Pyrolysis‐gas chromatography/mass spectrometry of pitch fractions recovered from preparative size exclusion chromatography: structural differences with increasing molecular size

Abstract

A sample of high-temperature coal tar pitch has been fractionated by preparative scale size exclusion chromatography (SEC). Twenty-four fractions corresponding to narrow molecular mass distributions were collected at sequential elution times of 3 minutes. The preparative scale column is thought to have filtered out most of the very large molecular mass material and the analysis focused on smaller mass components. The recovered fractions were characterised by analytical SEC. Pyrolysis-gas chromatography/mass spectrometry (pyrolysis-GC/MS) of the fractions was used to evaluate changes in main structural features with changing molecular size.As expected, analytical SEC of the fractions showed a systematic shift to smaller molecular sizes with increasing elution time. UV-fluorescence spectra of the samples showed parallel structural shifts to shorter wavelengths and gains in intensity with increasing elution times. Pyrolysis-GC/MS showed parent polycyclic aromatic components considered typical of pitch. The major fragments detected were pyrenes and fluoranthenes (m/z 202) and chrysenes (m/z 228). Benzopyrenes (m/z 252) and larger aromatics were not prominent amongst the pyrolysis products. Taken together, molecular structures were found to change significantly from fraction to fraction, but no systematic pattern emerged with changing elution time. Some components were identified as having formed during the analytical pyrolysis procedure itself. As aromatic products larger than dibenzopyrene (m/z 302) would not elute from the column, detected fragments are thought to have detached from larger pitch molecules.The pyrolysis of the largest species gave only alkanes and alkenes, with the aromatic fragments thought to be too large to elute through the GC column. The work provides information regarding the aromatic groups that are readily lost from larger molecules. Larger aromatic structures are thought to condense (to char) during the analytical pyrolysis step; the method is limited in not being able to provide information about their structural features.

Analytical Methods for Characterizing High-Mass Complex Polydisperse Hydrocarbon Mixtures: An Overview

“…143 The fractionation methods used included planar chromatography, 23 solvent solubility, 143,173 column chromatography, 173 ultrafiltration, 15 and fractionation by SEC. 27,29,76,114 In all cases an increase was observed in average mass as determined by LD-MS with earlier elution times observed by SEC. A near quantitative correlation between SEC (by PScalibration) and LD-MS mass estimates was found up to ∼3000 u.…”

“…The same pitch was separated into 24 fractions by preparative SEC, 27 and each fraction was examined by Py-GC-MS. Reinjecting the time-resolved fractions into an analytical SEC column showed the fractions eluting at times similar to their times of collection. Furthermore, the UV−F spectra ranged from short wavelength peaks for the smallest molecules (which elute late in SEC), to early eluting material excluded from the column porosity that showed maxima at longer wavelengths.…”

Characterization of Molecular Mass Ranges of Two Coal Tar Distillate Fractions (Creosote and Anthracene Oils) and Aromatic Standards by LD-MS, GC-MS, Probe-MS and Size-exclusion Chromatography

“…2,3 Several mass spectrometric techniques have been used by us to supplement information gained by SEC. These include GC-MS, probe-MS, laser-desorption mass spectrometry (LD-MS), matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS), and, less frequently, pyrolysis-GC-MS. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Each of these techniques presents particular limitations that must be evaluated within the context of individual sample properties and the type of data that needs to be acquired. With 1-methyl-2-pyrrolidinone (NMP) used as eluent, size exclusion chromatograms of the heavier fractions of coal liquids and petroleum residues consistently show bimodal distributions.…”

Positive-Ion Electrospray Ionisation Mass Spectrometry of Acetone- and Acetonitrile-Soluble Fractions of Coal-Derived Liquids

“…Previous work in this laboratory on the characterisation of coal derived liquids has involved the use of use of size-exclusion chromatography (SEC) as well as several different types of mass spectrometry. [1][2][3][4][5][6][7][8][9] The latter techniques have included gas chromatography/mass spectrometry (GC/ MS), 10 heated-probe-MS, 11 liquid chromatography (LC)-MS, [12][13][14] fi eld ionisation (FI)-MS, 14 fi eld desorption (FD)-MS, 14 fast atom bombardment (FAB)-MS, 14,15 252 Cf plasma-MS, 16,17 laser desorption (LD)-MS 10 and matrix-assisted laser desorption/ionisation (MALDI)-MS. 5,[18][19][20][21][22] The work has focused on examining both the structural features and the molecular mass distributions of these samples.…”