Time-resolved quantification of phenolic antioxidants and oxidation products in a model fuel by GC-EI-MS/MS

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.