Closed-vessel conductively heated digestion system with diluted nitric acid for the preparation of biomass and biochar samples.

A method using a closed-vessel conductively heated digestion system (CHDS) was developed for preparing lignocellulosic biomass and biochar samples for elemental analysis by inductively coupled plasma optical emission spectrometry (ICP-OES). The use of 1 mL of H₂O₂ plus 2 mL of HNO₃ 3.5 mol L^-1 was suitable to produce clear digests for determining Al, C, Cu, Fe, K, Mg, Mn, P, S, and Zn. The performance of the method was evaluated by analyzing certified reference materials and reference materials. Most CHDS results were in agreement with reference values at a 95 % confidence level (t-test). The method was subsequently applied to the analysis of biomass samples as renewable energy sources (leucaena, pine, sorghum, sugarcane, bamboo, eucalyptus, elephant grass) and their respective biochars. The relative standard deviations (n = 3) were in the 1-15 % range, and the concentration of analytes (in mg kg^-1) were between 52.3 and 3666 (Al), 5.1-15.7 (Cu), 66.6-2133 (Fe), 2691-35391 (K), 864-3275 (Mg), 29.9-559.1 (Mn), 407-3219 (P), 423-2661 (S) and 10.2-66.9 (Zn). Residual carbon contents in digests (935-1907 mg L^-1) were considered acceptable for ICP determinations (<2000 mg L^-1 C). The analytical greenness metric for sample preparation (AGREEprep) was used for the assessment of the proposed CHDS procedure, and the final score of 0.53 was higher than the values reported in similar applications in the literature employing CHDS (0.48), microwave-assisted digestion (0.46), and classical wet (0.23) and dry-ashing (0.30) methods. The proposal aligns with Green Analytical Chemistry (GAC) principles, highlighting its environmental and sustainability benefits for biomass and biochar analysis.