Microwave-Assisted Digestion Method and Dispersive Magnetic Solid-Phase Microextraction for the Determination of...

This paper proposes a sample preparation method for the determination of major (Ca, Cu, Fe, Mn, Mg, Na, and Zn) and trace elements (As, Pb, and Se) in lignocellulosic biomass by inductively coupled plasma optical emission spectrometry (ICP-OES). Major composition was determined directly after microwave-assisted digestion with dilute nitric acid, while trace elements were determined using dispersive magnetic solid-phase microextraction (DMSPE). For DMSPE, maghemite nanoparticles were synthesized using the alkaline hydrolysis coprecipitation method and characterized by high-performance scanning microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis. Optimization of the microwave-assisted digestion and DMSPE procedures was performed using a central composite design, with optimal conditions achieved at a sample mass of 280 mg, 2.5 mL of HNO₃, and 2.0 mL of H₂O₂ for MAD and a sorbent mass of 10 mg, a stirring time of 100 min, and a pH of 9.5 for DMSPE. The analytes were recovered with a HCl solution of 5% (w v^-1). The accuracy of the method was assessed by recovery tests and certified reference material analysis. The limits of detection of the method were, in μg g^-1, As (0.01), Pb (0.03), Se (0.01). Ca (2), Cu (1), Fe (0.8), Mg (0.2), Mn (0.2), Na (5), and Zn (0.8). The enrichment factors determined were 21, 31, and 42 for As, Pb, and Se, respectively. Five different lignocellulosic biomasses were analyzed using the proposed method, which yielded concentrations, in μg g^-1, for Ca (367-834), Cu (2-33), Fe (39-3,339), Mg (170-822), Mn (11-58), Na (21-3,967), and Zn (8-27), while As, Se, and Pb were lower than the LOQs.