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An alternative method for the determination of calcium and magnesium in biodiesel samples by flame atomic absorption spectrometry (FAAS) is proposed. The samples were prepared as water-in-oil microemulsions, and aqueous standards microemulsified in the same way were used for calibration. A study of long-term stability shows that the analytes remained stable in the microemulsions for at least 5 days. To assess the accuracy of the results, other methods were applied, such as recovery tests, as well as a comparison to the Brazilian Standard Method ABNT NBR 15556, which consists of the sample dilution with an organic solvent and a determination by FAAS. Limits of detection (LOD) of 0.04 and 0.1 μg g-1 and characteristic concentrations of 30 and 6 μg L-1 were obtained for calcium and magnesium, respectively. The method showed better LODs and precision in relation to dilution with organic solvent. The results obtained were statistically in agreement with those obtained with the Brazilian Standard Method at a 95% confidence level, and the recovery tests presented values between 92 and 117% for biodiesel samples. The main advantages observed in relation to the dilution method were the use of inorganic standards in the microemulsions for calibration, no use of carcinogenic organic solvents, and the higher stability of the analytes, besides the higher sensitivity. These advantages make the proposed method more appropriate for biodiesel routine analysis. The method was also tested for vegetable oil analysis.

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Lubricating oils are used to decrease wear and friction of movable parts of engines and turbines, being in that way essential for the performance and the increase of that equipment lifespan. The presence of some metals shows the addition of specific additives such as detergents, dispersals and antioxidants that improve the performance of these lubricants. In this work, a method for determination of calcium, magnesium and zinc in lubricating oil by flame atomic absorption spectrometry (F AAS) was developed. The samples were diluted with a small quantity of aviation kerosene (AVK), n-propanol and water to form a three-component solution before its introduction in the F AAS. Aqueous inorganic standards diluted in the same way have been used for calibration. To assess the accuracy of the new method, it was compared with ABNT NBR 14066 standard method, which consists in diluting the sample with AVK and in quantification by F AAS. Two other validating methods have also been used: the acid digestion and the certified reference material NIST (SRM 1084a). The proposed method provides the following advantages in relation to the standard method: significant reduction of the use of AVK, higher stability of the analytes in the medium and application of aqueous inorganic standards for calibration. The limits of detection for calcium, magnesium and zinc were 1.3 μg g(-1), 0.052 μg g(-1) and 0.41 μg g(-1), respectively. Concentrations of calcium, magnesium and zinc in six different samples obtained by the developed method did not differ significantly from the results obtained by the reference methods at the 95% confidence level (Student's t-test and ANOVA). Therefore, the proposed method becomes an efficient alternative for determination of metals in lubricating oil.