John E. Parker

Identification of large molecular mass material in high temperature coal tars and pitches by laser desorption mass spectroscopy

Abstract Molecular masses of up to 12 000 Da have been identified by laser desorption mass spectroscopy (l.d.-m.s.) in a number of coal tars and extracts. Observed molecular masses were greater than any hitherto detected in coal-derived liquids; fractions of high temperature coke oven tars have been used for verification of initial findings. Low molecular mass materials have been analysed using similar experimental procedures, to confirm the absence of laser or other apparatus derived high molecular mass clusters. Spectra extending to 12 000 Da show an envelope between 1000 and 3000 Da and a low molecular mass envelope between 12 and 400 Da, the latter apparently indicating the presence of carbon clusters. At high laser power densities, the 1000 to 3000 Da envelope was observed to lose intensity, with parallel gain in intensity of the low mass carbon cluster envelope, suggesting increased breakdown of substrate molecules under increasing laser power density. Comparisons with results from g.c.-m.s. and probe m.s. are presented, showing masses up to 600 Da. Limited structural information about the large molecular mass materials can be inferred from the present results, suggesting the presence of small aromatic groups with mass differences corresponding to methyl groups, ethylene bridges and benzo groups.

Matrix-assisted Laser Desorption/Ionization Mass Spectrometry of Pitch Fractions Separated by Planar Chromatography

Molecular mass distributions of coal-tar pitch fractions separated by planar chromatography (P-C) have been compared by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, carried out in the absence of added matrix. Solvent pairs used in the P-C separation were pyridine-acetonitrile, pyridine-N,ZVdimethylformamide and tetrahydrofuran-toluene. Molecular complexity and average molecular masses were found to increase with decreasing mobilities of the fractions. UV-fluorescence spectroscopy showed shifts of peak intensities to longer wavelengths and decreasing quantum yields, suggesting the presence of progressively greater concentrations of large polynuclear aromatic systems with decreasing mobility of samples in P-C. Fractions immobile in pyridine gave MALDI mass spectra of low intensity, indicating that larger molecules are less easily desorbed or ionized. The observed similarity of upper-mass limits of planar-chromatographic fractions of different mobility is thought to suggest the presence of relatively large-molecular mass (MM) materials of variable polarity in coal-tar pitch. The results were consistent with earlier investigations showing the presence of high molecular mass materials in coal-derived liquids. MM-distributions determined by size exclusion chromatography (SEC) were also observed to increase with decreusing mobility of the fractions on P-C plates. This qualitative agreement with results from MALDI-MS indicates that reported shifts of polar molecules to shorter elution times in SEC (i.e. to apparently larger-MMs) were not of a sufficient magnitude to distort the relative ordering of MM-distributions of PC-fractions observed by SEC. A number of problems relating to the refinement of MALDI-MS determinations on complex mixtures are discussed.

Molecular masses up to 270 000 u in coal and coal-derived products by matrix assisted laser desorption ionization mass spectrometry (MALDI-m.s.)

Abstract This paper describes the application of matrix assisted laser desorption ionization (MALDI) to coals and coal-derived materials using sinapinic acid as the matrix. The mass range of molecules in coal and coal-derived materials has been extended by a factor 100 compared with prelaser desorption mass spectrometric measurements. A peak of intensity is observed for coals and coal-derived materials in the mass range 1000–5000 u which is sample dependent. The upper mass ranges of the spectra vary according to sample, ranging from over 260 000 u for Point of Ayr coal and 200 000 u for a coal tar pitch, to 20 000 u for a maceral concentrate liquefaction extract. These results confirm earlier results using laser desorption mass spectrometry and are in broad qualitative agreement with size exclusion chromatography results. Detailed quantitative agreement, however, requires further work. The implications of this work for the debate on coal structure and models of coal conversion are considerable.

A study of the spectroscopic and thermodynamic properties of furan by means of photoabsorption, photoelectron and photoion spectroscopy

Abstract Three experimental techniques (photoabsorption, photoelectron, and photoion spectroscopy) have been used to study the spectroscopic and thermodynamic properties of furan. The absolute photoabsorption cross-sections of furan-h4 and furan-d4 have been measured using a double ion chamber and a new Rydberg series converging onto the G 2 A 1 ionisation threshold has been observed. HeI-excited photoelectron spectra of the X 2 A 2 , the A 2 B 1 and the G 2 A 1 states of furan-d4 have been recorded. Vibrational structure has been observed in all three bands and has allowed the energies of the ν3, ν4, ν6 and ν8 vibrational modes to be determined. Time-of-flight mass spectra have been recorded using monochromatic synchrotron radiation, and appearance energies have been measured for 19 fragment ions and the doubly charged parent ion. The fragmentation processes leading to the production of several high-intensity fragment ions have been modelled using ab initio and semi-empirical methods.

A study of the unimolecular decomposition of internal-energy-selected furan molecular ions by threshold-photoelectron-photoion coincidence spectroscopy

Abstract The unimolecular decomposition of internal-energy-selected furan molecular ions has been studied by means of threshold-photoelectron–photoion coincidence spectroscopy. Monochromatic synchrotron radiation was used as the ionisation source, and the molecular ion internal energy was established through the detection of a threshold electron. A pulsed electric field was applied to extract the ions from the interaction region and direct them towards a time-of-flight mass spectrometer. Breakdown curves were measured for photon energies up to 30 eV, and these have allowed appearance energies for a wide range of fragment ions to be determined. In the threshold region the breakdown curves have been measured for various ion residence times by introducing electronic delays between the detection of the threshold electron and the application of the ion extraction field. The breakdown curves have been modelled using the RRKM (Rice, Ramsperger, Kassel and Marcus)/QET (quasi-equilibrium theory) approach, and this has allowed activation energies and transition state geometries to be deduced. The threshold photoelectron spectra of furan-h 4 and furan-d 4 have been measured from the ionisation threshold to 28 eV, and vibrational structure has been observed and assigned in the bands due to the X 2 A 2 , the A 2 B 1 and the G 2 A 1 states. Vibrational progressions discernible between 16.2 and 17.3 eV have been attributed to autoionisation from a p-type Rydberg series converging onto the G 2 A 1 state ionisation threshold.

A photoabsorption and mass spectrometry study of pyrrole

Abstract The absolute photoabsorption cross-section of pyrrole has been measured between the ionisation threshold and 35 eV using a double ion chamber and monochromated synchrotron radiation. An interpretation has been proposed for some of the observed structure in terms of Rydberg series converging onto the A 2 B 1 state threshold. In addition, the possibility that several broad features may be attributed to π→π∗ transitions is discussed. Time-of-flight mass spectra have been recorded for excitation energies between 11.8 and 27.5 eV, and appearance energies have been determined for seventeen fragment ions and the doubly charged parent ion. These have enabled, previously unknown, heats of formation to be estimated for five ions, including the doubly charged parent ion. A high resolution mass spectrometer, equipped with an electron impact ionisation source, has been used in B/E and B2/E linked scanning modes to identify reactant/product pairings. Fragmentation processes leading to the production of several high intensity fragment ions have been modelled using ab initio and semi-empirical methods.

An experimental and theoretical study of the spectroscopic and thermodynamic properties of toluene

Abstract Three experimental techniques – photoabsorption, photoelectron and photoion spectroscopy – have been combined with many-body Greens function calculations to investigate the spectroscopic and thermodynamic properties of toluene. The absolute photoabsorption cross section has been measured from the ionisation threshold to 350 A using a double ion chamber and monochromated synchrotron radiation. Some of the structure has been arranged into Rydberg series. He I excited photoelectron spectra of toluene-h8 and toluene-d8 have been recorded and vibrational progressions have been observed in three bands. Fragmentation processes have been studied by measuring time-of-flight spectra and appearance energies have been determined for many small fragments, and the doubly-charged parent ion. The many-body Greens function approach, specially adapted for the outer valence region, has been used to calculate ionisation energies and pole strengths.

Some gas-phase ion–molecule reactions of acetone

Photoionization of acetone is used to produce the ground electronic state molecular ion with little internal or translational energy. Ion–molecule reactions occurring under thermal energy conditions lead to the formation of the proton bound dimer of acetone at mTorr pressures, the rate constant of which reaction has been measured. The structures of various reactant and product ions are investigated using collision-induced dissociation (CID) reactions at collision energies up to 15 eV. These low-energy CID results are compared with 8 keV CID structures obtained using a conventional high-pressure double-focussing mass spectrometer.

Charge-transfer reactions of carbon tetrafluoride

Abstract The dissociative charge-transfer reactions of CF 4 with a beam of He + , Ne + or H 2 + ions have been investigated under single collision conditions. The translational energy of the ion beam was varied in the range of 700 to 5000 eV. The charge-transfer cross-sections and the product branching ratios varied with translational energy. This variation is interpreted as an initial formation of CF 4 + D and C states by He + , CF 4 + C state by Ne + and CF 4 + X, A and B states by H 2 + . All of these states are then fully dissociated.

Energy transfer and primary product ion excitation in dissociative charge-transfer and penning ionization reactions

Abstract The near-adiabatic theory has been applied to experimental data obtained for some dissociative charge-transfer reactions of propane. The results support the proposition that the electronic energy levels of the C3H8+ primary product ion from dissociative charge-transfer correspond to published values of the appearance potentials of fragment ions from propane. The internal energy distribution of the C3H8+ primary product ion is obtained by plotting the relative abundances of the fragment ions against their appearance potentials.