Fullerenes from kerogen by laser ablation fourier transform ion cyclotron resonance mass spectrometry

Raw oil shale, kerogen (demineralized shale) and carbonaceous residues from kerogen pyrolysis in the range 350–700°C (at 50°C intervals) were studied by laser ablation Fourier transform ion cyclotron resonance mass spectrometry using the fundamental frequency of Nd: YAG laser (1064 nm). Normally, pyrolysis of the raw materials produces oil and the resulting residues have decreased hydrogen to carbon ratios and exhibit relative increases in aromatic carbons. Raw shale and kerogen give positive-ion spectra with mainly protonated species of m/z 100–400. Laser ablation positive-ion mass spectra of the pyrolysis products of the kerogen show the presence of C₆₀, C₇₀ and other fullerene ions with a distribution of higher mass fullerene ions up to m/z 4000. Using high laser powers (100–3000 MW cm^?2), the residue from pyrolysis at 350°C initially did not produce any fullerene ions (apart from traces of C₆₀ and C₇₀), but after continued ablation a cavity was formed in the target and a wide distribution of fullerene ions was obtained with subsequent laser pulses. Residues obtained from the pyrolysis of kerogen at 400–500°C produced fullerene ions at both low (4–200 kW cm^?2) and high laser powers. The 550°C pyrolysis residue gave only small amounts of C₆₀ and C₇₀ positive ions at low laser power whereas residues from the pyrolysis of kerogen above 550°C did not give fullerene ions over a wide range of laser powers. It is proposed from the above results that the changes in the aromatic nature of the kerogen residues with increasing pyrolysis temperature are directly related to the ease of fullerene formation. This is possibly due to the formation of large polycyclic aromatic systems at pyrolysis temperatures above 400°C, formed in the residues. It should be noted that the shale samples (raw or pyrolysed) did not generate fullerene ions under any of the conditions employed in these experiments.