Molecular mass distributions and structural characterisation of coalderived liquids

A.A. Heroda,* , M.-J. Lazaroa, M. Dominb, C.A. Islasa, R. Kandiyotia

aDepartment of Chemical Engineering and Chemical Technology, Imperial College, University of London, London SW7 2BY, UKbDepartment of Pharmaceutical and Biological Chemistry, School of Pharmacy, University of London, Brunswick Square, London WC1N 1AX, UK

Abstract

Calibration data for size exclusion chromatography and results from the analysis of MALDI–TOF mass-spectra of coal derived liquidshave been presented. The work provides the means for obtaining much more quantitative information from these two techniques than hashitherto been possible. The polystyrene based calibration has been matched against elution times of a wide range of model compounds (PAH,azaarenes, other nitrogen bearing compounds, several dyes, polars and numerous oxygenated compounds), covering a molecular mass rangeup to 1086 u. Compounds in all groups appeared to elute with a predominantly size dependent mechanism. With the exception of thesomewhat atypical fullerene mixture,noneof the model compounds eluted at times so early as to be detected in the excluded region of thechromatogram (i.e. near 10.5 min). Several semi-quantitative methods have been used to establish the upper mass limit of correspondingMALDI–TOF mass-spectra, which can be safely considered as representing signal. Spectra of a coal tar pitch and its pyridine insolublefraction were analysed. The highest estimates of the high mass limits based on the calculation of number and weight average parameters was.300,000 u. A conservative estimate obtained with a new method based on subtracting multiples of the standard deviation from the signalgave 42,000 and 95,600 u, for the two samples, respectively. Fractions of the same coal tar pitch (separated by planar chromatography) havebeen characterised, to test for changes in structural features with changing MM-distributions. SEC and MALDI–TOF–MS showed decreas-ing MM-distributions with increasing mobility in planar chromatography. However,13C NMR and pyrolysis–GC–MS showed that theimmobile fraction contained greater proportions of aliphatic material compared to more mobile fractions. The mobile material consists of thearomatic systems normally associated with coal tar. The relatively immobile (larger-MM) fractions consisted of aromatic systems too large toelute through the chromatographic column (in pyrolysis–GC–MS) and linked together by aliphatic chains, which were released on pyrolysisand detected as major pyrolysis products.q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Coal derived liquids; Pyrolysis; Size exclusion chromatography; Mass spectrometry

1. Introduction

Most recent advances in work on molecular mass distri-butions of coals and coal-derived materials have taken placethrough the parallel use of size exclusion chromatography(SEC) and laser-desorption mass spectrometry. Progress inthe use of these two independent techniques have oftenleap-frogged each other—as well as providing supportingevidence for each other. In this paper, this sequence ofdevelopments (e.g. cf. Refs. [1–4]), will be reviewedbriefly, before describing our recent results.

In early characterisation work [5–7] by SEC, using tetra-hydrofuran (THF) as eluent, the upper limit of detectedmolecular masses (MMs) in pyrolysis tars and liquefactionextracts was in the 4000–6000 u range [8–10]. These results

were comparable with those from similar research involvingthe use of SEC coupled to calibrations based on the vapourpressure osmometry (VPO) of coal derived liquid fractions.Using pyridine as eluent, Haenel and Zander [11] foundaverage values up to 2500 u, whilst Larsen and co-workers[12,13] reported average MM-values of 5050 u, with indi-cations of masses up to possibly 15,000 u.

Neither THF nor pyridine may be safely assumed tocompletely dissolve many common coal derived liquids.When using, either THF or pyridine as the eluent, theprogressive increase in back pressure across SEC-columnshas been widely recognised as a symptom of the precipita-tion of sample out of solution. Clearly, some of the highermolecular mass materials were not being observed by SECin THF or in pyridine. Further, insufficient solvent power(notably of THF or pyridine) leads to adsorption effectsbetween solute and column packing, known to distortchromatograms—and lead to smaller apparent MM-distri-butions. The shortcomings of still weaker solvents, such as

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* Corresponding author. Tel.:144-0171-589-5111; fax:144-0171-594-5604.

E-mail address:a.herod@ic.ac.uk (A.A. Herod).

toluene, chloroform, dichloromethane, benzonitrile anddimethylformamide, have recently been reviewed [4]. Itdoes not appear possible to dissolve tar or extract samplesin these solvents completely. Results fromany solutionbased technique, obtained with samples dissolved in thelatter solvents, must therefore be regarded as reflectingonly the more soluble parts of the samples.

Nevertheless, on the basis of (albeit somewhat unsatisfac-tory) data from SEC alone, it could be concluded that coaltars or extracts contain significant proportions of materialwith MMs above 1000 u and that smaller amounts of thesematerials could have MMs well above 5000 u. However,results from SEC using THF or pyridine as eluents haveoften (and justly) been criticised for the structure-depen-dence of measured retention volumes, inaccuracies inherentin VPO-based calibrations at high MMs and the scarcity ofmodel compounds above 300 u that could be used as cali-brants. Confirmation of results from SEC by independenttechniques was clearly necessary, but much of the earlywork based on various mass spectrometric methods didnot show MMs much above 1000–1200 (e.g. cf. Ref.[13]). It is instructive to review briefly the problems encoun-tered in evaporating and ionising coal derived materials bysome of the more common mass spectrometric techniques.

Electron-impact based mass spectrometry of samplesevaporated from a heated probe rarely show the presenceof MMs much larger than 550–600 u. This is a result of thelimited amount of material that can be thermally evaporatedfrom the probe, even in the high vacuum of the mass spec-trometer. Using this method it is nevertheless possible toderive some structural information regarding the more vola-tile parts of the sample.

Chemical ionisation has not been reported to significantlyincrease the range of MMs observed in coal derived liquids.Among other ‘soft’ ionisation techniques, field-ionisationmass spectrometry (FIMS) has provided a more promisingroute, showing evidence of MMs up to 1500 u, although therange of observable material is restricted to whatever can beevaporated under vacuum. A short but useful review of thisearly work may be found in Ref. [14].

In contrast to these results, two mass spectrometric tech-niques where ionisation is thought to precede the release/evaporation of sample molecules from the sample, viz. fielddesorption (FD) and fast atom bombardment (FAB), havebeen reported to show material with upper limits at wellabove the 1000 u range [14,15]. Pyrolysis FD–MS of acoal sample applied to the emitter as a suspension in acetonewas reported to give ionic components up to 3000 u. FAB–MS off a moving belt interface showed trace amounts ofmaterial up to 2200 u. Ions up to nearly 4000 u have beenobserved in a different set of coal derived samples by FAB–MS [16].

At this early stage, SEC could still be considered to showlarger MM-distributions compared to all available mass-spectrometric techniques. The next phase of our work wasbased on laser-desorption mass spectrometry and showed

much larger upper MM-limits in coal derived materialsthan could be detected by SEC. Initial results from aLIMA (laser ionisation mass analysis) instrument gavespectra for pyrolysis tars and a coal tar pitch, showinglarge peaks between 1000–2500 u and traces of materialat up to the limit of the instrument (12,000 u) [17–19].Detailed experiments were carried out to verify these resultsand to show that the reported large MM-signal did not origi-nate from the effect/action of the laser on the sample. Thiswork has been summarised in Ref. [1].

The next phase of the MS-based work was carried outusing a MALDI (matrix assisted laser desorption/ionisation)mass spectrometer, which—for this type of work—presented a number of advantages compared to the LIMAinstrument: lower laser power levels to avoid fragmentation,software enabling the co-addition of spectra, use of matrixand a much larger molecular mass detection limit(,270,000 u). It was shown that removal of the instru-ment’s reflectron from the flight path improved sensitivityto high-mass material. Results obtained with this instrument[20–24] showed agreement with what seems to be knownabout the nature of the samples (e.g. with coal rank, effect ofhydrocracking on extracts, age and maturity of kerogens).For pyrolysis tars and liquefaction extracts, large peakswere observed between 1000–5000 u with progressivelydiminishing signals at up to 20–30,000 u. In the case ofcoal samples, traces of material up to the high mass limitof the instrument were detected (cf. summary in Ref. [7]).

Considerable effort has been expended in investigatingthe nature of experimental parameters affecting the qualityof MALDI–MS spectra of coal derived materials [3,25].

1. Laser fluence:Laser fluence has been shown to affectspectra. High power levels lead to the formation ofcarbon clusters [26] whilst, at lower power levels,small increases have been observed to assist in accentu-ating features of the spectra.

2. Ion-extraction voltages:Relatively little high mass signalcan be observed below ion-extraction voltages of 20 kV.Increasing the ion-extraction voltage has been shown toassist in detecting higher masses by providing a morecomplete inventory of already ionised species [3]—with-out otherwise disturbing the sample. To date, ourexperiments have been limited to 30 kV, the upperlimit of the ion extraction voltage available on mostmodern commercial instruments.

3. Chemical composition of the matrix:For some samples(e.g. pyridine insoluble fraction of the coal tar pitch),greater MM-distributions may be observed by alteringthe chemical composition of the matrix. Using thesame matrix, it has been observed that differences inthe chemical structure and composition of the samplecould in some instances be instrumental in showinggreater or lesser molecular mass distributions, apparentlydue to the ways in which the sample and matrix interactunder laser radiation [27].

A.A. Herod et al. / Fuel 79 (2000) 323–337324

4. Fractionation of complex mixtures:(e.g. tar pitch) bysolvent separation or planar chromatography has enabledobservation of larger MM materials with greater clarity[28,29]. The effect is well known from prior work inother laboratories on polymer mixtures [30–36].MALDI–MS spectra of mixtures of polystyrene stan-dards [37] have been used to explain the extent towhich spectra were sensitive to the polydispersity ofsamples. We have found that relative concentrations ofhigh mass materials in coal derived liquids may begreatly underestimated, when determinations take placein the presence of a greater abundance of low mass mate-rial (i.e. ,2000 u) [38].

More recently, a comparison of results from MALDI–MSand252Cf-Plasma Desorption MS has been carried out usingcommon samples. The two MS-instruments differed in theirionisation systems (i.e. plasma vs. laser desorption) and inthe maximum available ion extraction voltage: 30 kV for theMALDI–MS instrument and 15 kV for the PD–MS. Thecomparison of plasma vs. laser desorption mass spectro-scopy could not therefore be carried out at comparable ionextraction voltages. Work at up to 30 kV in the MALDI-instrument indicated greater sensitivity to high-massmaterials at higher ion extraction voltages. The qualitativesimilarity of results from the two MS-techniques was never-theless apparent; the range of MMs observed in PD–MS aswell as in MALDI–MS were far larger than those hithertoreported by any other MS-based technique [39].

Returning to developments in SEC, removal of solubilitylimitations imposed by the use of THF as solvent and aseluent constituted the logical next step [2,28]. The use of1-methyl-2-pyrrolidinone (NMP) as eluent in SEC is due toLafleur and Nakagawa [40]. NMP appears to completelydissolve all the coal derived ‘liquids’ we have used todate, including a coal tar pitch—which is a solid at roomtemperature. Significantly, NMP does not lead to the buildup of back pressure in SEC columns, even during extendedperiods of use.

In their work, Lafleur and Nakagawa reported operatingwith a polydivinylbenzene column at ambient temperature.Their use of NMP allowed the detection of an additionalrange of materials in coal derived liquids with considerablyshorter retention times (i.e. larger apparent MMs) than thoseobserved in SEC using THF as eluent. In particular a largepeak was reported near the exclusion limit of the column[40]. However, some polar model compounds eluted attimes, which were far shorter than would have beenexpected from the column calibration against polystyreneMM-standards. In attempting to characterise the materialeluting near the column exclusion limit, they used heatedprobe-MS, which showed little ion abundance between 250and 500 u, the upper limit of the scan. These findingswere interpreted in terms of a multimode mechanismfor SEC, based on molecular size but with earlier-than-expected elution of polar molecules. The overall

conclusion from the work was that materials observedat/near the exclusion limit of the column were aggre-gates of smaller, polar molecules, held together by ionicforces, andnot of large molecular mass. The addition ofLiBr to NMP has recently been claimed to dissipate theseionic forces; the LiBr was shown to have shifted chromato-grams to much longer elution times [41] (i.e. smallerapparent MMs).

In our work, the addition of LiBr to the eluent NMP wasfound to shift chromatograms of twonon-polarsamples (anaphthalene mesophase pitch and a mixture of C60 and C70fullerenes) to longer retention times, i.e. to smaller apparentMMs. We have interpreted this to mean that the LiBr wasaltering column performance, irrespective of samplepolarity. More to the point, LiBr addition caused SEC chro-matograms to shift to retention times longer than thepermeation limits of the analytical columns. LiBr additionappears to adversely affect the solvent properties of NMPand to promote surface sorption mechanisms rather than sizeexclusion. Pumping pure NMP for some hours was requiredto restore the column to its original range of exclusion andpermeation limits, with no permanent damage to thecolumn. In the same study, we have observed large‘excluded’ peaks in the SEC-chromatograms of thenaphthalene mesophase pitch, an observation clearly unli-kely to be related to ‘aggregates of smaller, polar molecules,held together by ionic forces’. MALDI-mass spectra of thesame naphthalene pitch have shown MM-distributions withlarge high-mass limits [42].

In view of the outline presented above, the perceivedabsence of high-MM materials in the heated probe-MSanalysis performed by Lafleur and Nakagawa [40] may bereasonably interpreted in terms of inherent limitations of theheated probe-MS method. Clearly, there are sharp differ-ences between results from heated probe-MS and thoseobtained by numerous workers using MS techniqueswhere ionisation is thought to take place prior to evapora-tion (e.g. FD, FAB, laser-desorption) [14]. In our work, wehave found MMs of material showing under the excludedpeaks in SEC [2] to correlate with MALDI mass-spectrashowing large MMs—and for some samples extending toabove 100,000 u (Refs. [3,28,43]).

In examining coal pyrolysis tars, extracts, coal tar pitchesand similar coal derived liquids the point has thus beenreached where:

1. A large proportion (at times exceeding 50% by weight) ofsome samples appear likely to be composed of materialwith MMs in the 1000–5000 u range.

2. Trace amounts of some of these samples appear byMALDI to have much higher MMs, possibly extending(depending on the nature of the sample) from 10,000 or20,000 u to values as high as 300,000 u or above. To datehowever, the evaluation of a safe ‘true’ upper mass limit,where signal is clearly distinguished from instrumentnoise in MALDI, has not been arrived at; these

A.A. Herod et al. / Fuel 79 (2000) 323–337 325

evaluations have largely been comparative andqualitative.

3. Overall, the results from MALDI appear qualitativelyreasonable: both the magnitudes and distribution of thesignal in the mass range 1000–5000 u, as well as appar-ent values of high mass limits appear to change inaccordance with what we think we know about thesamples we have used.

4. A catalogue of MALDI mass-spectra has been accumu-lated on a few well defined samples to demonstrate theeffect of experimental parameters which affect the data:laser fluence, ion extraction voltage, sample polydisper-sity, chemical composition of the matrix, the method ofmixing sample and matrix, and the relative proportions ofsample and matrix.

Methods have been devised to enhance MALDI mass-spectra by manipulating some of these parameters, viz.maximising laser fluence and ion extraction voltages inMALDI-instruments [3,43], empirically attempting to opti-mise the chemical composition of the matrix and the mannerof its application (and sample mounting) [3,27] and finallyreduction of sample polydispersity (notably by solvent orplanar chromatographic separation [44,45]) in order toenhance the intensity of high mass signal.

To date, therefore, laser desorption mass spectrometryhas provided a valuable tool, qualitatively supportingSEC-based findings and providing estimates of absoluteMM-distributions in coal derived liquids. For its part, SECusing NMP as eluent has indicated the presence of appar-ently very large MM-material eluting near the exclusionlimit of the column. Passing beyond inhibitions regardingpolar aggregates, SEC in NMP has proved a sensitive toolfor comparing chromatograms of different samples.However, the vista offered by Ref. [31], regarding shifts

in retention times according to polarity, has somewhatdiminished the scope for attempts at quantifying findingsand calibrating retention times. Further, whilst the combina-tion of SEC and MALDI–TOF–MS has proved useful inMM-determinations, it does not allow the evaluation ofother chemical structural features in any detail.

The present paper describes our more recent work on thecharacterisation of coal derived liquids. The work hasfocused on three interrelated areas. First, the activity inSEC and MALDI–TOF–MS has aimed to explore prospectsfor extracting more quantitative information from the datathan has hitherto been possible. The SEC study seeks toestablish a relationship between a wide range of modelcompounds and a polystyrene-standard based calibrationfor operation with NMP as the eluent. Secondly, MALDImass-spectra have been analysed, in order to establish theupper mass limit which can be safely considered as repre-senting the actual signal from the sample. The third aspectof the work has involved an attempt to distinguish betweendominantstructural features of sample fractions separatedby solvent fractionation or by planar chromatography. Theanalytical methods used,13C NMR and pyrolysis–GC–MS,are well known, but their application to separated samplefractions which have been also characterised by SEC andMALDI–TOF–MS has led to some unexpected results.

2. Experimental

2.1. Samples

The coal tar pitch sample has been studied previously(e.g. Refs. [25,29,44]). Its elemental composition was C:91.4%, H: 4.1%, N: 1.3%, S: 0.76% and O: 2.4% (by differ-ence). The coal tar pitch is used as the standard in this

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Fig. 1. The polystyrene calibration curve for the Mixed-D column with NMP as mobile phase at 808C.

laboratory due to its abundance and uniform composition;about 20% of it is insoluble in THF and about 15% insolublein pyridine. Both the pyridine insoluble fraction and thewhole pitch itself can be completely dissolved in 1-methyl2-pyrrolidinone.

2.2. Size exclusion chromatography

A 30 cm long, 7.5 mm o.d. column, packed with 5mmpolystyrene-divinylbenzene polymer particles (‘Mixed-D’;Polymer Laboratories Ltd., UK) was operated at 808C, at aneluent flow rate of 0.50 ml min21. The solvent was NMP(peptide synthesis grade; Rathburn Chemicals, UK).

Detection was carried out using a Perkin–Elmer LC250variable wavelength UV-absorption detector. As NMP isopaque at 254 nm, detection of standard compounds aswell as the calibration against polystyrene standards(Fig. 1) was performed at 264 nm, where NMP is partiallytransparent.

Fig. 1 shows the polystyrene calibration curve for theMixed-D column, operated at 808C with NMP as eluent.The exclusion limit for this column is around 10.5 min,representing the high mass end of the linear part of thecalibration curve and corresponding to between 250,000–400,000 u in terms of polystyrene standards. Larger-MMpolystyrene standards elute at shorter times, but not

A.A. Herod et al. / Fuel 79 (2000) 323–337 327

Table 1Standard compounds in SEC

Polycyclic aromatic compounds MW te tcalc Dtcalc

Benzene 78 22.2 21.0 1 1.2Anthracene 178 21.2 19.9 1 1.3Picene 278 20.2 19.3 1 0.8Perylene 252 20.8 19.5 1 1.3Dibenzopentacene 378 20.6 18.9 1 1.7Fluorene 166 21.6 20.0 1 1.6Rubrene 532 19.1 18.5 1 0.6Fullerene 720–840 9.0 17.5–17.8 2 8.5 to 8.8Toluene 92 22.7 20.8 1 1.81-Methylnaphthalene 142 21.9 20.2 1 1.69-Methylanthracence 192 20.4 19.8 1 0.6Heterocyclic nitrogen compoundsPyridine 79 21.9 21.0 1 0.92,6-Diphenylpyridine 231 20.1 19.6 1 0.5Indole 117 20.0 20.50 2 0.52,3-Dimethylindole 145 20.2 20.21 2 0.1Carbazole 167 19.9 20.03 2 0.1N-phenylcarbazole 243 20.6 19.52 1 1.0Quinoline 129 21.2 20.4 1 0.92-Mercaptobenzothiazole 167 20.7 20.0 1 0.75,6-Benzoquinoline 179 20.9 19.9 1 1.04,4-Dimethyl 2,2-bipyridyl 184 21.3 19.9 1 1.42-Aminofluorene 181 19.4 19.9 2 0.51-Acetaminopyrene 259 19.3 19.4 2 0.1N,N0-bis (3-aminophenyl)- 573 17.6 18.4 1 0.73,4,9,10-perylenetetracarboxylic diimideAlcian yellow 838 18.0 17.9 0.2Alcian blue (pyridine variant) 1086 21.2 17.5 1 3.7Oxygenates and polar compoundsAcetone 58 21.5 21.4 1 0.1Anthraquinone 208 20.7 19.7 1 1.01,2-Benzanthraquinone 258 20.5 19.4 1 1.13,4,9,10-Perylen tetracarboxylic dianhydride 392 19.4 18.9 2 0.5Benzoic acid 122 19.6 20.4 2 0.8Salicylic acid 138 16.9 20.3 2 3.43-Dimethylaminobenzoic acid 165 19.5 20.0 2 0.53,5-Dimethoxybenzoic acid 182 19.2 19.9 2 0.79-Anthracene carboxylic acid 222 16.4 19.7 2 3.2Phenol 94 19.9 20.8 2 0.9Pyrogallol 126 18.8 20.4 2 1.5Coniferyl alcohol 180 18.6 19.3 2 1.3Sinapyl alcohol 210 18.6 19.7 2 1.1Stearyl alcohol 270 19.4 19.4 0.0

according to the same linear relationship: log10�MM � �8:72352 0:3246× elution time �min� : (regression coeffi-cient R2 � 0:9958�. In this work, elution times of modelcompounds have been determined; differences betweenexperimental elution times (te) and those calculated fromthe polystyrene calibration (tcalc) have been used to defineDtcalc:

Dtcalc� te 2 tcalc:

2.3. MALDI–TOF–MS

MALDI mass-spectra were acquired with a Fisons VGTOFSPEC mass spectrometer (VG Organic, Manchester,UK) fitted with a nitrogen UV laser (337 nm) and a VAX4000-base data system with OPUS software. The linear TOFmode was used at an accelerating voltage of 28 kV, withmaximum available laser power. For each sample, spectrawere calculated from 10, 30, 50 and 100 co-added scans.During this procedure, operation was visually checked toensure that the signal did not decay to noise—due to localsample depletion. The full mass range was examined. Thesample was applied to the target in NMP (or pyridine) solu-tion and dried in a vacuum oven for 2 h before insertion inthe mass spectrometer ion source. The use of the matrix didnot appear to alter the spectra of the coal tar pitch; thespectra presented below have been obtained in the laserdesorption mode (i.e. no added matrix). Spectra of the pyri-dine insoluble fraction have been obtained with 2-mercap-tobenzothiazole (MBT) as matrix; when deposited first,MBT was lost along with the NMP solvent during vacuumdrying at about 1008C. The matrix solution was added afterdeposition of the sample.

2.4. Thin layer chromatography

Procedures for the fractionation of samples by thin layerchromatography have been presented elsewhere [29,44,45].The coal tar pitch was separated by planar chromatographyinto the following fractions: (A) immobile in pyridine; (B)mobile in pyridine but immobile in acetonitrile; and (C)mobile in both solvents.

2.5. Pyrolysis–GC–MS

Analyses were performed using a Chemical Data SystemCDS1000 pyroprobe connected to a Carlo Erba 4130 gaschromatograph, in turn coupled to a Finnigan 4500 massspectrometer (Finnigan, Hemel Hempstead, UK). The GCfeatured a 50 m CP Sil-5 CB column (0.32 mm i.d., filmthickness 0.4mm). The procedure has been described else-where [46]. Briefly, samples were dried to remove NMP,loaded onto quartz wool held in quartz tubes, and heated in aflow of helium for 10 s. The whole pitch and the planarchromatographic fractions were examined at 7708. Thepyrolysis interface was held at 2508C and the GC injectorheld at 2508C. The temperature of the GC–MS transfer line

was 3108C. The temperature of the GC oven wasprogrammed as follows: isothermal at 358C for 5 min;raised at 48C min21 to 3108C; isothermal at 3108C for15 min. The MS was operated in full scan mode over the40–650 u range at 1 scan s21, 70 eV electron energy.

2.6. Solid state13C NMR

High-resolution solid-state13C NMR spectra of the threefractions of pitch derived from thin layer chromatographywere recorded at 75.5 MHz (7.05 T) on a Bruker MSL300spectrometer, as described previously [46].

3. Results and discussion

3.1. Correlating elution times of model compounds with thepolystyrene-standard based calibration

In our recent work [4], there have been indications thatSEC in NMP may not suffer from absorptive effects asseverely and that the level of structural dependence foundby earlier workers [40] may be, at least in part, avoided.Differences between our operating temperature (808C vs.ambient) and in the packing material of the analyticalcolumn itself (‘Mixed-D’ analytical column describedabove) suggested that a more detailed investigation maybe worthwhile.

A new study was thus undertaken to match the polystyr-ene-based calibration against elution times of a range ofmodel compounds. Table 1 compares measured andcalculated retention times for a selection of neutral andmoderately-polar polycyclic aromatic compounds, azaar-enes and other nitrogen compounds, oxygenated and polarcompounds; a more complete list is shown in Ref. [47].

3.1.1. Polycyclic aromatic compoundsFor planar, predominantly cataannelated PAH,Dtcalc

values were found to be positive and about one-minutelonger than expected from the polystyrene calibration. ThelargestDtcalc value was found for toluene at 1.84 min. Theeffect of the incorporation of five-membered rings (fluorene,fluoranthene, rubicene), pendant phenyl groups (rubrene) ornaphthyl groups (1,1-binaphthyl) had little overall effect onelution times in comparison with other PAH standards.Taken together, compounds in this group appeared toelute with a predominantly size dependent mechanism.The major exception was the mixture of fullerenes�90%C60 1 10%C70�; which eluted at 8.97 min—quitedifferent from the theoretical value (of around 18 min),but in this respect fullerenes shows a somewhat odd beha-viour: the estimated solution volume is around 0.66 nm3

(Ref. [48]), corresponding to the a diameter of 1.1 nm,much larger than expected for PAH of similar mass but ofplanar rather than spheroidal shape.

For the PAH in Table 1, the elution times in the currentwork are much closer to the calibration curve obtained for

A.A. Herod et al. / Fuel 79 (2000) 323–337328

polystyrenes than was found previously. Whilst Lafleur andNakagawa [40] also found that PAH eluted mostly withpositive values ofDtcalc (possibly due to their compact mole-cular shapes) theirDtcalc values were larger, e.g. fluoreneeluted 5.6 min ‘late’ compared with 1.6 min in Table 1.Further, some unsubstituted aromatics (benzene, naphtha-lene, naphthacene, pentacene) eluted earlier than expectedand alkylated aromatic derivatives showed marked devia-tions from the calibration curves: octyl benzene anddimethyl naphthalenes eluted withDtcalc values between 5and 7 min [40]. These differencescouldbe attributed to thehigher column temperature, tending to reduce sorptioneffects [49], or possibly to the different polymer used asthe packing (polydivinylbenzene) in that study.

3.1.2. Azaarenes [50] and other nitrogen compoundsMost of the pyridines, pyrroles and amino substituted

compounds eluted within 1 min of the expected time indi-cated by the polystyrene calibration. More complex nitrogencompounds such asN,N0-bis (3-aminophenyl)-3,4,9,10-perylene tetracarboxylic diimide and the dye ‘alcian yellow’also eluted within the same elution volume margins. Amongthose examined, the only compound which deviated moresignificantly from the calibration curve was the pyridinevariant of the dye ‘alcian blue’, which eluted 3.7 min laterthan expected. The structure of this compound is aporphyrin ring structure containing Cu, with pyridine chlor-ide groups pendant from the benzopyrrole groups. Theseresults differ from a previous work [40] which showedsignificantly earlier elution than expected from the calibra-tion, with largenegativeDtcalc values for indole (28.4 min),for pyridine (26.8 min) and for carbazole (25.0 min),while for 2,6 di-t-butyl-4-methyl pyridine theDtcalc valuewas15.1 min.

3.1.3. Polar and oxygenated compoundsSome of these compounds are those known to be present

in coal liquids, such as phenols and dibenzofuran; otherslike 3,5-dimethoxybenzoic acid are compounds used asmatrices in MALDI- and FAB-mass spectrometry and arenot likely to be found in coal derived liquids.

Most of these polar compounds eluted by what appears asa size-dependent mechanism with elution times, within1 min of the expected value. Larger differences in elution

volumes were obtained for 9-anthracene carboxylic acid (amatrix material) and salicylic acid, both of which elutedearlier than expected by more than 3 min.

Differences from values expected on the basis of thepolystyrene calibration were significantly and systemati-cally smaller than those reported in Ref. [40], where acetonewas found to elute earlier than expected by 10.7 min anddihydroxybenzenes by between 10.7 and 13.3 min; thegreatest deviation was observed for succinic acid, whicheluted 20 min earlier than expected.

3.1.4. Effect of column temperatureIn order to investigate the effect of the column operating

temperature in isolation, elution times of four compoundswhich gave largeDtcalc values in Ref. [40], but not in thepresent work (acetone, pyridine, carbazole and fluorene)were examined at ambient temperature. Table 2 showsthat the effect of temperature on retention times of all fourcompounds and a polystyrene standard was small. At ambi-ent temperature, the relatively low mass standards elutedslightly later, with differences ranging from 0.6 to0.9 min; these shifts were minor compared to a previouswork [40], where acetone eluted 10.7 min earlier, pyridineeluted 6.8 min earlier, carbazole eluted 5.5 min earlier andfluorene eluted 5.6 minlater than predicted by thepolystyrene calibration. These data suggest that thecolumn-operating temperature alone is not the reason forthe different behaviour observed here; other factors mayinclude the material of the column packing.

The standards used in this work are all of relatively smallmolecular mass with the largest at 1086 u, alcian blue; theelution times all correspond to those of relatively smallmolecules and there are no serious differences from theapproximation presented by polystyrene standards andnooverlap into elution times corresponding to exclusion fromcolumn porosity. However, the SEC profiles of coal-derivedsamples in NMP solution all show material excluded fromthe porosity of the columns used, suggesting that the MMsare far larger than any of the standards either used here oravailable in the chemicals catalogues. It would appear thatthe polystyrene calibration in the present column representsthe most reasonable compromise for SEC in NMP solvent.

3.2. Definition of the high-mass limit in MALDI mass-spectra

The information that may be gleaned from MALDI mass-spectra of coal derived liquids appears quite sensitive to theexperimental parameters used. Further work is required toimprove and systematise the quality of spectra that can beobtained from different types of fossil fuel derived samples.However, it is also desirable to evaluate and quantify infor-mation that can be extracted from these spectra. To date, theidentification of the high-mass limit in individual spectrahas largely been a qualitative procedure relying largely oncomparisons between spectra.

A.A. Herod et al. / Fuel 79 (2000) 323–337 329

Table 2Effect of column temperature on elution times

Compound Elution time (min) attemperature

Difference (min)

808C Ambient

Acetone 21.5 22.7 1.2Pyridine 21.9 22.7 0.7Carbazole 19.9 20.6 0.7Fluorene 21.6 22.7 1.0Polystyrene (580 u) 19.6 20.6 1.0

In order to analyse the results numerically, mass spectrawere converted into text files and accessed using the remoteVAX data system via the OPUS software. The data couldthen be examined using an ordinary desktop computer. Soft-ware was written in TurboPascal to calculate the baseline,standard deviations, number and mass average molecularweight averages (Mn and Mw, respectively) and the mole-cular weight dispersion (MWD) or polydispersity.

Fig. 2 presents MALDI mass-spectra composed of 10, 30,50 and 100 co-added scans of the pyridine insoluble fractionof the coal tar pitch; MBT was used as the matrix. The noiselevel in the spectra decreased with increasing numbers ofco-added scans. Standard deviations calculated for the last2% of the spectrum (near the detection limit of the instru-ment) diminished from 31.0 (arbitrary intensity units) for 10scans to 16.2 for 30 scans, 12.3 for 50 scans and 10.6 for 100scans. Reproducibility of the results was satisfactory: forspectra recorded on any one day; the average noise leveland the averaged signal at the detection limit remainedapproximately constant.

Several semi-quantitative methods have been used toestablish the upper mass limit, which can be safelyconsidered as representing the actual signal in MALDImass-spectra.

3.2.1. The ‘modified’ method of Montaudo et al. [51]The baseline was defined as the average signal intensity at

the limit of detection of the instrument. In the first approx-imation, any intensity greater than the baseline wasconsidered as the ‘real’ signal. Because of the higher levelof noise in our spectra our base line was calculated from theaverage value of the last 5% of the raw data at high mass.The procedure then consisted of calculating the Mw and Mn

as a function of increasing mass, starting with the intervalbetween the low-mass value and the peak of maximumintensity. The true high mass end of the spectrum wasthen determined by identifying the molecular mass wherethe slopes of the Mw and Mn vs.m/zplots tended to zero (seeRef. [51] for details). As may be noted from Tables 3 and 4,this method gave by far the largest values of the true high-mass limits.

3.2.2. Standard deviation of the signal at the limit ofdetection

The average value of the signal and the standard deviationwere calculated for the final part of the spectrum, nearthe detection limit of the instrument (usually the last 5%of the whole interval), were calculated. The average value ofthe signal thus calculated was taken as the height of thesmoothed baseline. ‘Signal above noise level’ could thenbe defined as the signal with greater intensity than the base-line value plus the value of the standard deviation,s .Increasingly more conservative estimates were obtainedby considering the true high mass limit of the spectrum asthe point where the signal exceeded the baseline value plusmultiples of (2, 3, 5 times) the value of the standard devia-tion (s ). Although an informed (arbitrary!) decision isinvolved in the procedure, we consider that the baselinevalueplus5s provides a sufficiently conservative criterion,allowing the resulting true high mass limit to be consideredas entirely safe. Mass and number averaged MMs were thencalculated for data from the selected low-mass limit up tothe newly calculated true high-mass limit.

3.2.3. Slope of signal lifting-off from baselineIn the third evaluation method, lift-off from baseline at a

pre-set angle was taken as the criterion for determining thetrue high-mass limit of the spectrum for the particularsample. The baseline was calculated as outlined above.The slope was determined by a ‘least-squares’ calculationover a data interval corresponding to 2% of the whole dataset; in the initial calculation, the true high mass data wasconsidered to have commenced when the slope exceeded anangle of one-degree above horizontal. When smaller angleswere selected, the high-mass limit rapidly moved to highervalues. At the time of writing, ongoing work suggested thata less arbitrary choice of the upper mass limit may evolvefrom this approach.

3.2.4. Comparison of the three methodsThree procedures have been described in great detail

A.A. Herod et al. / Fuel 79 (2000) 323–337330

Fig. 2. MALDI mass spectra of the pyridine insoluble fraction with MBT asmatrix. 10, 30 50 or 100 spectra were summed, as marked—showing theintensification of the signal with respect to instrument noise as the numberof co-added spectra is increased.

elsewhere [52]. Tables 3 and 4 present results from theapplication of these methods to MALDI mass-spectra,respectively, of the (‘whole’) coal tar pitch and the pyridineinsoluble fraction of the coal tar pitch, the latter acquired inthe presence of the matrix (MBT).

Tables 3 and 4 show that the true high mass limits calcu-lated by the ‘Montaudo method’ indicated by far the highestvalues of the three methods considered. The method appearsto detect the same high-mass limit that can be observedvisually—as the just perceptible shift from the baseline,near the detection limit of the instrument (e.g. cf. Fig. 3 inRef. [52]).

Considering the method of subtracting multiples of thestandard deviation from the signal: as the criterion for iden-tifying signal (above noise level) was made progressivelymore severe, progressively smaller true high mass limitswere calculated. However, Tables 3 and 4 show that evenwith the most conservative estimate (involving 5s abovethe baseline), the high mass limits for the two samples(the pitch and its pyridine insolubles fraction) wereobserved near 42,000 u and between 95,000–96,000 u,respectively. Although absolute upper mass limits stillremain a matter for debate, the estimate involving the useof 5s appears to be a reasonably safe estimate of the truehigh mass.

Comparing results between Tables 3 and 4, MMs of the(whole) coal tar pitch (Table 3) were found to extend tosmaller ranges of MMs. This result is in line with previouswork on these samples. The whole pitch clearly contains thesame range of higher mass (pyridine-insoluble) material asthe pyridine-insoluble fraction itself, but the pyridine-soluble fraction makes up about 85% of the present pitchsample. Material from the more abundant (in this case thepyridine-soluble) fraction generally tends to dominate thespectrum. The MALDI mass-spectra of the (whole) pitchand that of the pyridine-soluble fraction have previouslybeen found to be quite similar; signal due to the highermass material may be observed much more clearly in frac-tions with a narrower range of MMs [25,38].

Tables 3 and 4 also show that, (for an angle of 18) themethod of tracking the signal lift-off from the baseline gavethe lowest estimates of the true high mass limit between thethree methods. The choice of angle is clearly arbitrary butthe method is capable of development.

3.3. Differences in chemical structural features of pitchfractions separated by planar chromatography

Apart from MM-distributions, relatively little structuralinformation can be inferred from the use of SEC and

A.A. Herod et al. / Fuel 79 (2000) 323–337 331

Table 3Parameters calculated from the laser desorption spectra (no matrix) of the coal tar pitch

Method ofcalculation

Number ofscans summed

Calculated ‘true’high mass limit (u)

Mw (u) Mn (u) Mw/Mn Elution timea

(min) for Mn

Montaudo method (Ref. [51])10 399,800 87,000 2900 30.4 16.230 170,500 35,200 1600 21.9 17.050 270,200 69,200 2300 30.1 16.5

100 350,600 85,100 3600 23.5 15.9Standard deviations methodSt dev 1s 10 54,400 15,200 1400 10.9 17.2

30 54,400 14,000 1200 11.9 17.450 109,300 31,700 1800 17.8 16.8

100 177,300 55,600 2900 18.9 16.2St dev 2s 10 24,600 6,100 1000 5.9 17.6

30 31,500 7,700 1000 7.9 17.650 57,700 15,400 1400 11.3 17.2

100 109,300 35,100 2400 14.7 16.5St dev 3s 10 14,600 3400 900 3.9 17.8

30 8,800 1900 700 2.7 18.150 39,400 9700 1200 8.3 17.4

100 79,500 24,900 2100 12.1 16.6St dev 5s 10 7,600 1800 700 2.4 18.1

30 8,800 1900 700 2.7 18.150 16,600 3400 900 3.9 17.8

100 42,200 11,700 1500 7.6 17.1Slope methodSlope: 1-degree 10 10,000 2100 800 2.8 17.9

30 8,000 1600 700 2.5 18.150 9,700 2000 800 2.6 17.9

100 12,400 2900 1000 3.0 17.6

a Elution time corresponding to Mn in the polystyrene calibration (Fig. 1).

MALDI–TOF–MS alone. The parallel utilisation of UV-fluorescence spectroscopy has enabled comparison of therelative abundance of polynuclear aromatic ring systemsby shifts to longer wavelengths and diminishing fluores-cence quantum yields for large polynuclear aromatic ringsystems. In order to establish the methodology of relatingstructural features of individual sample fractions to theirMM-distributions, the same coal tar pitch has been sepa-rated by planar chromatography into three fractions: (A)immobile in pyridine; (B) mobile in pyridine but immobilein acetonitrile; and (C) mobile in both solvents. More shar-ply defined fractions can be obtained by planar chromato-graphy compared to separation by solvent solubilityfractions [53].

Fig. 3 compares size exclusion chromatograms of thethree fractions, with UV-absorbance detection at 300 nm,showing marked differences between their molecular massdistributions. Comparing with polystyrene standards, at theexclusion limit of the column, the polystyrene standard ofmass 300,000–400,000 u eluted at about 10 min and thepolystyrene standard of mass 1.84 million u at about9 min. The fraction immobile in both solvents (A) showeda big peak near the exclusion limit of the column with asmall one between 16 and 22 min showing that this fractionis composed mainly of material of large molecular mass.

However, fraction (C) (mobile in both solvents) showed anintense peak between 17 and 23 min with almost no materialnear the exclusion limit. The partly mobile fraction (B)showed a peak near the exclusion limit and a peak between16 and 21 min. The three fractions have been examined bypyrolysis–GC–MS [46] at 770 and 13008C and structuralconfirmation sought by NMR methods [solution state1HNMR and solid state13C NMR (CPMAS TOSS)].

Fig. 4(a) and (b) present pyrolysis–GC–MS profiles at7708C of (a) the whole pitch and (b) Fraction (A) (immo-bile). Polycyclic aromatic hydrocarbons identified in thepyrolysis chromatograms of the (whole) coal tar pitch andits fractions have been have been labelled in the figures bynumbered peaks and are listed in Table 5; solvent-derivedpeaks have been marked in the figures as S, while alkenepeaks are shown by A. All of the pyrograms are shown inRef. [46].

3.3.1. Pyrolysis at 7708CThe chromatogram of the whole pitch in Fig. 4(a) shows a

series of polycyclic aromatics from naphthalene�m=z�128� up to benzopyrenes�m=z� 252�; the upper limit ofthe present column. The components observed includediphenyl �m=z� 154�; dibenzofuran�m=z� 168�; fluorene�m=z� 166�; phenanthrene and anthracene (m/z 178),

A.A. Herod et al. / Fuel 79 (2000) 323–337332

Table 4Parameters calculated from the MALDI mass-spectra of the pitch pyridine insolubles spectrum acquired using MBT as the matrix

Method ofcalculation

Number ofscans summed

Calculated ‘true’high mass limit (u)

Mw (u) Mn (u) Mw/Mn Elution timea

(min) for Mn

Montaudo method (Ref. [51])10 291,400 61,600 7600 8.2 14.930 421,500 87,700 10,900 8.1 14.450 371,300 73,700 8200 9.0 14.8

100 391,400 82,300 10,000 8.2 14.6Standard deviations methodSt dev 1s 10 14,700 38,300 6300 6.1 15.2

30 252,100 68,600 10,100 6.8 14.550 293,700 70,100 8500 8.2 14.8

100 266,300 68,300 9800 7.0 14.6St dev 2s 10 89,000 23,300 5300 4.4 15.4

30 165,200 48,400 8900 5.5 14.750 167,700 45,200 7400 6.1 15.0

100 181,600 51,400 8900 5.8 14.7St dev 3s 10 69,300 10,900 4200 2.6 15.7

30 110,100 33,400 7700 4.4 14.950 112,500 35,300 6800 5.2 15.1

100 144,400 42,200 8200 5.1 14.8St dev 5s 10 38,400 10,900 4200 2.6 15.7

30 65,300 21,200 6300 3.4 15.250 96,700 27,600 6200 4.4 15.2

100 95,500 28,400 7100 4.0 15.0Slope methodSlope: 1-degree 10 23,000 7300 3800 1.9 15.8

30 22,000 7800 4300 1.8 15.750 20,000 6400 3600 1.8 15.9

100 22,000 7500 4100 1.8 15.7

a Elution time corresponding to Mn in the polystyrene calibration (Fig. 1).

carbazole�m=z� 167�; fluoranthene and pyrene�m=z�202�; benzofluorenes�m=z� 216�; benzo (ghi)fluoranthene�m=z� 226� and chrysene isomers�m=z� 228�: Other peaksinclude the methyl and some dimethyl derivatives of thearomatics. Some of the aromatic components may beconsidered as indicative of a high degree of pyrolysis,such as cyclopenteno (def) phenanthrene�m=z� 190� possi-bly formed by cyclisation of a methyl phenanthrene, and athird isomer of m=z� 202 which may be a benzoacenaphthylene and elutes between fluoranthene andpyrene. No significant alkane or alkene components weredetected in the pyrolysis product.

The pyrolysis chromatogram of the most mobile fraction,(C), (not shown) was similar to that of the (whole) coal tarpitch, being composed of polycyclic aromatics fromnaphthalene to benzopyrenes; the components detectedhave been discussed elsewhere [46]. Briefly, a major differ-ence in comparison with the whole pitch was the loss oflight ends from the planar chromatographic fraction, duringthe evaporation of the NMP used for extracting the samplefrom the plate silica. Also, the more highly pyrolysedaromatics such as acenaphthenem=z� 154 and acenaphthy-lene m=z� 152 were detected in the fraction, as well assulphur containing aromatics, identified on the basis oftheir m=z values in comparison with a previous work [54]using high resolution GC–MS and a sulphur-specific detec-tor. Two alkene peaks appeared neither of which wereobserved in the pitch. The mass spectra suggested one ofthem to be an isomer ofm=z� 168; possibly dodecene; theother alkene gave no molecular ion although the elutionposition near to the C17 alkane may suggest a similar

carbon number. The presence ofm=z� 73 in the spectrumhowever would indicate an oxygen containing compound. Afurther alkene was detected underlying the peak foracenaphthenem=z� 154; which would correspond to aC15 compound with no molecular ion. In addition, smallpeaks corresponding to 1-methyl-2-pyrrolidinonem=z� 99and its oxidation product [55] 1-methyl succinimide weredetected.

The chromatograms of the other two TLC fractions (Aand B) were different from that of the mobile fraction (C), inshowing mainly aliphatic components with only smallsignals for aromatics within the chromatographic range.For both fractions, the peaks corresponding to NMP and1-methyl succinimide are of major intensity in the chroma-tograms, partly because relative intensities of othercomponents were reduced. For fraction (B), mobile in pyri-dine (not shown), the pattern of major PAH was differentfrom Fig. 4(a): smaller aromatics including benzene,toluene and xylenes were detected before the peaks derivedfrom the extraction solvent, NMP, with naphthalene andmethyl naphthalenes appearing at longer times. The majoraromatics detected have been described in detail elsewhere[46]. The three alkenes or cycloalkanes observed in themore mobile fraction were observed in this fraction butwith different relative intensities.

The chromatogram of the pyridine immobile fraction (A)in Fig. 4(b), showed fewer solvent-derived peaks thanobserved from the pyridine mobile fraction and the NMPpeak was less intense. The alkene peaks were observed butin addition, an extra aliphatic peak corresponding tom=z�170; C12 alkane was detected. Only minor traces of

A.A. Herod et al. / Fuel 79 (2000) 323–337 333

Fig. 3. Area normalised size exclusion chromatograms of pitch fractions separated by planar chromatography into the following fractions: (A) immobile inpyridine and in acetonitrile; (B) mobile in pyridine but immobile in acetonitrile; and (C) mobile in both solvents. Detection by UV-absorption at 300nm.Reproduced from Ref. [46] (Correlation of pyrolysis–GC–MS and NMR spectra of pitch fractions separated by planar chromatography.q John Wiley & SonsLtd).

A.A. Herod et al. / Fuel 79 (2000) 323–337334

Fig. 4. Pyrolysis–GC–MS profiles at 7708C of (a) the whole pitch and (b) Fraction (A) (immobile). Peak numbers refer to Table 5. A indicates alkenes whilst Sindicates solvent-derived peaks. Reproduced from Ref. [46] (Correlation of pyrolysis–GC–MS and NMR spectra of pitch fractions separated by planarchromatography.q John Wiley & Sons Ltd).

aromatic peaks were observed, includingm=z� 128; 142,202 and 216.

These results indicate that the relatively immobile frac-tions (B and A) are different chemically from the mostmobile fraction (C). The mobile material consists of thearomatic systems normally associated with coal tar withno extensive alkyl substitution beyond methyl and ethylgroups, in agreement with results on fractions from TLCof the same pitch by GC–MS and by probe-MS. The rela-tively immobile fractions appear to consist of aromaticsystems too large to elute through the chromatographiccolumn after pyrolysis, linked together by aliphatic chains,which are released on pyrolysis and detected as the majorproducts. The aliphatic groups have been observed as onlythree different alkene pyrolysis products and one alkane,with proportions, which differ in the three fractions.

3.3.2. Solid state13C NMR characterisation of the threefractions

Fig. 5 presents the solid state13C NMR spectra of thethree pitch fractions from planar chromatography, clearlyshowing a relativereduction in aliphatic content—ongoing from fraction (A) (material immobile in bothsolvents) to fraction (C) (material mobile in both solvents).In terms of their SEC chromatograms, the aliphatic contentsappear to be greatest in the largest MM fraction. Althoughsomewhat counter-intuitive, these data are consistent with

the pyrolysis–GC–MS data outlined above and with paral-lel observations made by solution state1H NMR (notshown). The py–GC–MS and NMR experiments havebeen presented in greater detail elsewhere [46].

Most analytical techniques applied to the coal tar pitchwould only observe the material of the most mobile fraction,while that of the less mobile fractions would be effectivelyinvisible in the presence of the mobile material. However,the use of Planar Chromatography as a fractionation methodallows the relatively immobile material to be examined inisolation.

4. Summary and conclusions

New calibration data for SEC and results from the analy-sis of MALDI–TOF mass-spectra have been presented. Thework provides the means for obtaining a much more quan-titative information from these two techniques than hashitherto been possible. An attempt has been made toestablish methodology for relating structural features ofindividual sample fractions to their MM-distributions.

1. A polystyrene based calibration has been matchedagainst elution times of a range of model compoundscovering a molecular mass range up to 1086 u (the dyealcian blue). The work was carried out using compoundclasses ranging from polycyclic aromatic hydrocarbons,

A.A. Herod et al. / Fuel 79 (2000) 323–337 335

Fig. 5. Solid state CP–MAS TOSS13C-spectra of the coal tar pitch fractions separated by planar chromatography: (a) Fraction (A)—immobile in both solvents.(b) Fraction (B)—immobile in pyridine but mobile in acetonitrile. (c) Fraction (C)—mobile in both solvents. Reproduced from Ref. [46] (Correlationofpyrolysis–GC–MS and NMR spectra of pitch fractions separated by planar chromatography.q John Wiley & Sons Ltd).

azaarenes and other nitrogen bearing compounds, severaldyes, polars and numerous oxygenated compounds.Compounds in all groups appeared to elute with a predo-minantly size dependent mechanism. Results show muchless structural dependence than previous work; elutiontimes were much closer to the polystyrene calibrationcurve than was found previously [40]. With the exceptionof the somewhat atypical fullerene mixture,noneof themodel compounds eluted at times so early as to bedetected in the excluded region of the chromatogram(i.e. before 10.5 min).

2. The work was carried out with a column packing andexperimental conditions which differed from previouswork [40]. Our data show that the difference in columnoperating temperature does not explain the differentbehaviour observed here, other factors might includedifferences in the material of column packing.

3. Noise levels and standard deviations of MALDI–TOFmass-spectra decreased with increasing numbers (10,30, 50 and 100) of co-added scans. Several semi-quanti-tative methods have been used to establish the uppermass limit, which can be safely considered as represent-ing the actual signal. The methods have been comparedby analysing MALDI–TOF mass-spectra of a (whole)coal tar pitch and the pyridine insoluble fraction of thesame pitch, with the latter spectrum acquired in thepresence of the matrix (MBT). Among the three methodsconsidered, high-mass limits calculated by the modifiedmethod of Montaudo et al. [51] represented by far thehighest values (.300,000 u). This method appears todetect the same high-mass limit that can be observedvisually—as the shift from baseline, near the detectionlimit of the instrument. A more conservative estimateobtained with a new method based on identifying thesignal (above noise level), by subtracting multiples ofthe standard deviation from the signal, gave 42,000 and95,600 u, for the two samples, respectively.

4. The same coal tar pitch has been separated by planarchromatography into three fractions and characterisedby SEC, MALDI–TOF–MS,13C NMR and pyrolysis–GC–MS in order to test for changes in structural featureswith changing MM-distributions.

SEC and MALDI–TOF–MS have shown cleardifferences between the three samples, with apparent MM-distributions decreasing with increasing mobility in planarchromatography. Somewhat unexpectedly, however,13CNMR and pyrolysis–GC–MS have clearly indicated thatthe immobile fraction contained significantly greaterproportions of aliphatic material compared to fractionscontaining lower MM-materials which were more mobilein planar chromatography. The mobile material consists ofthe aromatic systems normally associated with coal tar withno extensive alkyl substitution beyond methyl and ethylgroups. The relatively immobile fractions appear to consistof aromatic systems too large to elute through the chroma-tographic column (in pyrolysis–GC–MS), linked togetherby aliphatic chains which are released on pyrolysis anddetected as major pyrolysis products. The aliphatic groupshave been observed as only three different alkene pyrolysisproducts and one alkane, with proportions, which differ inthe three fractions.

Acknowledgements

The authors would like to thank the European union for aMare-Curie Fellowship to M.J. Lazaro, the NERC OrganicMass Spectrometry Facility at Bristol University for pyro-lysis–GC–MS and the University of London IntercollegiateResearch Service (ULIRS) for provision of NMR facilities.Support for this work by the British Coal UtilisationResearch Association (BCURA) and the UK Department

A.A. Herod et al. / Fuel 79 (2000) 323–337336

Table 5Polycyclic aromatics identified in pyrolysis–GC–MS of the pitch fractions(peak numbers refer to Fig. 4(a) and (b))

Peak no. m/z Formula Name or type

1 78 C6H6 Benzene2 92 C7H8 Toluene3 106 C8H10 Xylene isomers4 128 C10H8 Naphthalene5 142 C11H10 Methyl naphthalenes6 156 C12H12 Dimethyl naphthalenes7 152 C12H8 Acenaphthylene8 154 C12H10 Acenaphthene9 168 C12H8O Dibenzofuran10 166 C13H10 Fluorene11 184 C12H8S Dibenzothiophene12 178 C14H10 Phenanthrene, anthracene13 167 C12H9N Carbazole14 192 C15H12 Methyl phenanthrene isomers15 190 C15H10 Cyclopenteno[d,e,f]phenanthrene16 204 C16H12 Dihydrofluoranthene, phenyl

naphthalene17 206 C16H14 Dimethyl phenanthrene isomers18 202 C16H10 Fluoranthene, pyrene and isomers19 208 C14H8S Phenanthro[4,5–b,c,d]thiophene

isomer20 218 C18H10O Benzonaphthofuran21 216 C17H12 Methyl fluoranthenes,

benzofluorenes22 232 C17H12O Methyl benzonaphthofuran23 230 C18H14 Dimethyl fluoranthene isomers24 234 C18H10S Benzonaphthothiophene25 226 C18H10 Benzo[ghi]fluoranthene26 228 C18H12 Chrysene, benzophenanthrenes

and anthracenes27 248 C17H12S Methyl benzonaphthothiophenes28 244 C19H16 Trimethyl fluoranthene isomers29 242 C19H14 Methyl chrysene isomers30 256 C20H16 Dimethyl chrysene isomers31 252 C20H12 Benzofluoranthenes,

benzopyrenes, perylene32 266 C21H14 Methyl benzofluoranthene

isomers

of Trade and Industry under BCURA Contract Nos. B32aand B44 is gratefully acknowledged.

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