Chemical Characterization and Quantitation of Phenols in Fuel Extracts by Using Gas Chromatography/Methane Chemical Ionization Triple Quadrupole Mass Spectrometry

Abstract

The ability to detect, characterize, and quantify phenols in aviation fuels is critical as these compounds can negatively affect the storage stability of the fuel. However, some phenols inhibit free-radical autoxidation and hence are beneficial. The development of a method based on gas chromatography coupled with positive-ion-mode methane chemical ionization/triple quadrupole mass spectrometry [GC/(methane CI) QqQ] for the characterization and quantitation of phenols in fuels is reported here. Sixteen phenols, ranging from phenol to phenols with up to three alkyl groups of different chain lengths, were studied. Natural phenols (defined as phenols without tert-butyl groups) were protonated upon methane CI, which was associated with diagnostic fragmentation. For example, upon protonation, 2-, 3-, and 4-ethylphenols fragmented via the loss of ethylene, which facilitated their distinction from the isomeric 2,4-, 2,5-, and 3,4-dimethylphenols. Furthermore, the natural phenols formed stable adducts with C2H5+ and C3H5+ ions generated from methane upon CI. In contrast, additive phenols (with one or more tert-butyl groups) were predominantly ionized via electron abstraction to yield molecular radical cations and did not form adduct ions and generated some diagnostic fragment ions, such as ions of m/z 57 (tert-butyl cation). Most of the molecular radical cations of additive phenols also exhibited a methyl radical loss, while protonated additive phenols exhibited water loss. The limit of detection and the limit of quantitation were determined to be 1.3 μM (0.33 ppm) and 4.2 μM (1.08 ppm), respectively, and the linearity of quantitation was between 5 and 160 μM. Measurements of equimolar mixtures composed of additive and natural phenols demonstrated similar (within 10%) but not identical ionization efficiencies for these two compound types. Intra- and interday measurements of the signal intensities of the phenols were highly repeatable, with average relative standard deviations of 1.7 and 5.2%, respectively. The same method was employed to successfully characterize and quantify unknown phenols in alternative and petroleum-based jet fuel extracts. The petroleum-based Jet A fuel was found to contain a 1.5 times greater concentration of phenols than the alternative fuel. Jet A contained monomethylated phenols (108 Da), while the alternative fuel did not. Several phenols with molecular weights of 122, 136, and 150 Da were detected in both Jet A and the alternative fuel. The CI mass spectra suggested that some of these phenols had multiple methyl substituents. The information that can be acquired by using the GC/(CI) QqQ method will facilitate establishing links between the chemical compositions of fuels and fuel properties. Overall, the method was found to be robust and repeatable.