## 2 Materials and Methods
### Analytical Approach
#### 2.4.2 Advanced Analytical Methods  
In order to identify the degradation products, samples obtained from artificial alteration and in fresh condition were analysed with a capillary GC-EI-MS consisting of a TriPlus autosampler and a Trace capillary Ultra GC coupled to a TSQ Quantum XLS mass spectrometer, all of which were sourced from Thermo Scientific (Austin, TX, USA). Base components were applied as reference substances at various product concentrations: ester-based base oil component bis(2-ethylhexyl) adipate was injected with 0.02 (\(w/w\)) %, 4,4'-methylene-bis(2,6-di-_tert_-butylphenol) with 0.008 (\(w/w\)) %, and the blend of base oil component containing AO with 0.01 (\(w/w\)) %, which were all diluted in dichloromethane (DCM). The dilutions of all samples after artificial alteration were prepared with a concentration of 4 (\(w/w\)) % in DCM. The degradation products containing active hydrogen atoms such as acids were treated by silylation. The silylation reagent \(N\),\(O\)-bis(trimethylsilyl) trifluoroacetamide (BSTFA) was applied to enhance the volatility for GC-MS and to avoid the undesired adsorption of the polar degradation products on the column material. BSTFA was added in a 2:1 mass excess of (\(w/w\)) % to the sample and aliquots were kept at 70 \({}^{\circ}\)C for 1 h to complete the reaction.  
Samples were subjected to GC-EI-MS by injection via a programmable temperature vaporization injector (PTV) with a volume of 1 \(\mu\)L and a constant injector temperature of 300 \({}^{\circ}\)C. Split was selected with a ratio of 1:25, and the carrier gas was helium and kept at a constant flow of 2 mL/min. To separate the sample prior to mass spectrometric analysis, a TG-17 MS capillary column obtained from Thermo Fischer Scientific (Waltham, MA, USA) was used, consisting of a 50%-diphenyl/50%-dimethylpolysiloxane stationary phase with a length of 30 m, a 0.25 mm inner diameter, and a film thickness of 0.25 \(\mu\)m. An oven temperature program starting at 50 \({}^{\circ}\)C for 2 min with a ramp of 7 \({}^{\circ}\)C/min up to 300 \({}^{\circ}\)C was applied, and the end temperature was kept constant for 15 min. The separated compounds were transferred to the mass spectrometer via a transfer line thermostated at 280 \({}^{\circ}\)C and were subsequently ionized in an EI source at 70 eV. The source temperature was set to 200 \({}^{\circ}\)C, and the analyser scan range was between \(m/z\) 40 and 650 for the full scan mode. Data acquisition and data processing were done with Xcalibur v2.0 software. For subsequent identification based on a full scan mass spectra comparison, the NIST library 2.0 (2011) with an update from the 2014 and reference substances was applied.  
For the main degradation products, the calculated monoisotopic \(m/z\) value was given, based on the sum formula and the respective amount of \({}^{16}\)O or \({}^{18}\)O atoms based on the mass spectrometric data, as well as the detected monoisotopic mass pattern, i.e., the molecular ion peak. For analytes without a clearly visible, i.e., abundant molecular ion peak with a relative intensity (RI) of more than 1%, the highest unambiguously detected \(m/z\) value was reported.