MolecularCharacterization of Hydrocarbons in AviationFuels via Two-Dimensional Gas Chromatography/Methane Chemical IonizationMass Spectrometry

Aviation fuels are complex mixtures consisting mostly of hydrocarbons that belong to several different classes, predominantly, <i>n-</i>alkanes, branched alkanes, cycloalkanes, and aromatic hydrocarbons. Two-dimensional gas chromatography (GC×GC) coupled with positive-ion mode electron ionization mass spectrometry (EI MS) has been used in the past to attempt to classify hydrocarbons in jet fuels. However, identification of hydrocarbons by using EI MS is unreliable due to the extensive fragmentation of their molecular ions, which often prevents obtaining molecular weight (MW) information. Furthermore, larger aliphatic hydrocarbons often yield identical fragment ion patterns. To address these issues, the utility of positive-ion mode methane chemical ionization (CI) coupled with GC×GC/high-resolution MS was examined for the analysis of hydrocarbons in aviation fuels. Initially, several individual model compounds and a mixture of 46 model compounds were studied. In contrast to EI mass spectra (also measured), the CI mass spectra showed pseudomolecular ions ([M+H–H₂]⁺) and/or protonated molecules ([M+H]⁺) for all but two of the examined model compounds, which facilitates the determination of the MW of unknown compounds. Further, CI caused different ionization and fragmentation reactions for model compounds belonging to the different hydrocarbon classes, thus providing valuable structural information. For example, all <i>n-</i>alkanes, branched alkanes, and cycloalkanes generated [M+H–H₂]⁺ pseudomolecular ions via protonation followed by elimination of a hydrogen molecule, with the exception of the two most highly branched alkanes. In sharp contrast, aromatic hydrocarbons generated primarily stable protonated molecules ([M+H]⁺) but also molecular radical cations (M^+●). Additionally, the detected fragmentation patterns were found to provide structural information that, when combined with the MW information, the double-bond-and-ring equivalent values and elemental compositions obtained from highly accurate mass measurements, facilitated the unambiguous classification and identification of unknown compounds. This approach was used to classify 93% of the hydrocarbons detected in Jet-A and to identify several of them.