Determination of Iron, Copper, Zinc, Aluminum, and Chromium in Biodiesel by Flame Atomic Absorption Spectrometry Using a Microemulsion Preparation Method

This work proposes a flame atomic absorption spectrometry (FAAS) method for Fe, Cu, Zn, Al, and Cr determination in biodiesel using water in oil (w/o) microemulsion (ME) as a sample preparation method. A less toxic solvent (n-propanol), instead of those used in the standard method (ABNT NBR 15556), the absence of surfactant, and calibration using inorganic reference standards were investigated. A ternary phase diagram of biodiesel as oil, water, nitric acid, and n-propanol showed a single-phase ME region. The composition of ME adopted for analysis was 1.7 g of biodiesel and 1.1 mL of 1.4 mol L-1 HNO3, completed with n-propanol to 10 mL volume. Linear calibration graphs with R > 0.99 were obtained using ME prepared with oleic acid and inorganic standards. Analytes were stable for at least 3 days both in sample and in metallic standards MEs. The limits of detection obtained were 0.3, 0.1, 0.07, 1.7, and 1.0 mg kg(-1) for Fe, Cu, Zn, Al, and Cr, respectively. These limits of detection were lower than those obtained by the reference method of dilution with organic solvent, due to lower fluctuation in the measurements. Recovery test values (93-105%) indicated absence of matrix effects. In order to assess the method accuracy, several real samples were analyzed by the proposed method, by the standard method, and also by acid digestion, without significant differences between results when applying the student's t test. Relative standard deviations lower than 5% were obtained for standard and samples MEs. Therefore, combination of w/o ME with the FAAS technique and calibration with inorganic standards proved to be an accurate, simple, fast, and practical strategy, with greater analyte stability and appropriate sensitivity, so that the proposed method is suitable for biodiesel routine analysis.