Electrospray ionization and atmospheric pressurematrix-assisted laser desorption/ionization massspectrometry of antioxidants applied in lubricantst

摘要

The aim of this study was to investigate the utility of ion trap mass spectrometry (ITMS) incombination with the two desorption/ionization methods, electrospray (ESI) and atmosphericpressure matrix-assisted laser desorption/ionization (AP-MALDI), for the detection of antioxidantswhich are applied in lubricants. These experiments should form the base for future investigations ofantioxidants in tribologically formed thin layers on the surface of frictional systems. Seventeendifferent antioxidants were selected out of the group of hindered phenolic and aromatic aminiccompounds. Practically all antioxidants could be characterized by positive ion ESI- and AP-MALDI-ITMS, forming various types/species of molecular ions (e.g. [M]~+, [M+H]~+ ,[M+Na]~+ or [M-2H+H]~+). A few compounds could be analyzed by negative ion ESI-MS, too, but none by negativeion AP-MALDI-MS. The influence of target materials in AP-MALDI-MS (gold- and titanium nitride(TiN)-covered stainless steel, micro-diamond-covered hard metal, hand-polished and sand-blastedstainless steel targets) with respect to the molecular ion intensity and type of molecular ion of twoselected antioxidants was evaluated. The surface properties are of particular interest because infriction tests different materials with different surface characteristics are used. However, the MSresults indicate that optimal target surfaces have to be found for individual antioxidants in AP-MALDI-MS but in general smooth surfaces were superior to rough surfaces. Finally the gold-coveredstainless steel MALDI target provided the best mass spectra and was selected for all the antioxidantsinvestigated.