A miniaturized liquid-liquid extraction method for further Na, K, Ca, and Mg determination in crude oil by FAAS

In this study, a miniaturized liquid-liquid extraction (LLE) method for pre-concentration of Na, K, Ca, and Mg in crude oil was proposed. Analytes in crude oil were quantitatively extracted to the aqueous phase, followed by flame atomic absorption spectrometry (FAAS) determination. The following parameters were evaluated: type of extraction solution, sample mass, heating temperature and time, stirring time, centrifugation time, and the use of toluene and chemical demulsifier. Accuracy was evaluated by comparing the results obtained by the proposed method (LLE-FAAS) with those obtained after high-pressure microwave-assisted wet digestion and FAAS determination (reference values). No statistical difference was observed between the reference values and those using the optimized conditions for LLE-FAAS: 2.5 g of sample; 1000 μL of 2 mol L^-1 HNO₃, 50 mg L^-1 of chemical demulsifier in 500 μL of toluene, 10 min of heating at 80 °C, 60 s of stirring, and 10 min of centrifugation. Relative standard deviations were lower than 6%. The limits of quantification (LOQ) were 1.2, 1.5, 5.0, and 0.50 μg g^-1 for Na, K, Ca, and Mg, respectively. The proposed miniaturized LLE method presents several advantages, such as ease-of-use, high throughput (up 10 samples can be processed per 1 h), uses a high sample mass reaching low LOQs. In addition, the use of a diluted solution for extraction reduces the amount of reagents (around 40 times) and consequently laboratory residue generation, becoming an environmental friendly method. Suitable LOQs were achieved for analyte determination at low concentration even using a simple and low-cost sample preparation system (miniaturized LLE method) and a relatively low-cost determination technique (FAAS), avoiding the use of microwave ovens and more sensitivity techniques, which are required for routine analyses.