Avaliação da microextração líquido-líquido dispersiva reversa como estratégia para o preparo de amostra de biodiesel para determinação de Ca, Mg, Na e K por espectrometria atômica

Abstract

Sample preparation procedures that use a minimum experimental manipulation have been a current trend in elemental chemical analysis. In this sense, a sample preparation strategy based on the reverse-dispersive liquid-liquid microextraction (R-DLLME) for the determination of Ca, Mg, Na and K in biodiesel samples was developed in this work. Seeds of canola (Brassica napus L.), crambe (Crambe Hochst abyssinica), sunflower (Helianthus annuus) and jatropha (Jatropha curcas) were used as raw material for the extraction of vegetable oil and biodiesel production. In addition to the elemental determination, other physicochemical parameters based of the ANP 45/2014 were also carried out to aggregate more information on the quality of the biodiesel produced. To determine the selected elements, the biodiesel samples were submitted to sample preparation using R-DLLME. The following variables were evaluated using the multivariate statistical analysis: sample mass, volume of the extraction solution, composition of the extraction solution and nitric acid concentration. The volume of the extraction solution and nitric acid concentration in the extraction solution composition were the variables that most influenced to obtain satisfactory results in R-DLLME for Ca, Mg, Na and K in biodiesel samples at the 95% confidence level. The accuracy and precision of R-DLLME were evaluated with the organometallic standard in B100 biodiesel using the conditions of ≈3.0 g of sample, 780 μL of the extraction solution, 5.0% (v/v) HNO3:isopropyl alcohol [3:1 (v/v)], thermostatic bath at 80 ° C, 30 s vortex sample agitation, 3 min in ultrasonic bath and 15 min centrifuge at 3600 rpm. Recoveries between 91.61 - 96.56% were obtained for the elements with relative standard deviations (% RSD) less than 5.36%. The proposed method was applied to the selected biodiesel samples and the contents determined were 0.9171 ± 0.0068 - 1.4562 ± 0.0337 mg kg-1 Ca, 0.0178 ± 0.0008 - 0.0576 ± 0.0009 mg kg-1 Mg, 0.4242 ± 0.0228 - 9.5958 ± 0.2963 mg kg-1 Na and 0.1097 ± 0.0010 - 2.6856 ± 0.0519 mg kg-1 K. Limits of detection of 0.0143 mg kg-1 (Ca), 0.0015 mg kg-1 (Mg), 0.0008 mg kg -1 (Na) and 0.0009 mg kg-1 (K). (RSD) less than 8.4% were obtained. It was demonstrated that the analytical determination technique has satisfactory sensitivity and can be used to determine these elements in biodiesel samples.