Characterization of Hydrocarbon Groups in Complex Mixtures Using Gas Chromatography with Unit-Mass Resolution Electron Ionization Mass Spectrometry

Complex
mixtures of hydrocarbons are ubiquitous as petroleum fuels
and, consequently, environmental contaminants. Because they contain
thousands of individual components with similar molecular structures,
detailed chemical characterization of hydrocarbon mixtures relies
on advanced analytical techniques that are not accessible to many
researchers. Many analyses of hydrocarbon mixtures instead characterize
them as “unresolved complex mixtures”, with quantification
limited to a small number of resolvable components and/or total observed
mass within specified volatility ranges. This work develops a new
analytical approach to characterize the hydrocarbon component of petroleum
and environmental mixtures by “hydrocarbon group” (defined
by carbon number, degree of unsaturation and, in certain cases, degree
of branching) using gas chromatography coupled to a unit-mass-resolution
electron ionization quadrupole mass spectrometer (GC/EI-MS), a standard
and widely available instrument. Average mass spectra of hydrocarbons
from a widely used spectral library are combined with chromatographic
signal representing the molecular ion of each hydrocarbon group to
recreate the magnitude and mass spectra of the chromatogram. Characterization
of hydrocarbons in diesel fuel by this approach is in good agreement
with state-of-the-art techniques relying on high-resolution and fast-response
mass spectrometers. Application of this approach to subsurface soil
gas samples from remediated sites of underground storage tank spills
demonstrates that composition of hydrocarbons in environmental samples
varies significantly and that the total signal of samples from contaminated
sites may contain a substantial fraction of oxygenated components.