Determinação de flúor em sementes de plantas leguminosas e oleaginosas após combustão iniciada por micro-ondas em frascos descartáveis

Abstract

In this work, the use of microwave-induced combustion in disposable vessels (MIC-DV) for fluoride determination in leguminous seeds and oilseeds (almond, peanut, hazelnut, Brazil nut, pecan nut, pea, pinto bean, chickpea, and lentil) was evaluated. The determination of fluoride in food is of great relevance since this element plays important roles in the organism. However, if ingested in excess, it can cause some adversities, like dental and bone fluorosis. Since in the literature, there are no works about the determination of fluoride after MIC-DV in food, the following parameters of this sample preparation method in leguminous seeds and oilseeds were evaluated: sample mass; type and concentration of the absorbing solution; volume of the ignition solution; purging time with oxygen; and evaluation of successive combustion cycles. The fluoride determination was performed by ion chromatography (IC) and fluoride by ion selective electrode (ISE). The microwave-induced combustion (MIC) was used to obtain the reference values. Values above the limit of quantification (LOQ) were obtained only for Brazil nut and pinto bean samples, and it was only possible to perform fluoride quantification by IC. In order to improve the LOQ of the method and be able to carry out the determination by ISE, successive combustions of the pinto bean sample were carried out and the results were in agreement with those previously obtained. The proposed method could be optimized with the following conditions: 30 mg of sample for those that form pellets and 15 mg for pastes; 5 mL total ionic strength adjustment buffer as absorbing solution; 40 μL of 10 mol L-1 NH4NO3 as combustion initiator; 10 s of purging time with oxygen (4 L min-1); possibility of carrying out up to 5 successive combustion cycles. Using 5 successive combustion cycles, it was possible to determine fluoride in Brazil nut and pinto bean samples. The accuracy of the MIC-DV/ISE method was evaluated by comparing the results obtained by MIC/IC and the results were in agreement (between 94 and 106%). Therefore, it was possible to determine fluorine concentration in solutions obtained after the MIC-DV of samples of leguminous seeds and oilseeds, and only pinto bean and Brazil nut samples presented fluorine concentration above the LOQ for samples that form pellets and for pastes, corresponding to 8.3 and 16 μg g-1, respectively.