The detection of high-mass aliphatics in petroleum by matrix-assisted laser desorption/ionisation mass spectrometry.

RATIONALE: The development of simplified procedures for isolating high-mass alkanes present in crude oils is described. The new procedures, which bypass the sample recovery step with hot toluene in the conventional alkane-isolation procedure, also provide an effective sample preparation route, prior to analysis by matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS). METHODS: Urea-alkane adducts are formed by mixing sample and urea solutions on chromatographic paper or silica-coated plates. Unreacted hydrocarbons are removed by developing the plates with chloroform. In a second development with water, adducts are broken up in situ and the liberated urea removed, leaving bands of isolated alkanes behind. For MALDI-MS, strips of paper/plates, carrying the isolated alkanes, are fixed on metal target plates. The samples are treated with matrix (AgNO(3)) and analysed by MALDI-MS. RESULTS: The observed signal represents silver ion adducts of the isolated alkanes. Silver appears to work without much fragmentation and to generate whole silver adduct ions. Much improved MALDI-MS detection sensitivity and a wider range of masses was observed when samples were ablated from paper/plate surfaces, than by ablation from bulk samples spread over a smooth surface--the conventional method. Chromatographic paper gave better resolution and a broader range of masses than silica-coated plates. CONCLUSIONS: The analytical sequences have been confirmed using standard alkanes (C(20)-C(60)) and Polywax. The proposed procedures enhanced the sensitivity and detection range of the MS analysis. The method was useful in detecting n-alkanes to m/z 1500 (C(100)) and required relatively small quantities of sample and reagents. It provides a promising qualitative analysis route for the rapid isolation and reliable determination of alkanes in crude oils.

Limitations of mass spectrometric methods for the characterization of polydisperse materials.

This paper is a review of work on the characterization of coal liquids and petroleum residues and asphaltenes over several decades in which various mass spectrometric methods have been investigated. The limitations of mass spectrometric methods require exploration in order to understand what the different analytical methods can reveal about environmental pollution by these kinds of samples and, perhaps more importantly, what they cannot reveal. The application of mass spectrometry to environmental problems generally requires the detection and determination of the concentration of specific pollutants released into the environment by accident or design. The release of crude petroleum or coal liquids into the environment can be detected and tracked during biodegradation processes through specific chemicals such as alkanes or polyaromatic hydrocarbons (PAHs). However, petroleum asphaltenes are polydisperse materials of unknown mass range and chemical structures and, therefore, there are no individual chemicals to detect. It is necessary to determine methods of detection and the ranges of mass of such materials. This can only be achieved through fractionation to reduce the polydispersity of the initial sample. Comparison of mass spectrometric results with results from an independent analytical method such as size-exclusion chromatography with a suitable eluent is advisable to confirm that all the sample has been detected and mass discrimination effects avoided.

Pyrolysis of waste polypropylene and characterisation of tar.

Waste polypropylene (PP) has been pyrolysed to obtain mainly a liquid tar product of high yield (83.5%) with the balance as gas (15.5%) and a little residue (1.0%). The elemental composition of the PP tar was: C: 87.1%, H: 12.6% and O+others: 0.4% (by difference). The tar samples have been characterised by gas chromatography/mass spectrometry, heated-probe mass spectrometry and laser -desorption mass spectrometry (LD- MS), to give molecular mass distributions for comparison with molecular mass ranges indicated by size-exclusion chromatography (SEC). About 50% of the tar was soluble in 1-methyl-2-pyrrolidinone, the solvent used for SEC. It appeared to consist mostly of low molecular mass materials with elution time at 20-27 min. Mass ranges from SEC and LD-MS agreed approximately in showing the upper mass limit of the tar to be about 1200 u, consisting of aromatics, alkenes, dialkenes and only minor quantities of alkanes.

The high-mass component (>m/z 10 000) of coal tar pitch by matrix-assisted laser desorption/ionisation mass spectrometry and size-exclusion chromatography.

The size-exclusion chromatography (SEC) of acetone-soluble, pyridine-soluble and pyridine-insoluble fractions of a coal tar pitch indicates a bimodal distribution in each fraction. The proportion of high-mass material excluded from the SEC column porosity increases with solvent polarity. The polymer calibration of SEC shows the mass range of the small molecules to be from approximately 100 u to approximately 6000 u, with the mass range of the large excluded molecules above 200 000 u and up to several million u. In contrast, matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS) shows a similar low-mass range of ion abundances (< m/z 6000), but with a smaller range of high-mass ion abundances, from approximately m/z 10 000 to 100 000. The large molecules may have three-dimensional structures to allow molecules of relatively low mass to behave as if they are of large size in SEC. Laser desorption mass spectrometry of the acetone- and pyridine-soluble fractions produced molecular ions of polycyclic aromatics that can be related to the known compositions from gas chromatography (GC) mass spectrometry. The experimental conditions used to generate the bimodal distribution by MALDI-MS involve reducing the ion signal intensities to avoid overload of the detector and enable detection of the high-mass ions, by reducing the high-mass detector voltage (i.e. sensitivity) and increasing the laser power.

Oligomeric carbon and siloxane series observed by matrix-assisted laser desorption/ionisation and laser desorption/ionisation mass spectrometry during the analysis of soot formed in fuel-rich flames.

Oligomeric carbon and siloxane series have been observed by matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS), during the analysis of the dichloromethane (DCM)-soluble fractions of condensable material recovered from fuel-rich flames. Laser desorption (LD) spectra showed a pattern of oligomeric dimethyl-siloxane structures with a spacing of 74 u. The siloxane series appears to have originated as contamination of samples by silicone oil used to lubricate connections of polymer tubing. This was confirmed by extracting silicone tubing and silicone grease with DCM followed by MALDI-MS analysis. A series of peaks with a mass spacing of 24 u was also observed, superimposed on the continuum of unresolved organic ions. This oligomeric series appears to correspond to polycyclic aromatics separated by (mainly) ethylene bridges. Thus LD-MS appears to have revealed a series of soot precursors, intermediate between polycyclic aromatics and particulate soot, which was not detected by MALDI-MS. More detailed work is necessary to define these species with precision.

Size-exclusion chromatography of large molecules from coal liquids, petroleum residues, soots, biomass tars and humic substances.

Size-exclusion chromatography (SEC) using 1-methyl-2-pyrrolidinone (NMP) as eluent has been calibrated using various standard polymers and model compounds and applied to the analysis of extracts of coal, petroleum and kerogens, to petroleum vacuum residues, soots, biomass tars and humic substances. Three separate columns of different molecular mass (MM) ranges were used, with detection by UV absorption; an evaporative light scattering detector was used for samples with no UV absorption. Fractionation was useful to separate signal from the less abundant high-mass material, which was normally masked by the strong signal from the more abundant low-mass material in the absence of fractionation. Fractionation methods used to isolate high-mass materials before SEC analysis included planar chromatography, column chromatography and solvent solubility. The apparently large molecules were concentrated into the fractions not soluble in common solvents and were relatively immobile in planar chromatography. All samples and fractions contained some material excluded from the column porosity. Evidence from other techniques suggests that the excluded material is of different structures from that of the resolved material rather than consisting of aggregates of small molecules. We speculate that the excluded material may elute early because the structures of this material are three-dimensional rather than planar or near planar.