## 4 Conclusions  
The unique usefulness of the isotope labelling approach based on mass spectrometry for the investigation of the thermo-oxidative stability of oxygen-containing lubricant components could be clearly demonstrated in this study. With a satisfactory reproducibility (in terms of the retention times and mass spectrometric peak pattern), neat dieser bis(2-ethylhexyl) adipate, and a blend containing 1 (\(w/w\)) % antioxidant, 4,4'-methylene-bis(2,6-di-_tert_-butylphenol) was artificially altered in a closed vessel under \({}^{16}\)O\({}_{2}\) and \({}^{18}\)O\({}_{2}\) atmosphere, and subsequent high-performance capillary GC-EI-MS analysis was applied to identify the main degradation products containing varying numbers of \({}^{18}\)O. Isotope labelling in combination with MS allowed a clear assignment of the location of the isotope-labelled oxygen in the degradation product, even in the case of a mix of \({}^{16}\)O and \({}^{18}\)O. The findings revealed the preferential bond cleavage sites in the dieser molecule between the C-O ester bond and subsequent oxidative attack, resulting in 2-ethylhexyl fatty acid esters and 2-ethylhexyl alcohol. Furthermore, 2-ethylhexanoic acid is generated by the bond scission between the O atom and C1 of the alcohol, and is followed oxidation during alteration under the applied conditions. Moreover, the main degradation products were built after oxidative cleavage on one side or on both sides of the initial ester groups, to give 2-ethylhexyl 2-oxobutyl adipate and 2-ethylhexyl 6-oxohexanoate, respectively. 2-ethylhexyl 2-oxobutyl adipate was produced in distinct abundance, based on the oxidation between C2 and C3 of the alcohol chain. An increased pressure during the alteration seems to have no considerable impact on the quality of the formed degradation products, but the quantities vary, depending on the pressure level. Base oil blends containing AO showed a far higher stability, resulting in an enhanced alteration duration until the end point. Although mainly the same degradation products were built, almost all of them showed a significantly lower abundance, in particular, short-chain fatty acids and esters thereof, as well as oxidized ester base oil molecules. The two main degradation products of the AO were obtained: 2,6-di-_tert_-butyl-_p_-benzoquinone and 3,5-di-_tert_-butyl-4-hydroxybenzoic acid. In sum, the advanced analytical approach combining artificial alteration, isotope labelling, and mass spectrometry, proved to be an appropriate and powerful tool to unambiguously determine the degradation products of oxygen-containing lubricant components. Hence, the foundations have been laid for the elucidation of the underlying oxidation mechanisms. Prospectively, the sampling of alteration aliquots at points of interest, e.g., before the pressure curve starts to drop, are planned. This opens up the way to develop a reliable prognostic tool (on the status of lubricant in certain devices for acting at an early stage) based on the early detection of certain quantities of degradation products long before NN or IR spectroscopy would indicate anything. GC-MS analysis shall reveal the time- and alteration state-dependent formation of major degradation products. For blends containing AO, the determination of the residual AO content at different alteration states is of high interest. For this purpose, various GC-MS techniques such as selected ion monitoring (SIM), multiple ion detection (MID), or low energy (collision induced dissociation) CID MS/MS will be applied. Moreover, this work showed that the conventional and advanced analytical data are in good accordance, e.g., lower NN values for samples containing less acidic compounds obtained with MS analysis. Based on the knowledge gained with such an advanced analysis, a correlation of the degradation products formed with conventional oil data such as NN and that obtainable by IR spectroscopy shall be achieved. The following are available online at www.mdpi.com/2076-3417/7/4/396/s1.