Analysis of the Thickening Agents in Automotive Greases by GC-MS

A laboratory procedure for the identification of the thickening agents in lubricating greases by GC–MS is described. The procedure entails isolation of fatty-acid soaps and other carboxylate salts by extraction of the base oil from the grease by hexanes. The carboxylate anions left in the residue are then converted to methyl esters by reaction with methanol/HCl and identified by GC–MS. A dozen different greases have been analyzed and the results tabulated. Each grease was examined for the presence of thirteen different methyl esters—six derived from saturated fatty acids (myristic, palmitic, stearic, eicosanoic, 12-ketostearic, and 12-hydroxystearic acids), three unsaturated fatty acids (oleic, linoleic, and linolenic acids), one carboxylic acid (benzoic acid) and three dicarboxylic acids (adipic azelaic, and sebacic acids). The identity of a methyl ester derived from the soap thickener in a grease was established by comparison of the retention time and mass spectral fragmentation pattern to that of a high-purity commercial methyl ester standard. Both second-semester general chemistry students and sophomore organic students have performed this lab. This lab experiment is part of a larger effort to develop a general chemistry sequence for engineering students using the theme of "chemistry and the automobile."