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.