NOVEL ANALYTICAL PROCEDURE USING REVERSED-PHASE DISPERSIVE LIQUID-LIQUID MICROEXTRACTION IN DIESEL FOR Na, K, Ca AND Mg DETERMINATION BY F AAS

In this work, a new procedure of sample preparation for Na, K, Ca e Mg determination at trace levels in diesel was developed. Analytes were extracted from sample using the reversed-phase dispersive liquid–liquid microextraction (RP-DLLME) method for further determination by flame atomic absorption spectrometry (F AAS). A mixture of diluted HNO3 and isopropanol was used as dispersant and extractant solvent, respectively. The procedure was carried out using a Falcon type conical tube (15 mL), and consisted of the following steps: (i) addition of a small volume of dispersant/extractant mixture on the heated sample; (ii) shaking on a vortex stirrer for 1 min; (iii) centrifugation at 5600 g for 15 min and (iv) removal of the aqueous phase with micro syringe and volume was completed to 5.0 mL with ultrapurified water. After, analytes were determined by F AAS, using the emission mode for Na and K and absorption mode for Ca and Mg. External calibration was performed using with aqueous standard solution (0.1 to 2.0 mg L-1 for all analytes). The influence of selected variables on the performance of the sample preparation step was evaluated. Evaluation of parameters were performed by addition of 1.0 µg of for Na, K, Ca e Mg using Conostan® (50 µg g-1 Na, K, Ca and Mg). Several parameters that can affect extraction efficiency were investigated using the factorial fractional design (25-2). Five variables were studied: (i) temperature (X1: 60, 70 and 80 °C), (ii) sample mass (X2: 5, 10 and 15 g); dispersant/extractor mixture volume (X3: 0.5, 1.0 and 1.5 mL); concentration of HNO3 solution (X4: 1.0, 1.5 and 2.0 mol L-1) and the ratio of the dispersant/extraction solvent (X5: 60, 70 and 80% (v/v)). As obtained results, it is important to mention that the variables X1 and X2 presented significant effects on the efficiency of analytes extraction. These two variables was used as central composite rotatable design (CCRD). Using these conditions was possible to obtain a validated model for the RP-DLLME method.Finally, the proposed procedure was applied to other diesel samples and results obtained were compared with the official method (EN 14538). Proposed procedure was considered suitable for determination of Na, K, Ca and Mg in diesel. The proposed procedure present simplicity of operation, low consumption of reagents, which is in according with green chemistry principles, suitable throughput for routine analysis, and very low limits of quantification.