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RAPID COMMUNICATIONS IN MASS SPECTROMETRY

Rapid Commun. Mass Spectrom. 2009; 23: 2087–2098

) DOI: 10.1002/rcm.4104
Published online in Wiley InterScience (www.interscience.wiley.com
Molecular mass ranges of coal tar pitch fractions by mass

spectrometry and size-exclusion chromatography

F. Karaca1, T. J. Morgan2, A. George2, I. D. Bull3, A. A. Herod2*, M. Millan2

and R. Kandiyoti2

1Department of Chemical Engineering, Marmara University, Goztepe Campus, 34722 Kadikoy, Istanbul, Turkey2Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK3Organic Geochemistry Unit (OGU), Bristol Biogeochemistry Centre (BBRC), School of Chemistry, University of Bristol, Cantock’s Close, Bristol

BS8 1TS, UK

Received 5 February 2009; Revised 26 April 2009; Accepted 27 April 2009

*Correspoeering, ImE-mail: a

A coal tar pitch was fractionated by solvent solubility into heptane-solubles, heptane-insoluble/

toluene-solubles (asphaltenes), and toluene-insolubles (preasphaltenes). The aim of the work was to

compare the mass ranges of the different fractions by several different techniques. Thermogravi-

metric analysis, size-exclusion chromatography (SEC) and UV-fluorescence spectroscopy showed

distinct differences between the three fractions in terms of volatility, molecular size ranges and the

aromatic chromophore sizes present. The mass spectrometric methods used were gas chromatog-

raphy/mass spectrometry (GC/MS), pyrolysis/GC/MS, electrospray ionization Fourier transform ion

cyclotron resonance mass spectrometry (ESI-FTICRMS) and laser desorption time-of-flight mass

spectrometry (LD-TOFMS). The first three techniques gave good mass spectra only for the heptane-

soluble fraction. Only LDMS gave signals from the toluene-insolubles, indicating that the molecules

were too involatile for GC and too complex to pyrolyze into small molecules during pyrolysis/GC/

MS. ESI-FTICRMS gave no signal for toluene-insolubles probably because the fraction was inso-

luble in the methanol or acetonitrile, water and formic acid mixture used as solvent to the ESI source.

LDMS was able to generate ions from each of the fractions. Fractionation of complex samples is

necessary to separate smaller molecules to allow the use of higher laser fluences for the larger

molecules and suppress the formation of ionized molecular clusters. The upper mass limit of the

pitch was determined as between 5000 and 10 000u. The pitch asphaltenes showed a peak of

maximum intensity in the LDMS spectra at around m/z 400, in broad agreement with the estimate

from SEC. The mass ranges of the toluene-insoluble fraction found by LDMS and SEC (400–10 000u

with maximum intensity around 2000 u by LDMS and 100–9320u with maximum intensity around

740u by SEC) are higher than those for the asphaltene fraction (200–4000u with maximum intensity

around 400u by LDMS and 100–2680u with maximum intensity around 286u by SEC) and greater

than values considered appropriate for petroleum asphaltenes (300–1200u with maximum intensity

near 700u). Copyright # 2009 John Wiley & Sons, Ltd.

Above the molecular mass ranges amenable to analysis by

gas chromatography/mass spectrometry (GC/MS), detailed

definitions of mass ranges andmolecular structures of heavy

hydrocarbon liquids are difficult to arrive at. Such data are

relevant for the processing of these materials and their

behaviour in refinery plant as well as being of academic

interest. There has been vigorous discussion about how best

to achieve the characterization of such materials, mainly

focused on petroleum-derived asphaltene.1–7 One part of the

debate has focused on comparing petroleum asphaltenes

with coal-derived asphaltenes. It has been found that

petroleum asphaltenes are of larger molecular size and

mass range than those from coal liquids.1 However, the main

coal-derived asphaltene sample used in that comparison

ndence to: A. A. Herod, Department of Chemical Engin-perial College London, London SW7 2AZ, UK.

[email protected]

appears to have been prepared from a coal liquefaction

sample, with supporting data from asphaltenes derived from

three coals following unspecified treatments. There is thus

some uncertainty regarding the level of degradation of the

coal-derived samples that have been used in these charac-

terizations. As outlined below, in the present study, a coal tar

pitch was fractionated to provide a more tractable coal-

derived asphaltene for further characterization.

In previous work, we fractionated a pitch sample, used as

laboratory standard, by column chromatography and

characterized the fractions alongside equivalent fractions

derived from a petroleum refinery atmospheric residue.8 The

toluene-soluble fraction of the pitch and those of two other

coal-derived liquids gave (by size-exclusion chromatog-

raphy (SEC)) smaller molecular size distributions than the

petroleum-derived toluene-soluble fraction. All four samples

contained somematerial excluded from column porosity; we

Copyright # 2009 John Wiley & Sons, Ltd.

Table 1. Mass balance from the solvent fractionation of pitch

Mass balance PHS PTS PTI Total

Total sample/g 3.0480 9.4220 8.7393 21.2093Mass fraction 0.144 0.444 0.412 1.000

PHS: Pitch heptane-soluble; PTS: Pitch toluene-soluble; PTI: Pitchtoluene-insoluble.

2088 F. Karaca et al.

know little about the masses and structures of these

‘excluded’ materials. As already outlined2,3 it is possible

that some of this material consists of molecules that have

adopted three-dimensional conformations. Synchronous

UV-fluorescence spectra of the four toluene-soluble fractions

indicated that the chromophores of all the samples covered

the same range of fluorescencewavelengths. The spectrum of

the petroleum-derived sample appeared closest to that of the

toluene-soluble fraction of a low-temperature tar. The pitch-

derived asphaltene and the asphaltene derived from a coal

extract showed more intense fluorescence at longer wave-

lengths, suggesting that these two fractions contained the

largest fused aromatic nuclei of the four asphaltene samples.

The petroleum-derived asphaltene fraction thus showed a

molecular mass distribution extending to larger molecular

masses than the coal-derived samples. However, toluene-

insoluble fractions were more abundant in the original coal-

derived samples and appear to represent much higher

molecular weight ranges than do the toluene-solubles.

Some of the toluene-insoluble fractions derived from

column chromatography of petroleum- and coal-derived

liquids have been discussed previously.9 Planar chromatog-

raphy10 was also used for the fractionation of a petroleum

residue and the same pitch sample that was used in this

work. The planar chromatograms were developed using

pyridine, acetonitrile, toluene and pentane. The fractions

revealed by examination in daylight and under UV-

fluorescence were recovered and characterized by SEC in

the eluent 1-methyl-2-pyrrolidinone (NMP). SEC of all the

samples indicated increasing apparent molecular sizes with

decreasing mobility on the plate. The pyridine-mobile

fractions showed considerably larger molecular mass

distributions than the pentane-mobile material. The pro-

portion of material excluded from the column porosity also

increased with decreasingmobility on the plates. The levels of

uncertainty in determining molecular masses of these heavy

fractions nevertheless remain high.

In the present work, our laboratory standard pitch sample

has been fractionated by solvent solubility rather than

column chromatography. This was because some of the

heaviest material contained in the samples was lost by

adherence on the chromatographicmedium.9,11 The heptane-

soluble, heptane-insoluble/toluene-soluble and toluene-

insoluble fractions were isolated and examined by SEC,

UV-fluorescence, thermal analysis, GC/MS, pyrolysis/GC/

MS, electrospray ionization mass spectrometry (ESI-MS) and

laser desorption/ionization mass spectrometry (LDMS) both

to estimate the molecular mass ranges of the fractions and to

show differences in chemical behaviour amongst them.

EXPERIMENTAL

SamplesA pitch from the high-temperature coking of coal, used as a

standard material in this laboratory,9–11 has been separated

by solvent solubility into heptane-solubles, heptane-insolu-

ble but toluene-solubles, and toluene-insolubles. The frac-

tionation used two aliquots of heptane, 300mL, followed by

two aliquots of toluene, 300mL. The fractionswere recovered

by drying the solvents; the fraction weights are shown in


Table 1. The asphaltene fraction comprised 44.4% by weight

of the pitch.

Size-exclusion chromatographyTwo columns of dimensions 300mm long, 7.5mm i.d.

packed with polystyrene/polydivinylbenzene particles – a

Mixed-D column with 5mm particles and a Mixed-A column

with 20mm particles were used (Polymer Laboratories,

Church Stretton, UK). The Mixed-D column was operated at

808C while the Mixed-A column was operated at room

temperature; both columns had a flow rate of 0.5mLmin�1

for all solvents used. NMP (Peptide synthesis grade;

Rathburn Chemicals, Walkerburn, UK) and NMP/chloro-

form mixtures (6:1 v/v NMP/CHCl3) were used as mobile

phases (CHCl3, HPLC grade, BDH, Poole, UK). Detection

was carried out using a LC290 variable wavelength UV-

absorbance detector (PerkinElmer, Beaconsfield, UK). As

NMP is opaque at 254 nm, detection of standard compounds

and samples was performed at 270 and 300nm, respectively,

where NMP is partially transparent. The calibration of the

SEC system using the mixed solvent has been described

previously.12,13 For both columns, bimodal distributions of

signal have been observed, with the first, earliest eluting

peak corresponding to the material of molecular size unable

to penetrate the porosity of the column packing, and referred

to as ‘excluded’ from the column porosity. The second

eluting peak corresponds to material able to penetrate the

porosity of the column packing. The exclusion limits of the

two columns, defined by the behaviour of the calibrant

molecules (polystyrene standards), are different, being

200 000 u for the Mixed-D column and about 1 000 000 u for

the Mixed-A column. These polystyrene molecular weights

define times of elution that separate the excluded molecules

from those retained by the column porosity but define a

molecular size rather than a molecular weight for the pitch

molecules. Molecular conformation2,3 is considered to be the

factor causing pitch molecules to become excluded from the

column porosity and not necessarily the molecular weight.

UV-fluorescence spectroscopyA LS55 luminescence spectrometer (PerkinElmer) was used

to obtain emission, excitation and synchronous spectra for

the pitch fractions, using NMP or CHCl3 as solvent in all

cases (only synchronous spectra are shown). The procedure

has been described elsewhere.14,15 The spectrometer was set

with a slit width of 5 nm, to scan at 500 nmmin�1;

synchronous spectra were acquired at a constant wavelength

difference of 20 nm. A quartz cell with a 1 cm path lengthwas

used. The solutions were diluted with NMP or CHCl3 to

avoid self-absorption effects: the dilution was increased until

the fluorescence signal intensity began to decay.


DOI: 10.1002/rcm

Molecular mass ranges of coal tar pitch asphaltenes 2089

Thermal analysisThermogravimetric analysis (TGA) of the fractions was

carried out with a Pyris 1 TGA instrument (PerkinElmer) to

determine the proximate analysis values – volatile carbon,

fixed carbon and ash content. A 1mg sample of each fraction

was heated under nitrogen gas from 508C to 1058C at

108Cmin�1, then heated at 1008Cmin�1 to 9008C, held for 1 h

to complete the release of volatiles followed by combustion

in air to indicate fixed carbon content and the ash content as

the residue. The weight losses to 1058C indicate residual

solvent (or moisture), with the weight loss to 5008Cindicating volatile carbon and the loss to 9008C the fixed

carbon or char residue after prolonged pyrolysis; the residue

after exposure to air is the ash content.

Mass spectrometry: GC/MSThe samples were run by GC/MS using a Saturn 2000GC/

MS ion trap instrument (Varian Ltd, Oxford, UK). Samples

were prepared by mixing approx. 1mg of sample with 1mL

of dichloromethane and agitating for 4 h. A volume of 1mL of

the solution was injected with a split ratio of 30:1 with the

injector at 2608C. The column was an HP1 (Agilent

Technologies Inc., Santa Clara, CA, USA) of length 25m,

i.d. 0.22mm and phase thickness 11 micrometres. The

temperature programme was: hold at 608C for 2min then

heated to 2808C at 58Cmin�1 and held at the upper

temperature; the transfer line into the ion trap was at this

upper temperature. Electron ionization (EI) at 70 eV was

used and the mass range scanned was from m/z 40 to 650.

Mass spectrometry: pyrolysis/GC/MSSamples were placed in quartz tubes and pyrolyzed at 6108Cin a flow of helium for 20 s in a resistively heated platinum

coil using a CDS AS-2500 pyrolysis autosampler (Analytix

Ltd., Peterlee, UK) interfaced to a PerkinElmer Turbomass

Gold GC/MS system (PerkinElmer). The pyrolysate was

introduced into a Chrompack CPSil-5CB fused capillary

column (Varian Ltd) 50m length� 0.32mm i.d.� 0.4mm

phase thickness via a split/splitless injector maintained at a

temperature of 3008C with a 5:1 split ratio. The GC oven

temperature was initially held for 4min at 408C then

increased at 58Cmin�1 to 3008C with a final isothermal time

of 15min. The transfer line to the mass spectrometer was

maintained at 3008C. The mass spectrometer was pro-

grammed to scan between m/z 50 and 650 in EI mode with a

scan time of 0.2 s. The source was maintained at a

temperature of 2008C and the ionization energy was set to

70 eV.

Electrospray ionization mass spectrometryThe Apex III Fourier transform mass spectrometer at Kings

College London (Bruker Daltonics, Bremen, Germany) was

used for this work in positive ion mode with a magnetic field

of 4.7 Tesla. Preparation of the solution for study involved

about 1mg of sample being dissolved in 1mL of NMP. From

this solution 40mL were diluted with 1mL of 50% MeOH/

water and 0.1% formic acid. This solution was directly

infused into the mass spectrometer at a rate of 120mLh�1.

Under these conditions, all three samples gave the same

spectrum.


A second set of runs in which the samples (1mg) were

mixed with acetonitrile (1mL) and infused into the carrier

solvent at a rate of 120mLh�1 was carried out. The carrier

was 50% acetonitrile, 10% dichloromethane and 40% water,

with 0.1% formic acid. These conditions gave good spectra

for the heptane-solubles and toluene-solubles, but not for the

toluene-insolubles. In each case, data points were collected

for about 5min while the sample solution was infused and

the results averaged. The instrument was operated in

accurate mass measurement mode throughout, ideally

accurate to 3 ppm, but generally to 12ppm for major ions

and to about 30 ppm for minor ions. The conditions of

operation of the ESI source were: pressure of drying gas

(nitrogen) 50 psi; pressure of sheath gas (nitrogen) 20 psi;

spray voltage 4 kV; capillary voltage 10V. Mass spectra were

obtained over the mass range m/z 80 to 900. Ions were

collected in a hexapole trap for 1 s before transfer to the

cyclotron cell by application of a 4 kV voltage pulse.

Mass spectrometry: laser desorption/ionizationMSA Bruker Reflex IV matrix-assisted laser desorption/

ionization time-of-flight (MALDI-TOF) mass spectrometer

was used in both linear TOF and reflector TOF modes. A

nitrogen laser at 337 nm ablated and ionized the solid

samples from the stainless steel target plate. An ion

accelerating voltage of 20 kV was used and the mass range

scanned was from m/z 0 to 300 000. Fractions were added to

the sample target without further fractionation. The laser

power was varied from 10% of available power to 70% with

changes in the high mass detector gain through variations of

the highmass accelerator (HMA) voltage both to enhance the

intensities of high-mass ions at low laser powers and to

reduce ion intensities at higher laser powers; this avoided

overloading the detector. Ion extraction with and without a

delay time was investigated using delayed ion extraction

(DIE) times of 200, 400 and 600ns. The high-mass detector

was only available in the linear TOF mode.

RESULTS AND DISCUSSION

Using petroleum nomenclature, the mass balances show that

the asphaltene fraction (defined as heptane-insoluble but

toluene-soluble) comprised 44% of the coal tar pitch, with

41% as preasphaltenes (defined as toluene-insoluble) and

14% as oil (defined as heptane-soluble). Little difference was

observed between the SEC chromatograms obtained when

using two different solvent systems as eluents (pure NMP

and an NMP/CHCl3 mixture). Similarly, the two different

SEC columns (Mixed-D and Mixed-A) showed small but

significant differences (Figs. 1(a) and 1(b), Table 2).

In both columns, the heptane-soluble fraction eluted later

than the other fractions. The chromatogram of the toluene-

insoluble fraction contained the earliest eluting peak. The

chromatograms of neither the heptane-solubles nor the

toluene-solubles contained any significant early-eluting

(excluded) peaks (between 10 and 12min on Mixed-D and

13–16min on Mixed-A). The chromatogram of the toluene-

insoluble fraction did, however, contain a significant

excluded peak. Estimates of the mass ranges of the later


DOI: 10.1002/rcm

Figure 1. (a) SEC chromatograms of pitch asphaltene frac-

tions dissolved in either pure NMP (curves labelled a) or 6:1

mixed solvent (curves labelled b), area normalized, UV absor-

bance detection at 300 nm, Mixed-D column. PHS: heptane-

soluble, PTS: toluene-soluble, and PTI: toluene-insoluble

fractions. (b) SEC chromatograms of pitch asphaltene frac-

tions dissolved in either pure NMP (curves labelled a) or 6:1

mixed solvent (curves labelled b), area normalized, UV absor-

bance detection at 300 nm, Mixed-A column. PHS: heptane-


fractions.

Figure 2. UV-fluorescencespectraofpitchasphaltene fractions

dissolved in CHCl3, peak normalized. PHS: heptane-soluble,

PTS: toluene-soluble, and PTI: toluene-insoluble fractions.

Figure 3. TGA profiles of pitch fractions. PHS: heptane-


fractions.


eluting (retained) peaks, calculated from a mass calibration

based on the elution times of polystyrene standards and

polycyclic aromatic hydrocarbon (PAH) standards, are listed

in Table 2.

As noted above, the heptane-solubles showed the smallest

molecular mass distribution, while the toluene-insoluble

fraction showed molecular mass distributions extending to

the highest range in the zone, showing material resolved by

column porosity (retained peak) as well as the majority of

material excluded from the column porosity. It is thought2,3

that these materials may be adopting three-dimensional

conformations, corresponding to large hydrodynamic

volumes and very large apparent molecular masses. The

Table 2. Mass estimates (u) of pitch fractions using polystyrene a

Fraction

Mixed-D column

Low mass Upper mass Most intense

Heptane-soluble 100 640 250Toluene-soluble 100 1450 250Toluene-insoluble 100 3280 550


upper mass limits on the two columns differ in terms of the

polystyrene masses equivalent to the observed lift-off of

signal from the baselines, but the trends observed using both

columns were similar.

Synchronous UV-fluorescence spectra in NMP (not

shown) and CHCl3 solutions, shown in Fig. 2, show that

the aromatic chromophore sizes increased from the heptane-

solubles to the toluene-insolubles. The fluorescence intensity

decreased in the same order, although Fig. 2 does not show

this effect since the spectra have been height-normalized to

emphasize the relative positions of the peaks. We have

shown elsewhere16 that typically only the fluorescence from

molecules below 3000 gmole�1 can be detected. Fluorescence

from earlier eluting molecules was too low to be detected.

The thermogravimetric analysis results are shown in Fig. 3.

With the temperature held constant at 1008C for 50min, the

nd PAH calibrations

Mixed-A column

mass Low mass Upper mass Most intense mass

100 1093 190100 2680 286100 9320 740


DOI: 10.1002/rcm


three fractions lost between 2 and 5% weight, corresponding

to residual solvent. The greatest weight loss was recorded for

the heptane-soluble fraction. After the temperature was

raised to 9008C, all three fractions showed rapid weight loss,

followed by a further slow weight loss. After 100min, the

extra weight losses were 93% from the pitch heptane-soluble

(PHS), 85% from the pitch toluene-soluble (PTS) and only

26% from the pitch toluene-insoluble (PTI). The toluene-

soluble fraction showed greater tendency to form char rather

than evaporate than the heptane-solubles while the toluene-

insolubles showed the greatest tendency for char formation.

The weight losses correspond to the volatile carbon contents,

with about 1% residue (fixed carbon) from the original PHS,

13% residue from PTS and 71% from PTI (definitions in

caption of Fig. 1). Air was finally admitted into the system to

combust the fixed carbon. None of the samples gave

significant residue after combustion that would correspond

to ash or trace metal oxides. The results show that the

molecules of the different fractions behave differently in

pyrolysis since the tendency to form char rather than to

evaporate increased from the heptane-solubles to the

toluene-insolubles.

As an aside, the observation of large (>3000 gmole�1)

molecules by SEC, a technique where extreme levels of

dilution are attained, has in the past been explained by other

researchers1,4 as resulting from the perceived ‘aggregation’

of small polar molecules. The thermogravimetric data

presented above show that progressively less soluble

materials are also progressively less volatile. This can be

explained in terms of a simple progression of increasing

molecular weights; that is also the trend observed by SEC.

Furthermore, in a recent 13C-NMR study,17 we reported that

the progressively diminishing solubility of pitch fractions

correlates with the rapidly increasing average number of

fused aromatic rings per polyaromatic hydrocarbon (PAH)

group. It is not clear how the concept of aggregates would fit

with these findings. In the present context, the concept of

aggregation would require us to believe that these ‘aggre-

gates’ do not thermally activate, and that the ‘small polar

molecules’ are somehow impeded from evaporating at (or

near) temperatures indicated by their boiling points.

Mass spectrometry of the pitch fractionsThe GC/MS chromatograms presented in Fig. 4 show that

the heptane-soluble fraction gave the most intense chroma-

togram. The components detectedwere the PAH compounds

normally expected of this pitch, ranging from fluorene as the

first significant peak, through phenanthrene, fluoranthene

and pyrene, chrysene isomers, benzopyrenes and benzo-

fluoranthenes to dibenzopyrenes eluting at about 60min; the

peaks are numbered in the figure. No aliphatic components

were observed. The chromatogram of the heptane-soluble

fraction closely resembles that shown for pentane-soluble

fractions of the same pitch prepared by elution from column

chromatography on silica.8

The chromatogram of the heptane-insoluble/toluene-

soluble fraction was much less intense as shown by the

sharp rise of baseline towards 60min. The identifiable

components ranged from phenanthrene to dibenzopyrene,

as in the heptane-soluble fraction. However, in this sample,


the peaks showed very low intensities. The major part of this

sample was clearly not able to pass through the chromato-

graphic column. It is expected that these materials would be

PAH compounds that are less volatile (i.e. larger molecular

mass) than dibenzopyrene. The chromatogram for the

toluene-insoluble fraction showed only very minor com-

ponents, assumed to represent contamination rather than

pitch components; the mass spectra indicate silicones and

phthalates.

These three chromatograms show that the small mol-

ecules were preferentially in the heptane-solubles and that

the mass ranges of the toluene-soluble and insoluble

fractions were shifted to higher masses, but they were still

highly aromatic as shown by several techniques used in this

study. Previous work8 also found that the fraction of the

pitch sample that was able to pass through the GC column

was contained in the pentane-soluble fraction. Very little of

the pentane- or heptane-insoluble/toluene-soluble fraction

was sufficiently volatile to pass through the column and

probably represented the residue of pentane- or heptane-

soluble material that was not completely separated by the

fractionation.

The pyrolysis/GC/MS chromatograms of the three

fractions are compared in Fig. 5. The chromatogram of the

heptane-soluble fraction resembles the GC/MS chromato-

gram of Fig. 4, suggesting that the components detected by

pyrolysis evaporated from the pyrolyzer stage rather than

being formed through the breakdown of larger molecules.

The components of the pyrolysis/GC/MS chromatograms

were naphthalene at 19min, methyl naphthalenes at 22 and

23min, acenaphthene at 27min, dibenzofuran at 28.4min,

fluorene at 30min, phenanthrene at 34min, fluoranthene and

pyrene at 39min and benzopyrene isomers at 50min. The

components common to Fig. 4 are numbered in the sameway

as in Fig. 5. It is noted that the components of Fig. 4 larger

than phenanthrene are at much lower relative intensities in

Fig. 5 for the pitch heptane-soluble (PHS) suggesting that

fluoranthene and larger aromatics may pyrolyze to form char

rather than simply evaporate in the pyrolysis procedure.

Similarly, the pyrolysis/GC/MS chromatogram of the

heptane-insoluble/toluene-soluble fraction (PTS) resembled

the early eluting section of the corresponding GC/MS

chromatogram, once again suggesting that the components

that could be identified had actually evaporated from the

sample, rather than being formed during the pyrolysis of the

sample. Components of higher molecular mass than pyrene

were at low relative intensities, indicating that they formed

char rather than simply evaporating on pyrolysis. The

difference between the PTS and PHS pyrograms was one of

intensity as judged by the relative intensities of the peaks in

Fig. 5 for the solvent, toluene, at about 6min; while this

solvent peak is hardly visible for PHS, it is the major peak for

PTS and the toluene-insoluble fraction (PTI). The pyrolysis/

GC/MS chromatogram of the PTI only gave a peak for the

residual toluenewith no significant fragmentmolecules from

the aromatic material of this fraction. The chromatogram of

the PTSwas of lower intensity than that of the PHS, reflecting

the same difference observed in Fig. 4, while the chromato-

gram of the PTI showed no sample-derived signal, as in the

case of the GC/MS chromatogram. These results strongly


DOI: 10.1002/rcm

Figure 4. GC/MS chromatograms of PHS: heptane-soluble (top), PTS: toluene-soluble (middle), and PTI: toluene-insoluble

(bottom) fractions. Peak identities and masses (m/z) are: 1 methylnaphthalenes (142); 2 acenaphthene (154); 3 dibenzo-

furan (168); 4 fluorene (166); 5 phenanthrene and anthracene (178); 6 methylphenanthrenes (192) and cyclopenteno-

phenanthrene (190); 7 fluoranthene and pyrene (202); 8 methylpyrene isomers (216); 9 chrysene isomers (228); 10

benzopyrene isomers (252); 11 benzo[ghi]perylene isomers (276); 12 dibenzopyrene isomers (302).


suggest that the heavier molecules in the heptane-insoluble

material are transformed into char with rising temperature

rather than going through a fragmentation process, as

indicated by TGA.

Electrospray ionization mass spectrometry (ESI-MS) in

positive ion mode was attempted initially using methanol,

water and formic acid as solvent/carrier. To ensure that the

pitch fractions were dissolved in the solution to be injected,

NMP was used as the solvent. However, for all three

fractions, the identical mass spectrum was obtained; these

mass spectra are not shown. The ions detected are thought to

be from components of the solvent NMP and are not from the

pitch samples. Thus dissolving the samples in NMP (an

excellent solvent for pitch fractions) did not appear to be a

viable option for ESI in methanol/water. Because no ions

relevant to the pitch samples were detected, the origin of the

few ions observed was not investigated further.

The three fractions dissolved in acetonitrile were

examined again using a solution of acetonitrile, dichloro-


methane and water with formic acid. The mass spectra

obtained for the heptane-solubles and toluene-solubles are

shown in Fig. 6. The heptane-solubles gave the greatest ion

intensity and the components can be defined as pyridinic

compounds or azaarenes, with their atomic compositions

calculable from the accurate mass measurement.

All the major ions were of even mass, equivalent to

protonation of an odd-mass molecule and derived by

substitution of a pyridino ring for a benzo ring in the

PAH of one mass lower than the aza compound. For

example, the ion at m/z 204 corresponded to C15H10Nþ

formed by protonation of the molecule C15H9N, acridine and

one mass larger than the PAH pyrene C16H10.

The compounds ionized in the toluene-soluble fraction are

likely to be pyridinic molecules not completely removed by

solution in heptane, since the mass range of the ions is only

slightly larger than those observed in the analysis of the

heptane-soluble fraction. Furthermore, the ion intensities

observed in the spectra of the heptane-insoluble/toluene-


DOI: 10.1002/rcm

Figure 5. Pyrolysis/GC/MS chromatograms of the pitch fractions: PHS: heptane-solubles, PTS: toluene-solubles, PTI:

toluene-insolubles. Peak identities as in Fig. 4.


soluble fraction were lower than those of the heptane-

solubles by a factor of about 3. The molecular types detected

in the heptane-soluble fraction and in the heptane-insoluble/

toluene-soluble fraction are listed in Tables 4 and 5,

respectively, although the accuracy of themassmeasurement


(within about 12ppm) was not sufficiently precise to define

the atomic compositions unequivocally. These tables show

the measured mass in the first column; the second column

shows the measured mass minus the mass of one H atom

giving the mass of the unprotonated molecule. The third


DOI: 10.1002/rcm

Figure 6. ESI spectra run from 50% acetonitrile, 10% dichloromethane, 40% water; top: PHS and bottom: PTS.


column shows the accurate mass of the pyridine compound

considered to be present while the fourth column shows the

mass difference (as ppm) between the measured mass and

the calculated mass. The fifth column gives the atomic

composition while the sixth column suggests a structural

type for themolecule. The data of Table 5 suggest that several

components containing two nitrogen atoms were detected.

However, the highest mass of Table 5, m/z 252, is lower than

the highest mass of Table 4, m/z 358, despite the fact that

many of the components of the two tables are the same. This

suggests that solubility alonemay not be the limiting factor in

the ESI method.

No significant ions were detected in the largest molecular

mass toluene-insoluble fraction and it is assumed that none

of the components were soluble in the carrier solution; in our

experience such materials would not be expected to be

soluble in acetonitrile/dichloromethane/water mixtures.

Solubility in the solvent to the ESI source has been shown

to be necessary18 for a complex micelle, where a membrane


around the complex provides the vital solubility factor to

allow the formation of ions from the complex. Previous

works on polar (pyridine-soluble) coal extracts,19 crude

oils20,21 and petroleum asphaltenes22 have used the addition

of pyridine or toluene-soluble materials to the ESI solvent

consisting of excess methanol with base or acid to facilitate

negative or positive ion formation. The mass spectra shown

for these materials19–22 are extremely complex, but of limited

upper mass limit, consistent with a solvent limited solubility;

our experience of such materials suggests they would not be

very soluble in methanol and that the largest molecules

would not be dissolved. Our previous work on the ESI23 of

coal tar fractions: (1) soluble in acetonitrile, (2) insoluble in

acetonitrile but soluble in pyridine, and (3) insoluble in

pyridine by direct infusion, only gave positive ions for the

acetonitrile-soluble fractions, similar to the ions detected in

this work. The fractions insoluble in acetonitrile gave no ions

at all, presumably because the fractions were insoluble in the

acetonitrile/water ESI carrier fluid. Because the solution


DOI: 10.1002/rcm

Figure 7. LDMS mass spectra of: (a) PHS: the heptane-

solubles in linear TOF mode at (1) 10%, (2) 30%, (3) 40%,

(4) 50%, and (5) 70% laser power; (b) PHS: the heptane-

solubles in reflector TOF mode at 40% laser power; (c) PTS:

the toluene-solubles in linear TOF mode at (1) 10%, (2) 20%,

(3) 30%, (4) 50%, and (5) 60% laser power and (d) PTI: the

toluene-insoluble fraction in linear TOF mode at (1) 10%, (2)

20%, (3) 30%, (4) 50%, (5) 60%, and (6) 60% [thick sample

layer] laser power.


prepared for ingestion into the carrier solvent is of low

concentration, it is not easy to determine a lack of solubility

after further dilution in the mixed solvents.

The LD mass spectra for the three fractions are shown in

Fig. 7 at different combinations of laser power and high-mass

detector voltage. The ion intensities have been restricted to

remain below 100 intensity units per laser shot.

For the heptane-solubles, the lowest laser power of 10% (of

available power) gave an ion distribution from aboutm/z 200

to 500 with a maximum intensity at about m/z 300. As the

laser power increased, the low mass and maximum intensity

mass did not change significantly, but the upper mass did

change. From 20% to 50% laser power, the upper mass was

between m/z 2000 and 3000 and this is thought to approach

the upper mass of the fraction. At higher laser power, the

high mass limit shifted to m/z 10 000 at 60% and towards

100 000 at 70% laser power, showing as a new high-mass

peak. However, at laser power above 40%, fragmentation of

the molecules became apparent through an increased

number of peaks at low mass. Thus, these spectra showing

material at and above m/z 10 000 are considered to be

distorted by ionized molecular clusters, probably formed

through excessive laser power desorbing clusters of

molecules rather than discrete molecules.

The spectra of the heptane-solubles discussed so far were

in linear TOF mode. The heptane-soluble fraction was also

examined in reflector TOF mode and the spectrum shown in

Fig. 7(b) is at a laser power of 40% of the maximum. The low-

mass limit was atm/z 200, the maximum intensity was atm/z

300 and the upper mass limit lay betweenm/z 1000 and 2000.

In reflector mode, higher mass ions (as well as molecular

cluster ions) are less likely to be detected than in linear TOF

mode. The reflector TOF spectrum can then be seen as

confirmation of the linear mass spectra.

The linear TOF mass spectra of the toluene-solubles under

similar conditions indicated that the low mass was around

m/z 200 for all spectra apart from those showing fragmenta-

tion at 60% laser power and above. The maximum intensity

remained at about m/z 400 throughout. The upper mass was

m/z 1000 for 10% laser power (low), but lay betweenm/z 3000

and 4000 for laser powers from 20 to 50%, depending on the

high mass accelerator voltage. At higher laser powers up to

70%, ionized clusters became evident as a new high-mass

peak was observed and fragmentation at low mass was

shown to be more severe.

The spectra of the toluene-insoluble fraction showed a

low-mass limit of about m/z 400 with the lowest mass ions at

10% laser power. Metal ions assumed to be Naþ and Kþ at

low mass were detected in all spectra. The maximum ion

intensity remained at about m/z 2000 for all the laser power

levels used. The upper ion mass limit of the spectra shifted

from m/z 5000 at 50% laser power to m/z 10 000 at 70% laser

power. However, fragment ions were detected at and above

60% laser power, indicating the onset of artefact formation.

At 70%, the highest laser power used, ionized molecular

clusters were observed as extra highmass peaks up to at least

m/z 100 000.

Selecting the ‘best’ spectra for each fraction gives the

results summarized in Table 3; the lowest laser power of 10%

is not thought able to desorb and ionize the whole range of

Copyright # 2009 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2009; 23: 2087–2098

DOI: 10.1002/rcm

Table 3. Mass estimates of pitch fractions from LDMS spectra

Fraction Low mass (m/z) Upper mass (m/z) Most intense mass (m/z)

Heptane-soluble 200 2000–3000 300Toluene-soluble 200 3000–4000 400Toluene-insoluble 400 5000–10 000 2000


masses present in the samples; the coincidence of results for

the spectra at laser powers higher than 10%, but insufficient

to produce the ionized cluster peak, has been taken to

indicate the most likely mass ranges. The definition of ‘best’

used here was the absence of significant fragment ions, the

absence of any cluster ion distribution at very high mass and

the stability of the shape of the ion distribution with no

significant shifts to low or high masses with increasing laser

Table 4. Measured masses of ions detected in ESI mass spectru

Measured mass -H molec. mass Accurate mass Diff, p

130.0882 129.08038 129.05781 174134.0557 133.04791 133.05272 36170.0941 169.08628 169.08909 16180.0778 179.06998 179.07345 19184.1091 183.10128 183.10473 18194.0930 193.08518 193.0872 10198.1243 197.11648 197.12037 19204.0770 203.06918 203.07345 21208.1082 207.10038 207.10473 21218.0921 217.08428 217.08908 22220.1080 219.10018 219.10473 21222.1236 221.11578 221.12037 21228.0802 227.07238 227.07345 5230.0917 229.08388 229.08909 22232.1073 231.09948 231.10473 23234.1230 233.11518 233.12037 22236.1389 235.13108 235.13601 21244.1067 243.09888 243.10473 24248.1384 247.13058 243.12037 41250.1544 249.14658 249.14976 13254.0908 253.08298 253.08909 24258.1220 257.11418 257.12037 24260.1380 259.13018 259.13601 22268.1060 267.09818 267.10473 24272.1375 271.12968 271.13601 23274.1534 273.14558 273.15165 22278.0905 277.08268 277.08909 23280.1058 279.09798 279.10473 24282.1214 281.11358 281.12037 24284.1371 283.12928 283.13601 24286.1530 285.14518 285.15165 23288.1687 287.16088 287.16729 22296.1367 295.12888 295.13601 24298.1524 297.14458 297.15165 24300.1682 299.16038 299.16729 23304.1048 303.09698 303.10473 26306.1207 305.11288 305.12037 25308.1361 307.12828 307.13601 25310.1519 309.14408 309.15165 24318.1201 317.11228 317.12037 26332.1352 331.12738 331.13601 26334.1503 333.14248 333.15165 27336.1664 335.15858 335.16729 26346.1497 345.14188 345.15165 28358.1501 357.14228 357.15165 26


power. These values are in reasonable agreement with the

equivalent values from SEC in Table 2; agreement within a

factor 2 in mass is considered good for SEC especially since

the material excluded from SEC is not taken into account.

It is interesting to compare the molecular mass ranges of

the coal tar pitch derived asphaltene fraction in the present

study with previously published results for asphaltenes.1

Themass rangemeasured in this work was from about 200 to

m of PHS fraction

pm Atomic composition Name or possible type

C9H7N QuinolineC8H7NO 2H-PyranopyridineC12H11N Methyl azadiphenylC13H9N AcridineC13H13N Alkyl azadiphenylC14H11N Methyl acridineC14H15N Alkyl azadiphenylC15H9N AzapyreneC15H13N Alkyl acridineC16H11N Alkyl azapyreneC16H13N AzadihydropyreneC16H15N Alkyl acridineC17H9N Azabenzo[mno]phenanthreneC17H11N AzachryseneC17H13N Alkyl azapyreneC17H15N Alkyl azadihydropyreneC17H17N Alkyl acridineC18H13N Methyl azachryseneC18H15N Alkyl azapyreneC18H19N Alkyl acridineC19H11N AzabenzopyreneC19H15N Alkyl azachryseneC19H17N Alkyl azapyreneC20H13N Alkyl azabenzopyreneC20H17N Alkyl azachryseneC20H19N Alkyl azapyreneC21H11N Azabenzo[ghi]peryleneC21H13N AzapentaceneC21H15N Alkyl azabenzopyreneC21H17N Alkyl azabinaphthylC21H19N Alkyl azachryseneC21H21N Alkyl azapyreneC22H17N Alkyl azabenzopyreneC22H19N Alkyl azabinaphthylC22H21N Alkyl azachryseneC23H13N AzadibenzopyreneC23H15N PhenanthroquinolineC23H17N Alkyl azapentaceneC23H19N Alkyl azabenzopyreneC24H15N Alkyl azadibenzopyreneC25H17N Alkyl azadibenzopyreneC25H19N Alkyl phenanthroquinolineC25H21N Alkyl azapentaceneC26H19N Alkyl azadibenzopyreneC27H19N Alkyl azacoronene


DOI: 10.1002/rcm

Table 5. Measured masses of ions detected in ESI mass spectrum of PTS fraction

Measured mass -H mol. mass Accurate mass Diff, ppm Atomic composition Name or possible type

134.05573 133.0479 133.05272 36 C8H7NO 2H-Pyranopyridine169.07489 168.06707 168.0687 10 C11H8N2 Pyrroloquinoline175.11767 174.10985 174.11562 33 C11H14N2 Alkylbenzimidazole180.07938 179.0734 179.07156 10 C13H9N Acridine183.09029 182.08508 182.08434 4 C12H10N2 Diphenyldiazene194.09484 193.08702 193.0872 1 C14H11N Methyl acridine197.10570 196.09788 196.09998 10 C13H12N2 Alkyl diphenyldiazene204.07890 203.07108 203.07345 12 C15H9N Azapyrene206.09470 205.08688 205.08909 11 C15H11N Azadihydropyrene208.11025 207.10243 207.10266 1 C15H13N Alkyl acridine218.09434 217.08652 217.08909 12 C16H11N Alkyl azapyrene219.08966 218.08184 218.08434 11 C15H10N2 Benzimidazoisoquinoline220.11014 219.10232 219.10473 11 C16H13N Alkyl azadihydropyrene222.12575 221.11793 221.12037 11 C16H15N Alkyl acridine230.09410 229.08628 229.08909 12 C17H11N Azachrysene232.10980 231.10198 231.10473 12 C17H13N Alkyl azapyrene234.08929 233.08147 233.0840 11 C16H11NO Methylphenanthrooxazole234.12558 233.11776 233.12037 11 C17H15N Alkyl azadihydropyrene244.10959 243.10177 243.10473 12 C18H13N Alkyl azachrysene246.12544 245.11762 245.12037 11 C18H15N Alkyl azapyrene254.09354 253.08572 253.08909 13 C19H11N Azabenzopyrene258.12516 257.11734 257.12037 12 C19H15N Alkyl azachrysene268.10922 267.1014 267.10473 12 C20H13N Alkyl azabenzopyrene280.10902 279.1012 279.10473 13 C21H13N Azapentacene282.12488 281.11706 281.12037 12 C21H15N Alkyl azabenzopyrene


2000 u with a peak of intensity around 400 u. The coal

asphaltenes cited1 have similar peak mass values (�400u)

derived from the fluorescence spectra but the upper mass

limits are not specified. The upper mass limit suggested by

ESI-MS for petroleum asphaltenes1,4 did not exceed 1200u.

CONCLUSIONS

A coal tar pitch was fractionated into heptane-solubles,

heptane-insoluble/toluene-solubles (asphaltenes) and

toluene-insolubles (preasphaltenes). Thermogravimetric

analysis, size-exclusion chromatography and UV-fluor-

escence spectroscopy show distinct differences between

the three fractions in terms of volatility, molecular size

ranges and the aromatic chromophore sizes. The smallest

size molecules and chromophores were found in the

heptane-solubles and the largest in the toluene-insolubles.

Several mass spectrometric methods were used to examine

the pitch fractions, including GC/MS, pyrolysis/GC/MS,

ESI-FTICRMS and LDMS. All methods have shown signifi-

cant differences between the heptane-soluble and heptane-

insoluble/toluene-soluble fractions. GC/MS, pyrolysis/

GC/MS and ESI-MS gave good mass spectra for the

heptane-soluble fraction but only low intensity spectra for

the heptane-insoluble/toluene-soluble (asphaltene) frac-

tions. The ions observed in the asphaltene fraction appeared

to be from a small proportion of heptane-solubles remaining

in the heptane-insoluble/toluene-soluble fraction.

Apart from LDMS, none of these mass spectrometric

methods gave any signal from the toluene-insolubles,

indicating that the molecules were too involatile for GC

and too complex to pyrolyze into small molecules during

pyrolysis/GC/MS which instead gave a char. The ESI-


FTICRMS technique gave no signal for this fraction. This is

thought to be due to the insolubility of the sample (toluene-

insolubles) in the methanol or acetonitrile, water and

formic acid used as solvent/carrier for electrospray. This

appears to be the primary reason why high-mass material

could not be observed in previous work, using this

technique.

LDMS was able to generate ions from each of the fractions.

The laser power was varied to desorb larger molecules using

higher laser fluences than would be required for smaller

molecules. This was made possible through fractionation of

the sample, in order toworkwith limited ranges ofmolecular

masses. In consequence, the formation of ion clusters was

avoidedwhile using relatively high laser fluences. The upper

mass limit of the pitch was determined to be between 5000

and 10 000u although it is possible that not all the toluene-

insoluble fraction was desorbed and ionized. The use of

matrix materials may be necessary to extend the mass range.

In comparison with the coal asphaltenes previously

mentioned, the pitch asphaltenes show a similar maximum

in the spectra, atm/z 400, in approximate agreement with the

estimate from SEC. However, the upper mass limit of the

pitch asphaltene of 2–3000 u is considerably higher than

those cited for coal-derived asphaltenes in previous work.

One of the major points emerging from the present study,

however, is that fractionation of complex samples is

necessary to remove smaller molecules and allow the use

of higher laser fluences for the larger molecules, thereby

avoiding the formation of ionized molecular clusters. The

mass ranges of the toluene-insoluble fraction by LDMS and

SEC are higher than those for the asphaltene fraction and

much greater than the values considered appropriate for

petroleum asphaltenes.


DOI: 10.1002/rcm


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DOI: 10.1002/rcm

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