Two-Dimensional Gas Chromatography Coupled with Electron and Chemical Ionization Mass Spectrometry for the Analysis of Aviation Fuels
Two-dimensional gas chromatography is a powerful method for the separation of compounds in very complex mixtures, including many petroleum products, such as transportation fuels. However, identification of compounds based on just their elution times is unreliable as very different types of compounds sometimes have similar elution times in both dimensions. Therefore, this method is often coupled to mass spectrometric analysis.
The identification of different types of unknown compounds in complex mixtures by using two-dimensional gas chromatography coupled with high-resolution time-of-flight mass spectrometry using positive-ion mode electron ionization (EI) and methane chemical ionization (CI) is discussed here. EI of saturated and chlorinated hydrocarbons as well as alcohols often produces extensive fragmentation. If a stable molecular ion is not generated for an unknown compound, which is true for large compounds of the types mentioned above, identification of the compound is challenging, even when using EI mass spectral libraries.
Several examples are provided and the limitations of identification of compounds via EI mass spectrometry library matching are discussed. However, the issues can be addressed by using CI with methane reagent gas as this approach usually yields ions that contain the intact analyte molecule (protonated molecules or their fragment ions formed by elimination of a hydrogen molecule) and hence provide MW information. This is true even for large saturated hydrocarbons, which enables their identification.
Furthermore, CI generates structurally informative fragment ions that complement the structural information obtained from EI mass spectra. Information obtained using both approaches for several additional compound classes, such as unsaturated and aromatic hydrocarbons and oxygen-, chlorine-, and nitrogen-containing compounds, is discussed.
Presenter: Hilkka Kenttamaa (Purdue University)
