Barry R. Johnson

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.

Absolute calibration of size exclusion chromatography for coal derivatives through MALDI-m.s.

This paper reports the fractionation and subsequent examination of two, very different coal derived materials, a coal tar pitch and a commercial liquefaction extract. These were separated on the basis of their retention in a typical preparative size exclusion chromatography apparatus using THF as the eluent. Matrix assisted laser desorption mass spectrometry, using the small molecules of the sample as matrix, has been used to examine the various fractions from preparative s.e.c. and the peak mass intensities have been used to define an absolute calibration of the s.e.c. separation. Limitations of the methods are discussed. The u.v.-fluorescence spectra of the fractions were recorded and trends in the spectra are described. The fractions were also analyzed by size exclusion chromatography employing N-methyl-2-pyrrolidinone as the chromatographic eluent. The main advantages of this system over traditional solvents is the avoidance of surface adsorption effects observed with THF and the more complete solution of coal-derived material.

N-methyl-2-pyrrolidinone as a mobile phase in the size-exclusion chromatography of coal derivatives

Abstract This paper reviews the current state of molecular mass determination for fossil fuel materials by chromatographic methods and describes improvements which can be made to both the chromatographic separation and the detection of eluting material. Most significantly, N-methyl-2-pyrrolidinone (NMP) offers a number of advantages as a mobile phase in the size-exclusion chromatography (SEC) of coal derivatives. Much more coal-derived material dissolves in NMP and solute-column packing interactions common with SEC solvents such as tetrahydrofuran are much reduced. NMP is compatible with UV absorption and UV fluorescence detection

Effect of oxygen on wave propagation in the ferroin-catalysed Belousov–Zhabotinsky reaction

An experimental investigation of the propagation of reaction-diffusion waves in thin layers of solution for the ferroin-catalysed Belousov–Zhabotinsky reaction is reported. It is shown that for layers of 1.5 mm or greater depth, there are no significant effects of oxygen on the wave speed, but that for more shallow layers the waves are sensitive to the presence of O2 in the gas phase above the solution and also to the concentration of malonic acid in the solution. Waves in solutions under N2 or solutions covered with a perspex lid show effectively no depth effects. Dispersion relationships, the dependence of the wave speed on wavelength, are determined and tested against a recently proposed ‘universal form’. Direct imaging of the waves shows that in the presence of O2, there is no wave propagation in the top 0.4 mm of solution, independent of the solution depth.

Modelling complex oscillations for the H2+ O2 reaction in an open system

The complex waveforms observed in the oscillatory ignition of a stoichiometric H2–O2 mixture in a well stirred, continuous-flow reactor are modelled using a 35-reaction Baldwin–Walker mechanism. The behaviour of this reaction is further studied through a series of reduced mechanisms of 17, 13 and 10 steps, respectively. In each case, complex waveforms require the self-heating of this exothermic reaction to be included, whereas simple oscillations arise even with isothermal models. The methods of bifurcation theory are applied to the smallest of these models and suggest a close connection between parameter ranges for which the simple oscillations in the isothermal scheme change rapidly in period and amplitude with the conditions for complex oscillations in the full, non-isothermal model.

252Cf plasma desorption and laser desorption mass spectrometry for the determination of molecular weight distribution of coal derivatives

A detailed knowledge of the molecular mass (MM) distribution in coal and its derived products is essential for a fundamental understanding of coal structure, and of the processes occurring during coal conversion. Fractionation using size exclusion chromatography (s.e.c.) using N-methyl-2-pyrrolidinone as the mobile phase has been applied to such materials and has provided improved MM distributions. Absolute calibration has been provided using matrix assisted laser desorption ionisation mass spectrometry (MAl.d.I-m.s.). An alternative method of volatilising and ionising large molecules for mass spectrometry (m.s.) is 252 Cf plasma desorption ( 252 Cf p.d.-m.s.). This involves the use of energetic fission fragments from the decay of 252 Cf and produces mass spectra consisting predominantly of molecular ions from a range of polymers and biomolecules. This has been used by other workers to determine the molecular weight distribution of heavy distillation residues obtained from coal liquefaction processes either unfractionated or fractionated into broad fractions. Generally, a good agreement was obtained between values of MM determined by 252 Cf p.d.-m.s. and s.e.c., A comparison is reported of MM distribution determined by 252 Cf p.d.-m.s. and laser desorption mass spectrometry (I.d.-m.s.) for narrower fractions separated by s.e.c. from a coal tar pitch.

Characterisation of oscillations in the H2+ O2 reaction in a continuous-flow reactor

The H2+ O2 reaction is one of the simplest chemical oscillators and of great practical significance. In a well stirred flow system, equimolar mixtures support oscillatory ignitions, with a simple periodic waveform, over a range of ambient temperature and operating pressure. Here we report a series of experiments in which the outputs from the monitoring thermocouple and photomultiplier are data captured onto computer. These ‘time series’ are then treated via a number of techniques from dynamical systems theory, including fast Fourier transforms and attractor reconstructions. This allows the routes by which oscillations set in and are extinguished to be recognised. New responses have then been predicted in the light of these theories and observed experimentally. With a stoichiometric 2H2+ O2 mixture, complex mixed-mode oscillations and birhythmicity are also observed over some range of experimental conditions.

A test of the MALDI-MS calibration of SEC with N-methyl-2-pyrrolidinone for coal derived materials

Abstract In this paper we test our proposed methodology for a calibration procedure for mass determination by size exclusion chromatography. A sample is coarsely fractionated on column and when the fractions are examined by matrix assisted laser desorption–ionisation mass spectrometry a direct variation of molecular mass with retention is found. Importantly the chromatograms produced when using N -methyl-2-pyrrolidinone as eluant are found to be free from solute association effects over a wide range of concentrations. The examination of a gasifier char is presented as a unique application of this chromatographic system.

Period doubling and chaos during the oscillatory ignition of the CO + O2 reaction

Experimental observations of period doubling accompanying the oscillatory ignition mode of carbon monoxide oxidation in a well stirred flow reactor are reported. In the presence of 0.5% added H2 and with a mean residence time of 16 s, we find oscillatory ignition for conditions above the second explosion limit. Further into this ignition region, the oscillatory waveform develops through period-2, period-4 ignitions etc., finally under-going a full ‘subharmonic cascade’ into chemical chaos. We have also observed subsequent periodic windows (period-5 and period-3). Simple oscillations are re-established via an inverse sequence back to period-1 ignitions. Our data capture allows reconstruction of the corresponding attractors and limit cycles and the Fourier power spectra.

Characterization of oil shales by extraction with N-methylpyrrolidone

Two Turkish oil shales were extracted at room temperature with N-methylpyrrolidone and with a mixture of N-methylpyrrolidone and carbon disulfide. The extracts were analysed by chromatography and spectroscopy. The aromatic compounds contained small clusters of aromatic rings highly substituted with branched alkyl groups. The alkane geomarkers are consistent with immature sediments derived from terrestrial higher plants and aquatic material deposited under saline conditions.