Quantitation of n -Alkanes, Branched Alkanes, Cycloalkanes, and Alkenes via GC×GC/(+)EI TOF MS

A quantitation method for n-alkanes, branched alkanes, cycloalkanes, and alkenes in mixtures has been developed based on two-dimensional gas chromatography (GCxGC)/positive ion-mode 70 eV electron ionization time-of-flight mass spectrometry ((+)EI TOF MS). Chromatographic peak areas for 50 standard model compounds in a concentration range from 0.5 up to 10 mM were obtained using GC×GC coupled with flame ionization detection (FID) and directly compared with the peak area response of the same standards obtained by using GC×GC/(+)EI TOF MS. Empirical normalized correction factors for each standard were calculated as the relationship between the peak area response for the FID and the (+)EI TOF MS at each concentration with respect to an internal standard of a fixed concentration, i.e., 1-methylnaphthalene at a concentration of 2 mM. Linear empirical correction factors to adjust the raw peak areas measured using (+)EI TOF MS with respect to the number of carbon atoms were found for n-alkanes, polysubstituted alkanes, and polysubstituted alkenes, while a quadratic relationship was found for n-alkylcyclohexanes. No linear or polynomial relationships were found between the normalized correction factors and the number of carbon atoms for disubstituted alkanes, disubstituted alkenes, and polycycloalkanes. However, specific normalized correction factors and weighing factors are proposed for their quantitation by using (+)EI TOF MS. The influence of the degree of branching of the analytes on their ionization efficiency upon EI and their response in FID and, therefore, on their empirical correction factors, is discussed.