Molecular mass ranges of coal tar pitch fractions by mass spectrometry and size-exclusion chromatography F. Karaca 1 , T. J. Morgan 2 , A. George 2 , I. D. Bull 3 , A. A. Herod 2 * , M. Millan 2 and R. Kandiyoti 2 1 Department of Chemical Engineering, Marmara University, Goztepe Campus,34722 Kadikoy, Istanbul, Turkey 2 Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK 3 Organic 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 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 10 000 u. 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 (400–10 000 u with maximum intensity around 2000 u by LDMS and 100–9320 u with maximum intensity around 740 u by SEC) are higher than those for the asphaltene fraction (200–4000 u with maximum intensity around 400 u by LDMS and 100–2680 u with maximum intensity around 286 u by SEC) and greater than values considered appropriate for petroleum asphaltenes (300–1200 u with maximum intensity near 700 u). 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 and molecular 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 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 some material excluded from column porosity; we RAPID COMMUNICATIONS IN MASS SPECTROMETRY Rapid Commun. Mass Spectrom. 2009; 23: 2087–2098 Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rcm.4104 *Correspondence to: A. A. Herod, Department of Chemical Engin- eering, Imperial College London, London SW7 2AZ, UK. E-mail: [email protected]Copyright # 2009 John Wiley & Sons, Ltd.
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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.
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
Copyright # 2009 John Wiley & Sons, Ltd.
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.
Rapid Commun. Mass Spectrom. 2009; 23: 2087–2098
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
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-
Copyright # 2009 John Wiley & Sons, Ltd.
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.
Rapid Commun. Mass Spectrom. 2009; 23: 2087–2098
DOI: 10.1002/rcm
2098 F. Karaca et al.
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