Molecular characterization of sulfur-containing species in petroleum is important because sulfur-containing
compounds are detrimental to the environment and the refining processes. In a recent report, the sulfur-containing
compounds in a vacuum bottom residue (VBR) were methylated to enhance their detectability by electrospray
ionization (ESI) mass analysis. The most abundant sulfur compounds exhibited relatively low double bond
equivalents (4 < DBE < 12). Alternatively, atmospheric pressure photoionization (APPI) mass analysis can
provide molecular characterization without chemical derivatization. Here, we compare the sulfur speciation of
a petroleum vacuum bottom residue by ESI and APPI with a 9.4 T Fourier transform ion cyclotron resonance
(FT-ICR) mass spectrometer. Even after methylation, ions produced by APPI extend to much higher DBE
than by ESI. Moreover, analysis of the saturates and aromatics fractions of underivatized VBR by APPI shows
comparable ionization efficiency across a broad DBE range. We conclude that methylation is hindered for
high-DBE species (DBE > 20), so that methylation followed by ESI MS is not suitable for sulfur speciation
of higher-boiling fractions from petroleum crude oil.
Elemental compositional analysis of processed and unprocessed diesel fuels is obtained with a 5.6-T Fourier
transform ion cyclotron resonance (FT-ICR) mass spectrometer coupled to an all-glass heated inlet system
(AGHIS). High-resolution mass spectra of electron-ionized diesel fuel samples are obtained from as little as a
500-nL septum injection into the AGHIS, to yield ∼500
peaks over a range 90 < m/z < 300, with as many as
seven peaks present at the same nominal mass. Molecular formulas (elemental compositions) are assigned from
accurate mass measurement with an average error less
than ±0.5 ppm. Comparison of the raw and processed
diesel spectra shows complete removal of the sulfur-containing species except for dimethyldibenzothiophene
and higher alkyl-substituted dibenzothiophenes. These
results confirm prior reports of the resistance of these
species to hydrotreatment due to steric hindrance of
catalytic desulfurization arising from 4,6 dimethyl substitution. A simple liquid chromatographic separation to
isolate N-, O-, and S-containing aromatics from processed
diesel fuel simplifies the mass spectrum and extends the
dynamic range of the analysis, making it possible to
identify many nitrogen and oxygen homologues of the
sulfur-containing species, as well as to confirm the presence of sulfur-containing species initially detected in the
unfractionated processed diesel fuel.
We report the use of a continuous-wave (CW) CO2 laser for the determination of relative activation
energy for unimolecular dissociation of large biomolecular ions. The [M + 5H]5+ and [M + 11H]11+ ions of
bovine ubiquitin and the [M + H]+ ion of bradykinin are irradiated with a CW CO2 laser and the rate constant
for dissociation at each of several laser intensities recorded. A plot of the natural logarithm of the first-order
rate constant versus the natural logarithm of laser intensity yields a straight line whose slope provides an
approximate measure of the activation energy (E
a) for dissociation. For dissociation of protonated bradykinin,
the absolute E
a value from infrared multiphoton dissociation (IRMPD) agrees with that obtained by blackbody
infrared radiative dissociation (BIRD), whereas the IRMPD-determined E
as for dissociation of the 5+ and
11+ charge states of bovine ubiquitin are lower than those obtained by BIRD. The relative E
a values for the
5+ and 11+ charge states of bovine ubiquitin from both BIRD and IRMPD are in good agreement. Master
equation modeling was carried out on the model peptide, (AlaGly)8, to characterize the nature of the internal
energy distribution produced from irradiation by a monochromatic IR source (e.g., CW CO2 laser) versus a
broadband IR source (e.g., blackbody). The master equation simulation shows that the internal energy distribution
produced by irradiation with the CO2 laser is essentially identical to that obtained by blackbody irradiation.
Our combined experimental and theoretical results justify the IRMPD technique as a viable method for the
determination of relative ordering of activation energies for dissociation of large (>50 atoms) ions.
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