At elevated temperatures mineral oil based lubricants are prone to oxidation. Thus, antioxidative stabilization of engine oils is an important issue in lubrication engineering. Although there are various tests in order to assess the oxidative stability of a lubricant, only very little is known about the depletion process of antioxidants on a molecular level. The current study presents a solid-phase extraction method capable of isolating antioxidants and corresponding degradation products from engine oils as well as a high performance liquid chromatography (HPLC) method for their subsequent separation. For detection the HPLC system was coupled to a UV detector and a high resolution quadrupole time-of-flight mass spectrometer (QTOF-MS). These methods not only enable the quantitation of antioxidants employed in lubricants but can also be used for investigations of their corresponding degradation products in used engine oils. By means of HPLC-QTOF-MS and MS/MS experiments it was possible to detect numerous reaction products formed from antioxidants during their service lifetime in engine oils. This allowed a deeper insight into the mode of action of the investigated stabilizers.
Hindered amine light stabilizers are essential for the stabilization of synthetic polymers, particularly for materials used for outdoor applications. Although up to now a considerable number of studies dealing with the analytics of this class of stabilizers had been published, especially the determination of oligomeric hindered amine light stabilizers is still an analytical challenge. In the current work, a fast and simple liquid chromatographic method for the quantitative determination of oligomeric hindered amine light stabilizers is presented. A key aspect of this method is their completely different retention behavior depending on the pH, enabling a single peak elution approach by a pH gradient run. This allows a quantitation with simple UV detection independent of the actual oligomeric composition. Calibration curves within the concentration range relevant for the analysis of real polymer samples (LOQ = 70 mg/L) were constructed with R values above 0.99. Spiked extracts from polyolefin samples showed recovery rates between 97.3 and 102.9% for five different commercial hindered amine light stabilizers. Relative standard deviations were between 2.0 and 3.9%. Furthermore, it was demonstrated that the employed approach can be easily adapted for mass spectrometry detection.
Durch direkte Ionisierung lassen sich Analyten aus der Dünnschichtchromatographie massenspektrometrisch untersuchen. Der Mechanismus der Ionisierung folgt den typischen Elektrospray‐Prinzipien, eine spezielle Ionenquelle ist nicht erforderlich.
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