Método de preparo de amostras em câmara única de reação assistida por radiação micro-ondas e determinação de elementos terras raras em azeite de oliva extra virgem por USN-ICP-MS

Abstract

The rare earth elements (REE) present in extra virgin olive oil (EVOO) have been considered as studied markers for traceability of the production origin. However, REE are present in low concentrations (ng g-1 range) in EVOO, requiring analytical methods sufficient for multi-element determination and with low limits of quantification (LOQ). In addition, the conditions used in the method must minimize matrix and residual acidity (RA) interferences and digest a high mass of EVOO. Thus, the objective of this work was to propose a method for sample preparation of EVOO using microwave-assisted wet digestion in a single reaction chamber (MAWD-SRC) and determination of REE by inductively coupled plasma mass spectrometry with an ultrasonic nebulizer (USN-ICPMS). Initially, the irradiation program was evaluated to allow the digestion of high masses of EVOO (1.0 to 2.0 g). The optimized irradiation program conditions were: (i) 5 min ramp to 115 ºC; (ii) 50 min ramp to 180 ºC; (iii) 10 min ramp to 270 ºC, and hold for 20 min at 270 ºC. The initial pressure with Ar gas and power applied was 40 bar and 1500 W, respectively. The digestion efficiency was evaluated from dissolved organic carbon (DOC) and residual acidity (RA). Considering the complexity of the sample, such as the presence of organic compounds that are difficult to decompose, especially linoleic acid, it is necessary to use concentrated HNO3 employing the MAWD-SRC. Thus, the digestion of 1.0, 1.5 and 2.0 g of EVOO in 6, 7 and 8 mL of 14.4 mol L-1 HNO3 was evaluated. Using the optimized irradiation program was possible to digest up to 2.0 g of EVOO completely. The optimal condition for REE determination was obtained using 1.0 g of EVOO and 8 mL of 14.4 mol L-1 HNO3, resulting in DOC and RA of 129 mg L-1 and 7%, respectively, requiring a low dilution factor of the digests (4 times) to match the acidity content with the calibration solution. The accuracy was evaluated by adding REE to the samples from the reference solution before digestion, obtaining 90 to 107% recovery. Accuracy was also evaluated by comparing the results obtained with the proposed and the microwave-assisted wet digestion (MAWD) reference methods, and showed no statistical difference (t-test, with 95% confidence level) between the results. Moreover, MAWD-SRC allowed the efficient digestion of the high mass of EVOO (1 g) compared to MAWD (0.4 g). MAWDSRC enabled lower LOQ (0.01 - 0.7 ng g-1 ) than MAWD (0.1 - 6.8 ng g-1 ), favoring the quantification of more REE analytes at low concentrations. Therefore, the proposed method is efficient for REE determination at very low concentrations in EVOO and can be used in olive oil authenticity studies.