Feasibility of dispersive liquid-liquid microextraction to determine Ca, Mg, K, and Na in biodiesel by atomic spectrometry

This work describes the development of a dispersive liquid-liquid microextraction (DLLME) sample preparation procedure for the determination of Ca, Mg, Na, and K in renewable biodiesel samples by atomic spectrometry. The proposed DLLME was based on the extraction of the analyte from biodiesel into a micro-drop of an acid:alcohol extraction aqueous phase at the bottom of a conical tube. Among the parameters that influence the extraction procedure, sample mass, extraction solution volume, acid concentration and composition of the extraction solution were evaluated to achieved satisfactory extractions. Using a 20 mg kg(-1) Ca-Mg-Na-K-P metallo-organic standard biodiesel B100, the optimized extraction conditions of DLLME were established using a 2(4) full factorial design and Doehlert matrix multivariate analysis. The best results were obtained with 3.0 g sample mass and 780 L of 5.0% (v/v) HNO3:isopropyl alcohol [3:1 (v/v)] extraction solution. The accuracy of the sample preparation procedure was evaluated by a recovery test, and the recovery percentages were within the 90.2-94.7% interval with a relative standard deviation (RSD) < 2.0%. The precision of the method was evaluated by analysis of 10 independent and multielement standard solutions, and the values were less than 2.0%-Ca, 2.5%-Mg, 1.7%-Na, and 1.3%-K. The limits of detection (LOD) were 32.3, 4.3, 3.0, and 5.8 g kg(-1) for Ca, Mg, Na, and K, respectively. Therefore, the developed procedure was successfully applied for Brassica napus L., Crambe Hochst abyssinica, Helianthus annuus and Jatropha curcas biodiesel samples, and the results were in the ranges of 0.92 +/- 0.07-1.46 +/- 0.04 mg kg(-1) (Ca), 0.42 +/- 0.02-9.60 +/- 0.29 mg kg(-1) (Na), 0.11 +/- 0.01-2.69 +/- 0.05 mg kg(-1) (K), and 17.84 +/- 0.76-57.63 +/- 0.95 (Mg) g kg(-1). The DLLME can be used as an alternative and non-destructive sample preparation procedure for biodiesel samples.