Use of silica gel in the preparation of used lubricating oil samples for the determination of wear metals by flame atomic absorption spectrometry

Abstract

Used lubricating oils were analysed for Cr, Fe, Mg and Pb using flame atomic absorption spectrometry after three types of sample preparation: direct dilution with an organic solvent, dry ashing-acid dissolution and dry ashing in the presence of a porous inert material (silica gel). The porous material prevented sputtering and volatilisation of the sample and hence a rapid charring-ashing programme could be employed. Results of the three methods of sample preparation indicated that direct dilution could be applied to highly volatile metals such as Pb which could be lost during the heating periods required for ashing. Dry ashing was particularly useful for all the metals studied provided that the rate of heating was slow. The proposed method, however, combines the advantages of dry ashing and rapid sample preparation because lubricating oil samples containing silica gel behave as solid samples and hence no loss of the metals occurs during heating, except for the highly volatile metals which could not be determined with sufficient precision using this method. The determination of metals in lubricating oils must be carefully carried out taking into account possible changes in their concentration during storage. In addition, metallic particles may precipitate on storage; hence vigorous shaking and other homogenisation procedures need to be employed prior to analysis.