Determination of Calcium and Magnesium in Biodiesel by Flame Atomic Absorption Spectrometry Using Microemulsions as Sample Preparation

Abstract

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.