17th International Mass Spectrometry Conference :: Prague, 2006
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|Presentation date:||Tue, Aug 29, 2006|
|Presentation time:||09:50 – 11:20|
Alan Alfred Herod1, Eiman Al-Muhareb1, Trevor J. Morgan1, Marcos G. Millan-Agorio1, Rafael Kandiyoti11 Imperial College London, London, United Kingdom
Correspondence address: Alan Alfred Herod, Imperial College London, Chemical Engineering Department, South Kensington Campus, London, SW7 2AZ United Kingdom.
Keywords: Fuel; High Mass Ions; Laser Ionization; MALDI.
Novel aspect: High mass molecules in polydisperse samples have been identified using mass spectrometry and size exclusion chromatography. A conformation change seems to occur above several thousand mass units.
Large molecules have been detected in environmental samples (coal- and petroleum-liquids) initially by size exclusion chromatography (SEC) and their presence subsequently confirmed by MALDI-MS. Although SEC does not measure molecular mass directly, it can be used to estimate molecular mass ranges of polydisperse samples and fractions. It is advisable to match results from two different techniques in estimating mass ranges of polydisperse samples and to approach the upper mass limit of the sample.
Mass spectrometric studies using lasers to ionise samples of wide polydispersity have shown that mass discrimination prevents ionisation of the largest molecular mass material. A higher laser power is needed to vaporise and ionise the larger mass molecules than for the small mass molecules present in polydisperse samples.
We have found that several MALDI- MS parameters needed to be adjusted for reproducible mass spectra for a pyridine-insoluble fraction of pitch showing a bimodal but continuous distribution of mass up to about m/z 100,000. The ion intensities of smaller molecules were reduced by diminishing the voltage on the detector while increasing the laser power to 90% of maximum, and operating at low total ion intensities with pulsed ion extraction.
Similar results have been obtained for petroleum asphaltenes. The low-mass ranges detected by LD-MS (below m/z 10,000) and SEC agree reasonably satisfactorily. The high-mass range of LD-MS is a factor 100 (or more) lower in mass than indicated by the excluded peak observed in SEC. For agreement to be reached between the two methods, the molecules detected by LD-MS above several thousand mass units must be three-dimensional in shape to elute from SEC in the excluded region as large molecules.
A requirement of SEC is a solvent that 1) minimises interaction between solute molecules and column packing and 2) can completely dissolve the sample. We have shown that SEC using 1-methyl-2-pyrrolidinone (NMP) as eluent and calibrated using polystyrene standards provides an estimate of molecular mass for coal tar pitch fractions, in good agreement with the peak masses measured up to m/z 3,000 for the same fractions by MALDI-MS and LD-MS. Size exclusion chromatograms in NMP eluent always show a bimodal distribution with an approximately zero-intensity gap between the peaks. This is true for coal liquids, petroleum asphaltenes, biomass tars, soil, peat and amber. The reason for this gap may be a change of conformation above a particular mass value such that molecules become excluded from the SEC column porosity as indicated by three-dimensional standards (fullerene, colloidal silicas and soot) and elute independent of density.
Further evidence of changes of structures with increasing molecular mass will be described using data from HT-GC-MS and pyrolysis-GC-MS of fractions of coal and petroleum liquids prepared by solvent solubility and column chromatography, as well as from UV-fluorescence.