## 3 Results and Discussion
### Degradation Behaviour Identified by Advanced Analytical Methods
Exemplarily, short-chain fatty acids such as propionic acid (a) and butanoic acid (b), were detected as TMS esters in only minor amounts. Accordingly, 2-ethylhexyl fatty acid esters (e, g-j) with various acid chain lengths are generated in noticeably lower quantities. The lower amount of acidic degradation products of blends with AO is in good accordance with the results from NN analyses, which show values which are only half of the ones without AO. However, (2-ethylhexyl)oxy TMS silane (d) and adipic acid TMS ester (k) were formed in similar quantities. Regardless of the amounts of degradation products formed, qualitative analysis revealed almost completely the same list of degradation products for both the neat base oil and the blend containing AO. In addition to the ester degradation products discussed above, the degradation products of the AO were also detected: 2,6-di-_tert_-butyl-_p_-benzoquinone (o) and 3,5-di-_tert_-butyl-4-hydroxybenzoic acid TMS ester (p) were found in TIC and are shown in Figure 10B. Figure 11 shows the positive ion mass spectra of \({}^{16}\)O\({}_{2}\) and \({}^{18}\)O\({}_{2}\) atmosphere alteration of the AO degradation product 3,5-di-_tert_-butyl-4-hydroxybenzoic acid TMS ester. Due to a shift of +4 Da for the molecular ion from \(m/z\) 322.0 to 326.0, as well as for \(m/z\) 307.0 to 311.0, the oxygen atoms in the carboxyl acid group can be identified as \({}^{18}\)O isotopes. Hence, one can conclude that the oxidation of the AO leads to a bond cleavage between the C bridge-atom and one of the phenols, and the C bridge-atom is oxidized to a carboxyl acid by oxygen from the atmosphere, detected as a TMS ester after silylation. Furthermore, the oxidation of the residual phenolic residue results in the second degradation product, detected as 2,6-di-_tert_-butyl-_p_-benzoquinone. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \({}^{16}\)O\({}_{2}\) & **Analy** & **Retention** & **Calculated Monoisotopic** & **Detected Monoisotopic** & **Highest Detected _m/z_** Value** \\  & & **Time (min)** & _m/z_ & **Value of Molecular Ion** & **Molecular Ion (RI \%)** & **in the Mass Spectrum** \\ \hline o & 2.6-di-_tert_-butyl-\(p\)-benzquinone & 18.11 & 220.3 & 220.0 (40.2) & 220.0 \\ p & 3.5-di-_tert_-butyl-4-hydroxybenzoic acid TMS ester & 25.50 & 322.20 & 322.0 (14.5) & 322.0 \\ \hline \({}^{18}\)O\({}_{2}\) & **Analy** & **Retention** & **Calculated Monoisotopic** & **Detected Monoisotopic** & **Highest Detected _m/z_** Value** \\  & & **Time (min)** & _m/z_ & **Value of Molecular Ion** & **Molecular Ion (RI \%)** & **in the Mass Spectrum** \\ \hline o & 2.6-di-_tert_-butyl-\(p\)-benzquinone & 18.11 & 220.3 & 220.0 (38.3) & 220.0 \\ p & 3.5-di-_tert_-butyl-4-hydroxybenzoic acid TMS ester & 25.52 & 326.2 & 326.0 (12.8) & 326.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main AO degradation products of artificial alteration under \({}^{16}\)O\({}_{2}\) and \({}^{18}\)O\({}_{2}\) atmosphere. Figure 11: Positive ion mass spectra of 3,5-di-_tert_-butyl-4-hydroxybenzoic acid TMS ester (p) obtained after alteration under \({}^{16}\)O\({}_{2}\) (**A**) and \({}^{18}\)O\({}_{2}\) (**B**) atmosphere, respectively. Peaks of fragments where a change of the \(m/z\) value due to isotope labelling occurred are labelled with the respective values. A simplified oxidation reaction is suggested for the respective molecule. * signifies assignment of \({}^{16}\)O or \({}^{18}\)O atoms according to the most likely degradation pathway. Figure 10: TIC chromatograms of alteration products obtained under 1 bar \({}^{16}\)O\({}_{2}\) atmosphere without (**A**) and with 1 (_w/w_) % AO (**B**). Main products (a–m), AO degradation products (o, p), and target molecule (n, starting material) are marked and given in Tables 1–3. Peaks under bracket with label 1 indicates oxidized molecules of ester-based base oil bis(2-ethylhexyl) adipate.