Journal of Chromatography A, 1024 (2004) 227–243 Thin-layer chromatography of pitch and a petroleum vacuum residue Relation between mobility and molecular size shown by size-exclusion chromatography W. Li b , T.J. Morgan a , A.A. Herod a,∗ , R. Kandiyoti a a Department of Chemical Engineering and Chemical Technology, South Kensington Campus, Imperial College London, London SW7 2AZ, UK b Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, China Received 12 June 2003; received in revised form 1 October 2003; accepted 9 October 2003 Abstract A coal tar pitch and a petroleum vacuum residue have been separated by TLC using pyridine, acetonitrile, toluene and pentane to develop the chromatograms. The bands of material detected were recovered in 1-methyl-2-pyrrolidinone (NMP) solvent and examined by size-exclusion chromatography (SEC) in NMP eluent. The relation between elution time in SEC and mobility on the TLC plate indicated that molecular size increased steadily with increasing immobility on the plate. This relation was reinforced by UV fluorescence spectroscopy in that the fluorescence moved to longer wavelengths with increasing immobility. The molecular size of the material excluded from the porosity of the SEC column remains undefined; some excluded material was found in all of the fractions from both samples. The valley of zero intensity separating the retained material from the excluded material may suggest a change of structure from near-planar in the retained region to three-dimensional in the excluded region. © 2003 Elsevier B.V. All rights reserved. Keywords: Coal tar; Petroleum 1. Introduction Previous work [1] has explored the relation between TLC mobility and elution time in SEC, but using tetrahydrofuran (THF) as eluent. In that work, the molecular mass of material in bands recovered from TLC increased when measured by solids probe mass spectrometry; in SEC using THF as eluent, however, the elution of the different bands covered virtually the same range of elution times, with the main difference being that the lift-off of signal from baseline at the earliest elution time varied slightly. More recent work with the same coal tar pitch [2] has shown that the range of molecular mass of the sample is greatly in excess of that accessible by probe mass spectrometry (about m/z 600), with material detected by matrix-assisted laser desorption ionization (MALDI) MS ∗ Corresponding author. Fax: +44-207-594-5604. E-mail address: a.herod@imperial.ac.uk (A.A. Herod). up to 100 000 u [3] and indications of masses up to the order of millions of mass units by static light scattering [4] and by SEC in 1-methyl-2-pyrrolidinone (NMP) solution calibrated using polystyrenes [5] and other polymers [6]. Where fractionation of the pitch, vacuum residues and other coal-derived samples has been achieved by thin-layer chromatography or by a column chromatography method using solvent such as acetonitrile, pyridine and NMP or toluene and tetrahydrofuran, the SEC profiles using NMP as eluent have shown shifts to earlier elution (larger molecules) in the solvent solubility order toluene, acetonitrile, THF, pyridine, NMP or immobile in pyridine on the TLC plate. The evidence from UV fluorescence indicated that the flu- orescence intensity decreased in the same order of solvent solubility, showing that the larger molecules did not emit the absorbed light as fluorescence, but distributed the energy into other pathways. The present work has repeated the experiment in TLC, using the coal tar pitch and a petroleum distillate residue, 0021-9673/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2003.10.020