The use of tetrahydrofuran (THF) as mobile phase in size exclusion chromatography (SEC) has been found to lead to partial loss of sample and to give anomalous results in the characterization of a liquefaction extract and its hydrocracking products. The problem has been resolved by using NMP (1-methyl-2-pyrrolidinone) as mobile phase in SEC, showing significant fractions of sample eluting at the exclusion limit of an identical SEC column. This fraction has not previously been observed in SEC chromatograms obtained in THF. Comparison of SEC chromatograms obtained by UV-absorption and UV-fluorescence detection (in NMP) suggests that the material observed at the exclusion limit of the column corresponds to larger, more complex polynuclear aromatic ring systems than those present in material separated by the column. In NMP, samples produced during progressively higher temperature hydrocracking experiments eluted, as expected, at longer times, indicating progressive molecular size reduction with increasing intensity of the reaction. These data are consistent with the UV-fluorescence spectra and TGA-derived boiling point distributions of the set of samples. A twofold mechanism for loss of material in THF-based SEC may be proposed: (i) not all the sample dissolves in THF and (ii) some of the larger/more polar molecules apparently soluble in THF tend to deposit on column packings and do not elute through the column. Considerable caution therefore appears necessary in using THF as mobile phase in SEC work for the characterization of complex coal-derived liquids.

Size exclusion chromatography (SEC) has been used extensively in the characterization of coal‐derived liquids to estimate molecular masses (MM), indicating the presence of considerably greater upper mass ranges compared to mass spectrometric methods based on heated probes and field ionization. In the present study, the use of tetrahydrofuran (THF), a solvent commonly used in SEC, has been shown to lead to partial loss of sample on column packings and to allow some separation by an adsorption mechanism rather than by size exclusion. In this paper, we have compared these results with data from SEC, where 1‐methyl 2‐pyrrolidinone was used as the mobile phase, indicating that the largest molecules (not observed in THF‐based work) appear at the exclusion limit, with a continuum of material down to the total permeation limit of the column. In terms of polystyrene standard masses, this is equivalent to a range from 100 u, at the lower limit, up to in excess of 30–40 000 u and possibly over 2 000 000 u. Our results from SEC in NMP suggest that limitations exist in much solution‐based work attempting to identify distributions of MMs in coal‐derived liquids. Matrix‐assisted laser desorption/ionization (MALDI) spectra of the fractions show similar features to those seen by SEC in NMP, with a continuum from high to low mass, and a significant shift to higher mass in the insoluble fraction compared with the soluble fraction. The new data are in line with observations by laser desorption mass spectrometry (LD‐MS) and MALDI‐MS, which have shown much greater upper‐MM limits than either SEC (in THF) or mass spectrometric techniques relying on thermal evaporation of coal‐derived materials from heated probes. The work would appear to have implications for other polydispersed natural polymer systems.

Two propositions relating to the interpretation of size-exclusion chromatograms (SEC) of coal-derived materials in 1-methyl-2-pyrrolidinone (NMP) have been examined. These were (i) that signal peaks showing up at exclusion (short retention time) limits of SEC columns are due to sample polarity alone and (ii) that shifts in SEC chromatograms to longer retention times, observed upon addition of LiBr to the eluent (NMP), are due to dissipation of ionic binding forces, causing disaggregation of polar clusters that would otherwise have appeared at retention times appropriate to larger molecular masses. In our experiments, effects due to polarity and molecular mass have been isolated by using two nonpolar samples (a naphthalene mesophase pitch and a mixture of fullerenes). In the presence of LiBr, precipitation of solute out of solution and shifts of chromatograms to longer retention times, unrelated to sample polarity, have been observed. A partial breakdown of the size exclusion mechanism was identified by the observed extension of chromatograms beyond the permeation limit of the column, similar to those observed when using eluents of insufficient solvent strength (e.g., THF, chloroform). Dosing LiBr into NMP sharply reduces the solvent power of NMP for coal-derived solutes. In the absence of LiBr, SEC chromatograms of the fullerene mixture, the naphthalene mesophase pitch, and its fractions separated by planar chromatography clearly showed significant signal under the “excluded” peak, entirely due to nonpolar material. The damage caused to the SEC column arising from precipitation of sample, in the presence of LiBr, was not permanent as had originally been feared. The balance of the evidence suggests that polarity of some molecules may cause shifts in their elution times to shorter values (larger apparent molecular masses) and that these may overlap with signal from large molecular mass material.

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.Recent attempts to reconcile molecular mass (MM) distributions identified in coal-derived liquids by laser desorption-based mass spectrometric techniques and by size exclusion chromatography (SEC) have passed through several stages.' Briefly, the first confirmation of the order of magnitude of MMs found by SEC, using tetrahydxofuran (THF) as mobile phase:-6 has been arrived at during work with a laser-ionization mass analysis instrument (LIMA)? Depending on the nature of the sample, clustered peaks were observed between mlz 500 and m / z 3000, with traces of material extending to the detection limit of the instrument (mlz 12 000). Follow-up work with a matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer (using sinapinic acid as matrix), on a set of coals, coal derived tars and extracts and demineralized kerogens*-" extended the observed MM-ranges to well beyond values observed by SEC in THE Depending on the nature of the sample, MM-ranges of the clusters of peaks were observed between mlz 1000 and mlz 6000. The upper mass limits of the trailing edges of trace material were found to extend to very high mlz values, reaching, in the case of the coal-tar pitch, almost to mlz 200 000.The range of coal-derived mate...

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The recent observations of transient complex oscillations in the Belousov-Zhabotinsky (BZ) reaction in a batch reactor are confirmed and an attempt is made to model this behavior using the Gyorgyi-Field mechanism (an extension of the classic Field-Koros-Noyes model). It is seen that the concentration of bromomalonic acid plays an important role, acting somewhat like a "slowly-varying parameter," causing the system to sweep through a region of oscillatory response even in the absence of reactant consumption. Complex behavior is not observed in the model if the usual "pre-equilibrium" assumption in made for the intermediate species BrO(2) but does arise if a full steady-state approximation is applied to this radical. These results may bring into question some assumptions made in the usual treatment of the cerium-catalyzed BZ system in closed reactors. (c) 1997 American Institute of Physics.

The inhibiting e †ect of dissolved oxygen on wave propagation in the ferroin-catalysed BelousovÈZhabotinsky (BZ) reaction is exploited to obtain three-dimensional scroll waves experimentally in this system. The development of the scrolls is monitored by imaging the wave evolution via a video camera and image acquisition software. Solutions for which the initial waves are initiated under an atmosphere of pure oxygen have a subexcitable or inhibited layer of typically 1 mm depth. Once the is replaced by this layer O 2 N 2 , regains excitability and a scroll wave structure can develop. For waves initiated under air, the depth of the inhibited layer is less and scroll waves do not develop. A simple extension of the ZBKE model for the ferroin system is proposed and shown to predict the oxygen inhibition in reasonable agreement with experiment.