Molecular mass ranges of coal tar pitch fractions by mass spectrometry and size‐exclusion chromatography

Abstract

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. Thermogravimetric 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 chromatography/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 insoluble 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).

Characterization of Tars from the Thermal Processing of Baganuur and Tavan Tolgoi Coals from Mongolia, Using SEC, UV-F, IR and Mass Spectrometry

“…Similar findings for the pitch [45] indicated the largest molecular ion masses, up to m/z 10,000 were in the toluene-insoluble fraction.…”

“…Recent work [45] has examined a coal tar pitch sample using similar methods to those employed here. The pitch was fractionated into heptane-soluble, toluene-soluble and toluene-insoluble materials.…”

Structural Characterization of Large Polycyclic Aromatic Hydrocarbons. Part 1: The Case of Coal Tar Pitch and Naphthalene-Derived Pitch

“…16 It has a H/C atomic ratio of 0.54. AR mesophase pitch, kindly provided by Mitsubishi Gas Chemical Company, is a 100% mesophase pitch synthesized by polymerization of naphthalene.…”

Characterisation and feasibility as carbon fibre precursors of isotropic pitches derived from anthracene oil

“…According to previous studies, this difficulty in the ionization is more acute in highly polydisperse samples, which seems to be the case in these fractions [9,15]. This effect is particularly noticeable when comparing the LDI-MS upper limit for the whole samples (around 12,000 m/z) with the upper limit of the PI fractions (about 20,000 m/z).…”

“…The optimum conditions for the analysis were selected taking into account 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 power [15]. As observed in Fig.…”