## 3 Results and Discussion
### Degradation Behaviour Identified by Advanced Analytical Methods  
Figure 7 shows the total ion current (TIC) chromatograms of ester-based base oil samples without additives, altered at 150 \({}^{\circ}\)C and a pressure of 1 bar under \({}^{16}\)O\({}_{2}\) (A) and \({}^{18}\)O\({}_{2}\) (B) atmosphere, respectively. In accordance with the observations from the RPVOT pressure curves and conventional analyses, the same degradation products in a highly similar distribution were obtained for both alterations. Thus, no impact of the oxygen isotope on the type of degradation products formed was found, and a good reproducibility for the applied alteration method was demonstrated. The verification of these facts is essential for the successful application of the isotope labelling approach and the evaluation of the findings obtained. Figure 6: Water content (**left**) and neutralisation number NN (**right**) of oil samples before and after artificial alteration (under different oxygen atmospheres). The values for the fresh samples are \(<\)0.1 (\(w/w\)) % and \(<\)0.1 mg KOH/g. The target molecule (n), i.e., the starting molecule, and the main degradation products marked in Figure 7 are listed in Table 1 for \({}^{16}\)O\({}_{2}\) atmosphere and Table 2 for \({}^{18}\)O\({}_{2}\) atmosphere, respectively. Moreover, the retention time in minutes, calculated monoisotopic \(m/z\) value of the molecular ion, detected monoisotopic mass of the molecular ion with a relative intensity in % (RI %), and the highest detected \(m/z\) value are given. Besides linear carboxylic acids with short chain lengths such as propionic acid (a) and butanoic acid (b), detected as the trimethylsilyl (TMS) ester due to a prior sample silylation with BSTFA, hexanediolic acid TMS ester (k) referring to adipic acid in the base oil was found. 2-ethylhexyl moiety in the ester base oil was transformed into 2-ethylhexyl alcohol, detected as oxysilane (d), and 2-ethylhexanoic acid, detected as TMS ester (f). Esters of 2-ethylhexanol from formic acid up to pentanoic acid (e, g-j) were formed in distinct amounts, together with other esters (l, m) based on the alcohol moiety of the initial ester. Moreover, oxidation products with a ketone (c) structure were obtained after alteration, as well as a broad variety of oxidized molecules (peaks under bracket carrying the label 1) derived from the initial bis(2-ethylhexyl) adipate (n). The benefit of using the isotope labelling approach to determine the degradation products related to the degradation mechanisms of the base oil is shown based on different molecules, either containing two (Figure 8), one (Figure 9 and Figure S1), or no (Figure S2) \({}^{18}\)O atoms. In Figure 8, the positive ion EI mass spectra of the degradation product 2-ethylhexanoic TMS ester (f) obtained under \({}^{16}\)O\({}_{2}\) alteration atmosphere (A) and \({}^{18}\)O\({}_{2}\) alteration atmosphere (B) are shown. When comparing mass spectrum A (top) and B (bottom), some particular, close-by ions clearly differ in their \(m/z\) value, e.g., 128.9 (A) and 132.7 (B), 159.9 (A) and 163.9 (B), and 201.0 (A) and 204.9 (B). This shift of + 4 Dalton (Da) is obtained as the molecule contains two \({}^{18}\)O atoms when altered under \({}^{18}\)O\({}_{2}\) atmosphere instead of \({}^{16}\)O\({}_{2}\). Figure 7: Total ion current (TIC) chromatograms of alteration products obtained under 1 bar \({}^{16}\)O\({}_{2}\) (**A**) and \({}^{18}\)O\({}_{2}\) (**B**) atmosphere without AO. Main products (a–m) and target molecule (n, starting material) are marked and given in Tables 1 and 2. Peaks under bracket with label 1 indicate oxidized molecules of ester-based base oil bis(2-ethylhexyl) adipate. Considering the structure of the initial molecule in addition to this information, the most likely pathway of degradation of the ester and formation of the degradation product can be revealed: the 2-ethylhexyl moiety cleaves between the alcohol's C1 and O atom, which is followed by the oxidative attack of O\({}_{2}\) from the atmosphere.