Time-Resolved Quantification of Phenolic Antioxidants and Oxidation Products in a Model Fuel by GC-EI-MS/MS

Abstract

Phenol-type components, such as butylated hydroxytoluene (BHT), are used as antioxidants (AOs) to enhance the thermo-oxidative stability of kerosene-type Jet A-1 fuel. Although the antioxidative effect of BHT is well known and often published, there is far less information about the degradation products of BHT in fuels and their impact on stability toward oxidation. In order to monitor a time-resolved depletion of BHT in model kerosene, an artificial alteration method adapted for regular sampling was applied. Subsequently, the molecular structure of degradation products of BHT was identified by gas chromatography with electron impact ionization mass spectrometry (GC-EI-MS). For the quantification of the residual BHT as well as the two representatives of degradation products, namely, 3,5-di-tert-butyl-4-hydroxybenzaldehyde (HBA) and 2,6-di-tert-butyl-p-benzoquinone (BQ), an analytical technique comprising a GC-EI triple quadrupole mass spectrometer run in the MS/MS mode was developed. The limit of detection (LOD) and the limit of quantification (LOQ) for BHT, BQ, and HBA were determined below 1 ppb. The formation of BQ and HBA was observed shortly after the nascent degradation of BHT, while an increase of oxidation products derived from the fuel ascended remarkably after a full depletion of both the initial AO BHT and the monitored oxidation products BQ and HBA. As the evolution of BQ and HBA followed a characteristic trend, these compounds can be used as markers to reliably predict the residual time until a total consumption or a predefined threshold of BHT is reached. This way, the quality management of in-service or stored kerosene-type fuels is enhanced.

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

“…Previous MS measurements took advantage of SIM and MRM for the chemical characterization of compounds in fuels (Bernabei et al, 2000(Bernabei et al, , 2003Frauscher et al, 2020;Webster et al, 2014). The continued use of SIM and MRM and novel use of precursor ion scan and neutral loss scan MS methods may help identify, characterize, and quantitate additional compounds in jet fuels at relatively low abundances.…”

“…The PA have hindered alkyl groups (i.e., tert ‐butyl groups) and a maximum concentration of 24.0 mg/L in jet fuels (ASTM D1655). Although PA are not natural compounds, many efforts were devoted to analyzing this class of OCCs in fuels (Bernabei et al, 2000; Frauscher et al, 2020; Loegel et al, 2014; Webster et al, 2014; Yuan et al, 2021). Therefore, this review will include MS studies that determined the compositions of natural OCCs and PAs in jet fuels.…”

“…As PA are used to prevent autoxidation reactions in fuels, establishing methods that determine their concentrations and reaction products after oxidative stress tests may be crucial to evaluate the stability of fuels. Several studies attempted to develop such methods (Frauscher et al, 2020; Webster et al, 2014; Yuan et al, 2021).…”

“…Both methods successfully quantitated PA postthermal oxidation, which showed significant decreases in PA concentration when compared to the nonstressed fuel. In a relevant study, upon the oxidation of a surrogate fuel doped with BHT, the antioxidant and oxidized reaction products (3,5‐di‐ tert ‐butyl‐4‐hydroxybenzaldehyde and 2,6‐di‐ tert ‐butyl‐ p ‐benzoquinone) were quantitated successfully to ppb levels by using GC/(EI)QqQ MS in the MRM mode (Frauscher et al, 2020) (Figure 10).…”

“…The measurements support the use of MS as an effective tool at assessing fuel stability as a function of time. Reprinted with permission from Frauscher et al (2020). Copyright 2020 American Chemical Society.…”