Large Molecular Mass Materials in Coal-Derived Liquids by ²⁵²Cf-Plasma and Matrix-Assisted Laser Desorption Mass Spectrometry

Abstract

The paper compares responses of 252 Cf-plasma desorption MS (PD-MS) and matrix-assisted laser desorption (MALDI) MS to identical samples. The two pairs of samples selected for the comparison were known from previous work to differ significantly in their high mass contents. MALDI-MS showed large differences in MM distributions within both pairs of samples. The PD-MS data showed a degree of similarity between one pair of samples (pyridine soluble/insoluble fractions of a coal tar pitch); for the second pair (a coal extract and its hydrocracked product), trends from the two MS techniques agreed closely. The MM range observed by PD-MS was somewhat narrower, extending to between 3000 and 5000 u. Significant differences within pairs of samples were observed by SEC and by UV-fluorescence spectroscopy, providing somewhat closer agreement with the MALDI spectra. The two MS instruments differ in two important respects: the ionization system (i.e., plasma vs laser desorption) and the maximum available ion extraction voltage: 30 kV for the MALDI-MS instrument and 15 kV for the PD-MS. The comparison of plasma vs laser desorption mass spectroscopy could not therefore take place at high ion extraction voltages. Work at up to 30 kV in the MALDI instrument indicated better sensitivity to highmass materials at higher ion extraction voltages. The qualitative similarity of results from the two MS techniques is nevertheless apparent; the range of MMs observed in PD-MS as well as in MALDI-MS were, furthermore, far larger than those reported by any MS technique, to date.

On the determination of molecular weight distributions of asphaltenes and their aggregates in laser desorption ionization experiments

“…1 (20-40 µJ/pulse), the measurements on the more diluted sample (0.03 µg cm 2 ) lead to LDI spectra showing a maximum at ¾500 amu and a tail extending to ¾3000 amu. Remarkably, these latter spectra are in excellent concordance with those reported previously employing plasma desorption and MALDI MS. 17,19 On the contrary, for the denser sample (1.2 µg cm 2 ) at the same laser energies, a roughly constant (or slightly decreasing) signal is observed within the 500-2000 amu range, and the distribution decreases slowly and extends above ¾8000 amu. For the higher laser energies, the measured distributions move to greater MW values, keeping in all cases a clear gap between the curves for the two sample concentrations.…”

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.…”

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

“…The coal tar pitch has been studied extensively 2,. 3,, 5,, 6,, 8–10,, 12–14,, 16,, 17,, 25–29 It is the residue from the destructive distillation of a coal tar produced in a high‐temperature coke oven and has been exposed to temperatures up to 1500 °C.…”