Study of the extraction of Ca, Mg and Zn from different types of lubricating oils (mineral, semi-synthetic and synthetic) employing the emulsion breaking strategy

This paper reports the study and development of a method for the extraction induced by emulsion breaking of Ca, Mg and Zn from unused lubricating oils. Three types of oils were evaluated: mineral, semi-synthetic, and synthetic lubricating oils. The analytes were determined in the extracts by flame atomic absorption spectrometry. The influence of several parameters on the sample preparation was evaluated (type and proportion of solvent used to dilute the samples and HNO3 and Triton X-114 (TM) concentrations in the extractant solution). The calibration conditions were also studied in detail. Under optimum conditions, the samples were diluted with toluene (mineral and semi-synthetic oils) and ethylbenzene (synthetic oils) prior to application of the extraction strategy. The proportion of solvent was always 40% (v/v) and maximum extraction was achieved when the concentrations of HNO3 and Triton X-114 (TM) in the extractant solution were 4.2 mol L-1 and 3% (m/v), respectively. Since the concentrations of the analytes in the samples were high, the final extracts had to be diluted prior to analysis, which allowed the use of aqueous solutions for methodological calibration. The limits of quantification of the method were 134, 15 and 25 ng g(-1) for Ca, Mg, and Zn, respectively. Ten samples of unused lubricating oils of different types were analyzed and an excellent agreement was observed between the values obtained by the proposed method and those obtained by the reference method based on the total digestion of the samples.