## 1 Introduction
The replacement of these hydrogen atoms, e.g., to give a quaternary \(\beta\)-carbon, disables this mechanism and degradation products have to be generated via the thermodynamically less favorable free-radical pathway. Thus, the stability is increased with a higher degree of substitution on this carbon atom [2; 10]. This strategy is exploited in polyol esters by replacing all of the hydrogen bonds to the \(\beta\)-carbon by alkyl groups in the alcohol moiety. Thus, the thermal decomposition of polyol esters occurs via a free-radical pathway and proceeds remarkably slower, resulting in a significantly higher stability [10; 11; 12]. The oxidative stability is increased with a decreasing mid-chain acid length, whereas the length of the end-chain alcohols shows less influence on the oxidative stability. However, branching of the alcoholic residue has a beneficial effect in terms of an improved oxidative stability, but is reduced by branched acid chains. While for the alcohol chain a complete H substitution at the \(\beta\)-carbons is particularly favourable, acid chain stability follows the order \(-\)CH\({}_{3}>-\)CH\({}_{2}->-\)CH- [2; 13]. Beyond these considerations on the optimum base oil chemistries, an improvement of the thermo-oxidative performance with aminic and/or phenolic antioxidative additives is the most  
Figure 1: Diester bis(2-ethylhexyl) adipate with highlighted (blue circle) \(\beta\)-carbon atoms in the alcohol moiety (end chain). common way to a achieve higher resistance against oxidation and a longer functional durability of lubricants [14; 15]. Lubricant degradation follows the free-radical chain reaction, based on four steps: (1) initiation; (2) propagation; (3) branching; and (4) termination. Within this study, the focus was laid on sterically hindered phenols suitable for high temperature applications to decelerate oxidation. In order to evaluate the resistance of a base oil against thermo-oxidative stress and, thus, the impact of an antioxidant, several methods are available--either standardised or customised--such as the thermo-oxidative stability test (TOST) (DIN 51587) [16], corrosion and oxidation stability test (DIN 51808) [17], or the rotating pressurised vessel oxidation test [18] (RPVOT, see Section 2.3). Subsequent to the accelerated alteration of lubricant samples in laboratory devices, appropriate analytical methods have to be applied to determine the degree of degradation by oxidation. On the one hand, parameters regarding the degree of degradation can be obtained via conventional analytical methods based, for example, on infrared (IR) spectroscopy and the neutralisation number (NN) (see Section 2.4.1). However, these analytical methods do not provide detailed information related to the structures of degradation products or the mechanistic aspects of the process. For this reason, more advanced instrumental analytical methods have to be applied that are able to provide structure identification capabilities. Capillary gas chromatography coupled with electron impact ionization mass spectrometry (GC-EI-MS) is considered suitable for this task (see Section 2.4.2). With this hyphenated technique, the identification and characterization of the degradation products formed can be achieved after the high resolution chromatographic separation of the oil sample [19; 20; 21]. Nevertheless, a clear assignment of the origin of the degradation products, as well as the mechanistic aspects from oxygen-containing base oils and additives by oxidation, cannot be solely provided by chromatographic and mass spectrometric methods. In previous research work, a novel approach combining artificial alteration, isotope labelling, and mass spectrometry was developed to elucidate and track oxidation reactions in oxygen-containing fuel components [22].