. These results highlight the need for some care in studying complex hydrocarbon mixtures by LD or MALDIMS. For the PAH mixture, a LP value of 5-10% proved sufficient to gain good ion intensity. Using LP values up to 30% was possible without any significant adverse affect on the observed mass spectrum. At LP > 40%, the formation of multimer and cluster ions becomes an issue for highly concentrated samples. In previous work39 PAH standards have been studied using laser desorption, using a high-power laser (NdYAG) that had

3.4.2. LD-MS Spectra of the Creosote and Anthracene Oils. Only results from the study of the neat original samples will be shown, as these results are thought to best represent the whole sample, as will be explained below. Of the reflector mode spectra acquired using the neat sample, only those with a DIE of 200 ns are shown. These conditions were chosen because they most clearly illustrate the highest and lowest mass regions of the samples. Results obtained using other DIE times showed an expected trend: as the DIE time was increased, more of the smallest ions were lost, and the observed mass spectrum shifted to higher m/z values. Only the linear mode LD-MS spectra of the PC-separated fractions are shown. In all cases, the HMA was set to the maximum level of 10 kV, and no extraction delay was used. All DIE times (200, 400, and 600 ns) were studied, as was the affect of laser power in reflector (not shown) as well as linear mode. Ten shots were summed for each spectrum shown. Only PC fractions 1, 4, and 5 were studied by LD-MS due to low amount of material in the other fractions (cf. Figure 1a-d), which made it impossible to get a satisfactory signal above noise level. In the SEC work it was possible to analyze all the fractions by recovering the sample from a large area of the PC plate into a small volume of solvent. The m/z ranges recorded by LD-MS for all the CO and AO samples (whole and PC fractions) are listed in Table 3 alongside the SEC-based mass estimates. The LD-MS derived m/z values for the whole samples are given as ranges, with the lower m/z values from the reflector mode (DIE 200 ns) and the higher values from the linear mode operation (DIE 600 ns). For the PC fractions only one value is given, which is based on the linear mode analysis with no DIE. Only spectra from on-scale runs (50% but not at lower laser powers. At 60% LP, ions were observed between m/z 500 and 600. Once again, these might either be sample-derived molecular ions or trimers. The spectra also showed increased intensities of fragment ions and possibly metal ions below m/z 80. There was no evidence of ions above m/z 600, and the main body of the spectrum (about m/z 100-300) remained as the most intense part throughout. Taking these data together, it seems difficult to distinguish from the analysis of the neat, whole sample whether the higher-mass ions are multimer ion artifacts or sample-derived molecular ions. This will be addressed below, in the discussion of the LD-MS results off the PC plates. Figure 10a shows spectra of the neat creosote oil in linear mode LD-MS from low laser power (10% of maximum) to 70% of maximum. Of the linear mode spectra acquired for the neat sample, only those with a DIE of 600 ns have been shown. These conditions were chosen because they most clearly illustrate the highest mass regions of the sample. As above, neat samples were placed directly onto the metal target. In these linear TOF mass spectra (Figure 10a), the voltage on the HMA was reduced as LP was increased, to keep the value of the maximum ion current intensity per laser shot below 100 arbitrary units. In Figure 10a (all linear mode spectra), the spectrum obtained at 20% laser power (LP) appears similar to that in Figure 9 for 40% LP reflector mode, but with poorer resolution. The range of maximum intensity of ions ranged from m/z 250 to 500, although fragment ions and metal ions below m/z 100 became more intense with increasing LP. At LP values greater than 30%, the spectra showed tailing toward m/z 1000 but with no



Figure 10. (a) Effect of increasing laser power on the LD-MS spectra of the neat creosote oil, where the HMA voltage was reduced to keep the ion count below 100 units per shot. Linear mode, 600 ns DIE, laser power (% of max.) and HMA voltage (kV) from 1 to 7: [LP/HMA] 10/10, 20/10, 30/8, 40/7, 50/6, 60/6, and 70/6 %/kV. Each mass spectrum is the sum of 10 scans. (b) Effect of increasing laser power on the LD-MS spectra of the CO PC fraction 1. Linear mode, no DIE, HMA 10 kV, at laser powers (% of max.) from 1 to 4: 10, 20, 40, and 50%. Each mass spectrum is the sum of 10 scans. (c) Effect of increasing laser power on the LD-MS spectra of the CO PC fraction 4. Linear mode, no DIE, HMA 10 kV, at laser powers (% of max.) from 1 to 5: 40, 50, 60, 70, and 80%. Each mass spectrum is the sum of 10 scans. (d) Effect of increasing laser power on the LD-MS spectra of the CO PC fraction 5. Linear mode, no DIE, HMA 10 kV, at laser powers (% of max.) from 1 to 4: 30, 40, 50, and 60%. Each mass spectrum is the sum of 10 scans.

significant intensity above the m/z 1000 level. At 70% LP, the fragment ions became the most intense group of ions. 3.4.4. LD-MS Linear Mode Results of the PC Fractions 1, 4, and 5. Although the spectra in Figures 9 and 10a were obtained under closely controlled conditions, the possibility of forming and observing dimers and higher multimer ions during the examination of the neat creosote oil sample cannot be excluded. These artifacts would be expected to overlap with any ionized high-mass material present in the sample. As already observed, the interference by artifact species is thought to worsen

when high laser power levels are combined with high HMA voltage values. Figure 10, panels b-d, presents a sequence of linear mode spectra of progressively heavier fractions of the creosote oil, as fractionated on the PC plate. The spectra were acquired by submitting strips of PC plates (on which the sample fractionation had been effected) stuck directly onto the MALDI target. Sample was thinly dispersed on the PC plate and the aim of this strategy was to reduce the amount of the sample reached and ionized by each laser shot. The rationale of this strategy was based on


Figure 11. Effect of increasing laser power on the LD-MS spectra of the neat AO. Reflector mode, 200 ns DIE, at laser power (% of max.) from 1 to 6: 20, 25, 30, 40, 50, and 60%. Each mass spectrum is the sum of 10 scans.

observations that dilute solutions applied to the target (Figure 8d) did not allow distortion of the spectra and the formation of artifacts. It may be immediately observed that no distortions of the spectra were observed when high LP values were combined with the maximum HMA setting of 10 kV. The good resolution and repeatability obtained with this sample introduction technique suggests that the amounts of sample desorbed by the lasershot, gas phase concentrations and the likelihood of gas phase aggregation are all significantly reduced. This in turn allows the use of the maximum HMA voltage (10 kV) without exceeding the 100 unit per shot limit. A clean silica surface from the PC plate was found to give no observable ion current under any of the LD-MS operating conditions used. It may be surmised, therefore, that signals observed at m/z > 400 actually originated from traces of high-mass species present in this sample. Higher laser powers have also been applied without increasing the observed mass distribution (compare Figure 10, panels c and d). At the highest laser powers used (g70%) some multimer ions appear to have formed. The results were consistent in reflector (not shown) and linear mode (Figures 10). Expected trends were also observed when the delayed ion extraction time was increased from 0 to 600 ns; the mass spectra shifted to higher m/z values as the DIE time was increased (not shown). Care needs to be taken when using the DIE method, as it possible that high concentrations of low-mass species are not observed due to the delayed extraction as mentioned above. These low-mass species could still form clusters ions that remained in the extraction zone, causing high-mass artifacts to be observed, although in practice this was difficult to achieve. In the present study, using either a neat sample or the PC subfractions, this affect was rarely observed. Using linear mode operation with a DIE time of 600 ns with the maximum HMA voltage was found to be the best method for observing the highest-mass ions. However, it is necessary to study spectra obtained with both short (0 ns) and long (600 ns) DIE times. When a long DIE time was used it was possible that the higher-mass ions detected were in fact artifacts caused by aggregation of small molecular ions that themselves were not detected. If the results from 0 to 200 ns show there were no

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or only low levels of ions at low m/z values, then there was a higher probability that the higher mass ions detected, at longer DIE times, were sample-derived molecular ions rather than aggregates. 3.4.5. LD-MS of Anthracene Oil. Figure 11 presents the LDMS spectra of the neat anthracene oil, in reflector mode, and Figure 12a shows the LD-MS of the same sample in linear mode. Here, the HMA voltage was reduced to maintain the ion intensity per mass spectrum below 100 arbitrary units of intensity. As in the case of the neat creosote oil (see above), sample was applied directly onto the metal target plate. Acquiring reflector mode spectra was useful for examining these relatively light samples. It allowed checking of the lowmass components in greater resolution, to make sure there was no small molecular mass material present that was not observed in linear mode. Figure 11 (reflector mode) showed sample ions between m/z 170 and 370, tailing off approximately to m/z 1500 for the neat AO. The intensity of fragment ions below m/z 170 increased with LP. The presence of ions between m/z 550 and 700 also increased at LPs higher than 20% in both reflector and linear modes. At LP 60% the ions in the m/z 500-1000 range became more intense with practically no ion intensity above m/z 1000. In view of observations outlined so far, it would seem likely that some of the higher mass signal is due to dimer and trimer ion formation as well as from molecular ions. The study of the PC fractions (below) would be expected to assist in distinguishing between these two possible origins for higher mass signal. In linear mode, the main band of ions from the neat AO sample (Figure 12a) was between approximately m/z 180 and 370 at low LP. Signals extended to about m/z 3000, similar to the range of mass observed in reflector mode. As the LP level was increased, the formation of multimer ions is likely to become more prominent, as is the formation of higher-mass molecular ions. At LP >50%, fragment ions could be clearly observed. At 60% LP and an HMA value of 6 kV, the detector intensity scale was not overloaded, but cluster ions at m/z 3500 and 35 000 were apparent. This was combined with a significant increase in fragment ions. At higher values of HMA voltages these clusters were observed at LPs as low as 40% (with the detector overloaded, not shown). The linear mode LD-MS results for the AO PC fractions F1, F4, and F5 are shown in Figure 12, panels b-d, respectively, and a direct comparison of spectra from the three fractions is presented in Figure 12e. Spectra from the most immobile fraction (F1) on the PC plate, in Figure 12b, show clear evidence for ions up to m/z ∼5000, possibly extending to m/z 10 000. The LD-MS spectra of the increasingly mobile PC fractions (F4 and F5, Figure 12, panels c and d) are similar to those seen for the whole sample and show a decrease in m/z (main band m/z 150-400) compared to fraction 1. The shift to lighter material can be clearly observed in Figure 12e and in Table 3. Because of the lack of ions below m/z 800, for fraction 1 (Figure 12b), and the low ion intensity per shot, the high-mass signal was considered to be from molecular ions and not clusters. As the laser power was increased, the mass range remained consistent up to LP 50%, where a high-mass shoulder was observed due to cluster ion formation. A similar formation of cluster ions was observed for the most mobile fractions (which contain molecules of much lower masses) when higher levels of laser power were used (Figure 12, panels c and d). Taken together, these data suggest that cluster ion formation could be triggered either by the use of relatively high laser power levels or by relatively high target concentrations;



Figure 12. (a) Effect of increasing laser power on the LD-MS spectra of the neat AO. The HMA voltage was reduced to keep the ion count below 100 units per shot. Linear mode, 600 ns DIE, at laser power (% of max.) [LP/HMA] from 1 to 7: 10/8, 10/10, 20/7.5, 30/7, 40/6.5, 50/6, 60/6 and 70/5.5 %/kV Each mass spectrum is the sum of 10 scans. (b) Effect of increasing laser power on the LD-MS spectra of the AO PC fraction 1. Linear mode, no DIE, HMA 10 kV, laser power (% of max.) from 1 to 5: 30, 35, 40, 50, and a repeat at 50% (4 was a lighter area on the PC plate, and 5 a darker area). Each mass spectrum is the sum of 10 scans. (c) Effect of increasing laser power on the LD-MS spectra of the AO PC fraction 4. Linear mode, no DIE, HMA 10 kV, laser power (% of max.) from 1 to 4: 30, 40, 50, and 60%. Each mass spectrum is the sum of 10 scans. (d) Effect of increasing laser power on the LD-MS spectra of the AO PC fraction 5. Linear mode, no DIE, HMA 10 kV, laser power (% of max.) from 1 to 4: 20, 30, 40, and 50%. Each mass spectrum is the sum of 10 scans. (e) LD-MS spectra comparing the AO PC fractions 1, 4, and 5 (spectra 1, 2, and 3, respectively). Linear mode, no DIE, HMA 10 kV, and laser power 40-50% of max. Each mass spectrum is the sum of 10 scans.


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Figure 13. (a) LD-MS spectra of the neat AO and its vacuum residues; exposure to the vacuum system for increasing lengths of time, from 1 to 4: 0, 2, 10, and 28 h. Linear mode, 600 ns DIE, HMA 10 kV, and laser power 20% of max. Each mass spectrum is the sum of 10 scans. (b) Area-normalized SEC chromatograms of the whole anthracene oil (1) and its vacuum residue after 28 h (2) at 300 nm UV-A, Mixed-A column in NMP eluent. (c) Peak-normalized synchronous UV-F spectra of the whole anthracene oil (1) and its vacuum residue after 28 h (2).

both factors can generate excessively high gas-phase concentrations in the extraction zone following the laser pulse, leading to the observation of cluster ions. In Figure 12b, the spectra labeled 4 and 5 refer to repeat runs on different areas of the same PC fraction (F1). In both cases, an LP level of

50% was used with no DIE. Spectrum 5 was acquired from a darker area (high sample concentration) than spectrum 4. This result (Figure 12b) shows that the cluster ion formation is a concentration-dependent effect and is not directly related to laser power alone.



Figure 14. LD-MS spectra of the whole AO where the HMA voltage was reduced to keep the ion count below 100 units per shot. Linear mode, 600 ns DIE: (a) effect of increasing laser power on the 20 ppm solution; LP (% of max)/HMA (kV) from 1 to 7: 10/10, 20/7.5, 30/7, 40/6, 50/6, 60/6, and 70/6 %/kV. Each mass spectrum is the sum of 10 scans. (b) Effect of increasing laser power on the LD-MS spectra of the 2 ppm solution; LP (% of max)/HMA (kV) from 1 to 8: 10/9, 20/7, 30/7, 40/6, 50/6, 60/6, 70/6, and 80/6 %/kV. Each mass spectrum is the sum of 10 scans.

The presence of high-mass species (m/z >1000) detected in this part of the work were also confirmed by the study of vacuum residues produced from the AO sample, presented in the next section. 3.4.6. Vacuum Residues of the AO Sample. Vacuum residues were produced by simply exposing the neat sample in the LDMS vacuum chamber and acquiring spectra after various lengths of time up to 28 h. A clear trend was observed between the length of time the sample was exposed to the high-vacuum system of the mass spectrometer (∼2 × 10-7 mbar) and the mass distribution of the sample, measured by LD-MS. A loss of signal at low masses was observed with increased time under vacuum, and an enhancement of signal at m/z about 1000 by LD-MS (Figure 13a). The observed trend was confirmed by recovering the residue from the LD-MS target with subsequent characterization by SEC (Figure 13b) and UV-F spectroscopy (Figure 13c). A small red-shift was observed by UV-F and

larger masses by SEC for the vacuum residues compared to the original oils. These changes are smaller than those observed for the PC fractions, which is not surprising as PC gives a better separation than vacuum distillation. Moreover, even after 28 h in the vacuum system, some low-mass species were still present. These smaller molecules dominate the observed SEC chromatograms and UV-F spectra due to their stronger UV absorbance and fluorescence compared to the higher-mass components. 3.4.7. Dilute Solution of the AO Sample. We have already seen in this work how the use of dilute sample solutions may help overcome the problem of possible gas-phase aggregation in the LD-MS analysis of coal tar and petroleum asphaltenes (after Hortal et al.33,34). The effect of sample dilution was studied for the whole AO sample. The results presented here (cf. Figure 14) provide evidence that if dilute solutions are spotted onto the LD-MS target then there is mass discrimination. Effective suppression of the high-mass signal occurs in favor of the


identification of the most abundant or most easily observed species in the mixture. The result of the work presented here suggests that a simple solution dilution approach33,34 is inappropriate for the investigation of complex mixtures of wide polydispersity and will underestimate average mass values by not counting (i.e., detecting) higher-mass species. It is necessary to factor in the higher-mass species detected from the analysis of the PC fractions to gain a more accurate average mass value. Once again, the fractionation of polydisperse samples appears necessary before attempting to measure their mass ranges using either SEC or LD-MS. 4. Summary and Conclusions This paper describes LD-MS method development work undertaken to improve estimates of mass ranges for complex hydrocarbon mixtures. The aim of the work was to get a better understanding of how laser power, sample concentration, and other operating parameters interact and influence the acquired spectra. Two relatively light distillate fractions derived from coal tar (a creosote oil and an anthracene oil) were examined. A mixture of known PAH compounds was studied under comparable operating conditions. Supporting analytical methods used included SEC, GC-MS, probe-MS, and UV-fluorescence spectroscopy. The data on the mixture of the four PAHs indicated the possibility of cluster ion formation, depending on spectrometer conditions. The combination of high laser power and high HMA voltage led to both the appearance of artifact peaks and the overloading of the detector. Furthermore, cluster ion formation of significant intensity in relation to the molecular ions was found to be possible without necessarily overloading the detector system. However, cluster ion formation could be nearly totally eliminated by submitting more dilute samples for analysis. It is thought that diluting the sample solution has the effect of lowering the gas phase ion concentration in the ionization zone when the laser is fired. No artifacts were observed when the diluted sample was used, even when high laser power levels and high mass accelerator voltages were applied. With careful balancing of sample concentration, laser power, total ion current, and delayed ion extraction, it has been shown that high-mass materials can be observed without generating high-mass multimer (i.e., artifact) ions. The key to suppressing cluster ion formation appears to be to keep to low target and gas-phase sample concentrations following the laser pulse. These findings are consistent with findings by earlier researchers.

Morgan et al.

As in earlier work, it was found that when samples of high polydispersity are characterized by size exclusion chromatography and LD-MS, the more abundant, lighter components swamp the signal. Once again, it becomes necessary to fractionate the samples. The principal method of characterization used was the fractionation of the samples by planar chromatography followed by successive LD-MS analysis of the separated fractions directly from the silica-coated surfaces of the chromatographic plates. Pieces of PC-plates bearing separated fractions were directly introduced into the spectrometer as the target. The method achieves two objectives: (i) fractionation of the sample, to reduce masking of signal from the less abundant fractions by the more abundant and (ii) reducing the concentration of sample, by spreading on the PC-plate, produces relatively low gas-phase sample concentrations at the ion extraction stage when the laser is fired. The technique demonstrably suppresses multimer formation and has greatly improved the reproducibility of the spectra. The mass distributions recorded from the PC fractions and vacuum residues of the oils by LD-MS, with or without extraction delay, show almost identical mass ranges and peak maxima in both linear and reflector modes. These mass range estimates were also in close agreement to those calculated from SEC. The material observed in the excluded region of the SEC chromatogram, however, remains unaccounted for in terms of mass. Molecules with ion masses up to at least m/z 5000 have been found to be present in the anthracene oil sample. The presence of the high-mass species (500-5000 u) was confirmed by SEC, and findings were consistent with changes observed in the UVfluorescence spectra of the samples. However, the proportion of higher-mass material in these samples appears to be rather small. The work presented in this paper suggests the method outlined above could be directly used for the characterization of heavier coal- and petroleum-derived liquid fractions. Acknowledgment. The authors would like to express their thanks to the European Union for funding this work under Project ref. RFC-PR04001, to the Spanish Scientific Research Council (I3P postdoctoral grant) for extending research leave to P.A., and to the Spanish Ministry of Education and Science (Jose Castillejo grant) for funding her stay. EF800333V

Characterization of Molecular Mass Ranges of Two Coal Tar Distillate

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