## 3 Results and Discussion
### Degradation Behaviour Identified by Advanced Analytical Methods
Thus, 2-ethylhexanoic acid is obtained, detected as a TMS ester via sample preparation with the BSTFA silylation reagent. Nevertheless, as the initial ester molecule is symmetric, this reaction can take place on both alcohol moieties of the adipate. In Figure 8, the simplified reaction steps are exemplarily shown on one side of the initial molecule. 3-heptanone (c) is a degradation product containing only one isotope-labelled oxygen atom, as shown in Figure S1. When comparing the mass spectra obtained under \({}^{16}\)O\({}_{2}\) and \({}^{18}\)O\({}_{2}\) atmosphere, the presence of one \({}^{18}\)O atom is confirmed: besides fragments containing oxygen, i.e., \(m/z\) 57.0 and 58.9, 72.0 and 73.9, 84.9, and 86.6, the molecular ion peak with a shift from \(m/z\) 114.0 to 115.8 is also effortlessly visible. It can be assumed, that thermal decomposition takes place via the six-membered cyclic intermediate involving the hydrogen on the \(\beta\)-carbon, as reported in the literature (see Introduction). This results in an alkene counting eight C atoms and an (di)acid. With these fragments, further reactions can take place. Due to oxidation, evocated by the oxygen from the atmosphere, 3-heptanone is built, carrying one \({}^{18}\)O atom. However, when looking closely at the mass spectra of this molecule, one may also see low abundant fragments without isotope labelling such as 114.0, 84.9, and 72.0 (Figure S1B). This observation may lead to a second possible formation mechanism of 3-heptanone, without involving \({}^{18}\)O\({}_{2}\). Hence, the application of the isotope labelling approach is not only feasible for explaining issues of interest, but also raises new ones. Figure S2 in the supplement shows an example of a degradation product without any isotope-labelled oxygen, 2-ethylhexyl alcohol detected as (2-ethylhexyl)oxy TMS silane (d). Due to the fact that in none of the mass spectrometric fragments found in the mass spectrum of the compound formed under \({}^{18}\)O atmosphere a shift of \(m/z\) values (+2 or 4 Da) is obtained, there is no hint of a participation of oxygen from the atmosphere in the formation of this product. Thus, by using the  
Figure 8: Positive ion mass spectra of 2-ethylhexanoic acid TMS ester (f) 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. isotope labelling approach, one can conclude that the molecule was formed by an unspecified cleavage of the C-O ester bond. Besides radical cleavage, the more likely mechanism can be hydrolysis of the ester bond. This mechanism requires conditions provided by proceeding alteration, in particular, an acid catalyst (see increased NN value for altered samples in Section 3.1) and a sufficient amount of water (see water formation for altered samples in Section 3.1). Accordingly, in the following passages, the term \"cleavage of the C-O ester bond\" refers to an unspecified mechanism of cleavage. Besides the degradation products containing either \({}^{18}\)O or \({}^{16}\)O at the sites of the oxidative attack, molecules with a mix of isotopes are also possible. Figure 9 provides a 2-ethylhexyl acetic acid ester (g) as an example of this. Based on the shift in the molecular ion peak, as well as the highest detected shift of +2 Da, the presence of one \({}^{18}\)O atom can be confirmed. A comparison with the NIST library identifies the molecule as 2-ethylhexyl acetic acid ester, containing two oxygen atoms. The correlation of the fragment ions with the structure only gives a shift for these fragments, where either both oxygen atoms are involved or exclusively the C=O group, i.e., the fragment ions at \(m/z\) 43.1 and 45.0. This observation leads to the finding that the C=O group contains the \({}^{18}\)O atom.