Separation and measurement of flame‐formed high molecular weight polycyclic aromatic hydrocarbons by size‐exclusion chromatography and laser desorption/ionization time‐of‐flight mass spectrometry

Abstract

The partial contribution of polycyclic aromatic hydrocarbons (PAH), capable of being detected by gas chromatography (GC-PAH), both to the total mass of the extractable organic fraction of flame-formed carbon particulates and to its UV-visible absorption and fluorescence spectra, has been determined by previous work. This contribution indicates the presence of PAH of molecular weight (MW) greater than 400 Da not accessible to conventional analysis. The detection of species in this higher MW range is important for both their potential toxicology and their possible role in soot formation. In the present work extracts of soots have been analyzed by linear mode laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) to extend the MW range that can be analyzed beyond the GC-PAH. The results have been compared with both analysis by reflector mode LDI-TOF-MS and the MW evaluation obtained by SEC analysis, as the shortcomings and advantages of both techniques appear to be complementary. Matching the results from the two techniques could give interesting insights in the molecular mass range between GC-PAH and the first soot particles (of mass > 2000 Da). Mass spectra in this molecular mass range have been obtained with a main ion sequence spacing of 24 Th and a minor ion sequence also with a spacing of 24 Th but off-set by 12 Th with respect to the main sequence. The two ion progressions have been interpreted by attributing the predominant peaks mainly to PAH with even-carbon numbers and the smaller ones to cyclopenta-fused ring PAH. These distributions indicate the occurrence of two competitive mechanisms in the growth of PAH and soot nucleation, i.e. the addition of acetylene (HACA mechanism) and the incorporation of pentagons by large polycyclic aromatic molecules into their aromatic bonding network.

Blue and green luminescent carbon nanodots from controllable fuel-rich flame reactors

“…The extent to which blue and green CDs are separated by DCM and NMP extraction can be evaluated by the separation in size-, MW-segregated fractions 36 obtained by SEC analysis coupled with UV-Visible absorption detector fixed at 350 nm. Figure 6 shows a typical MW/size distribution of blue and green CDs.…”

“…It has been found that two- to seven-membered ring PAH justify only a part of the organic species extracted with dichloromethane (DCM) from the carbon particulate matter formed in premixed flames 35 . This inference points towards the presence of heavier aromatic species as supported by size exclusion chromatography (SEC) 36 and spectroscopic analysis 24,25 . Looking for the heavier aromatic species that are probably involved in soot formation, we have previously discovered the presence of green-fluorescent aromatic species, that were strongly clung to the soot particles.…”

Laser desorption/ionization determination of molecular weight distributions of polyaromatic carbonaceous compounds and their aggregates

“…The refinement and upgrading of mass spectrometry (MS) techniques have provided a considerable impulse to the determination of the composition, molecular weight distribution (MWD) and structure of carbonaceous species 6–19. In particular, laser desorption/ionization mass spectrometry (LDI–MS) has currently achieved a mass resolution capable of discerning chemical composition and structure in complex carbonaceous mixtures, especially when applied in combination with chromatographic separation 13–15. The present article is focussed on the application of LDI–MS techniques to the characterization of specific classes of carbonaceous molecules, namely polycondensed hydrocarbons and heavy oil fractions (asphaltenes).…”

Distinction of gaseous soot precursor molecules and soot precursor particles through photoionization mass spectrometry

“…Soot precursor particles have been measured in the past mostly by microscopic4 or by several spectroscopic methods 5, 6. In addition, atomic force microscopy (AFM), size exclusion chromatography (SEC) and laser desorption/ionization mass spectrometry have served this same purpose 7–9. More recently, progress has been achieved particularly through the use of differential mobility analysis (DMA).…”