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 C 2 H 5 + and C 3 H 5 + 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.

Chemical compositional analysis of jet fuels: Contributions of mass spectrometry in the 21st century

“…The use of CI was shown to prevent fragmentation for natural phenols and PA by using (GC/methane CI) QqQ MS (Guthrie et al, 2022). Protonation [M + H] + was the dominant ionization reactions for natural phenols measured.…”

“…The results support similar ionization responses for the phenols. Reprinted (modified) with permission from Guthrie et al (2022). Copyright 2022 American Chemical Society.…”

Exploring Negative Chemical Ionization of Per- and Polyfluoroalkyl Substances via a Liquid Electron Ionization LC-MS Interface

“…Spectra in ECNCI are characterized by limited fragmentation and intense and easily recognizable molecular ions or intense fragment ions. Soft ionization techniques, such as CI, are therefore experiencing renewed interest, demonstrating their advantages and suitability in many applications. , GC-NCI-MS methods have gained significant interest in several fields, ranging from environmental and biological to food, due to the high sensitivity and specificity toward the compounds containing electronegative ions and functional groups. − …”

“…It was clearly demonstrated that most PFASs tend to be observed using NCI rather than EI or PCI modes. The strong electrophilic character of PFASs is given by the presence of F and the terminal carboxylic group, making these compounds amenable to NCI rather than EI or PCI modes. , The use of NCI as a PFAS detection technique has been described in numerous articles. − Considering the high selectivity of CI for halogenated compounds, − in this work an ultra-high-performance liquid chromatography (UHPLC) system was coupled with a CI source using the liquid electron ionization (LEI) interface. LEI has been amply demonstrated to allow the proper vaporization of LC-amenable compounds before reaching an under-vacuum ionization source, such as EI or CI.…”