SGMS Meeting 2001

Characterization of ageing products of ester based synthetic lubricants by LC/ESI-MS, ESI-MS/MS and HR-EI-MS

Martin Kohler and Norbert V. Heeb
Swiss Federal Laboratories for Materials Testing and Research, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland

Ester based synthetic lubricants such as phthalates, adipates, sebacates, neopentylpolyol esters and pentaerythritol tetraesters are widely used due to their excellent tribological properties. The key to improve the long-term stability of these products is to understand the chemical and physical processes induced by tribological stress (friction, wear, heat, water, and oxygen).

Ageing products of a commercial jet engine oil based on pentaerythritol tetraesters which were formed upon operation in an aviation turbine were detected by electrospray ionization mass spectrometry (ESI-MS) and characterized by LC/ESI-MS [1]. The fatty acid composition of these ageing products was investigated by ESI-MS/MS analysis. The ammonium adducts of the newly formed pentaerythritol tetraester degradation products were found to be suitable MS/MS parent ions to investigate the fatty acid composition of such esters. The tetraesters formed during tribological stress upon jet engine operation were found to have a different fatty acid composition, containing in parts, longer fatty acid chains. These findings were consistent with increased chromatographic retention times of these newly formed compounds on a reversed phase column. Exact mass measurements using high resolution direct-inlet electron impact ionization mass spectrometry (HR-EI-MS) gave additional insight on the processes involved in ageing of pentaerythritol tetraesters upon tribological stress.

ESI-MS, LC/ESI-MS, ESI-MS/MS and HR-EI-MS proved to be versatile tools to study the chemical composition (distribution of homologues) as well as the mechanism of ageing of ester based lubricants on a molecular level. Due to its high sensitivity, ESI-MS can also be used to characterize and identify trace levels of ester-based lubricants in other matrices such as mineral oils.

[1] M. Kohler, N. V. Heeb, J. Chromatogr. A, 2001, 926, 161-165