Application of the extraction induced by emulsion breaking for the determination of Cu, Fe and Mn in used lubricating oils by flame atomic absorption spectrometry.

A novel approach is proposed for the sample preparation of used lubricating oils to determine Cu, Fe and Mn by FAAS. The method is based on the extraction induced by emulsion breaking, in which the elements of interest are transferred to an aqueous phase before the measurement by FAAS. In the method, each sample of used lubricating oil was diluted with toluene (20% v/v) and the resulting solution was emulsified with a Triton X-114 solution containing HNO3. Further, the water-in-oil emulsion was broken by centrifugation for 30 min at 3500 rpm, originating a system with two well-separated phases: (i) the upper phase, containing the used lubricating oil diluted in toluene and (ii) the lower aqueous phase, containing the analytes that were extracted from oil. The lower phase was collected, diluted with water and the analytes were determined by FAAS. The optimization of the methodology was performed by studying the influence of different parameters that could affect the extraction efficiency such as the nature and concentration of the solvent used for sample dilution, the concentrations of HNO3 and Triton X-114 in the solution employed for emulsification and the operational conditions for extraction (extraction, centrifugation and sampling times). The limits of quantification for Cu, Fe and Mn were 2.9, 77 and 8.2 ng g(-1), respectively. The accuracy of the method was evaluated by comparison with the reference method based on the total digestion of the samples in a closed-vessel microwave oven. There were no statistical differences between the results obtained with the proposed method and the reference one, except for Fe in the cases where its concentration was higher than 80 µg g(-1).