The digestion of wood samples has specific challenges due to its complex organic matrix. The resistant structure of wood, composed of lignin, cellulose and hemicellulose, may lead to incomplete digestions and spectral and non-spectral interferences that compromise analytical precision. Therefore, it is essential to develop digestion procedures that not only ensure the complete decomposition of organic matter but also allow the detection of a wide range of chemical elements, while minimizing the residual carbon content and the residual acidity in the final solution [1]. Given this challenge, the aim of this study is to optimize and validate a microwave-assisted acid digestion procedure for multi-element analysis of wood samples comparing three different digestion procedures. Samples of the endangered tropical species Ipê (Handroanthus spp), a highly valued hardwood known for its exceptional durability and resistance, were collected in the Amazon Rainforest, Pará state, Brazil. Wood samples were ground in a knife mill with tungsten carbide blade and passed through a 250 µm nylon sieve. Analytical portions of 250 mg were weighed into PTFE vials and digested with HNO₃ (2 mol/L; 4.5 mol/L; 14 mol/L) and H₂O₂ (30% w/w) at 200 °C and 1800 W. The resulting solutions were analyzed in a triple quadrupole inductively coupled plasma mass spectrometer (TQ-ICP-MS) to determine the elements B, Ba, Ca, Ce, Co, Cr, Cu, Gd, K, La, Mg, Mn, Mo, Ni, P, Rb, Sb, Sm, Sr, Tb, W and Zn. Residual carbon was determined using inductively coupled plasma optical emission spectrometer (ICP OES). For analytical quality control, certified reference materials SRM1515 Apple Leaves, SRM1575a Trace Elements in Pine Needles from NIST and in-house quality control material [2] of tropical wood prepared at Radioisotopes Laboratory (CENA/USP) were used. The residual acidity varied from 2 to 14%, proportionally increasing with the concentration of HNO₃. Residual carbon content in wood digests ranged from 7.6% to 22.4%, significantly affecting the reproducibility of the results. Analysis of variance showed no significant difference (p > 0.05) in element concentrations between digestion with diluted and concentrated HNO3. The recoveries of all elements in the digestions carried out with 4.5 mol/L HNO3 were between 80-117%, demonstrating good accuracy and reliability of the procedure. Considering the results with highest precision and reproducibility, as well as critical factors such as cost and green chemistry principles, the procedure using diluted HNO₃ (4.5 mol/L) has superior performance. Additionally, the use of concentrated reagents presents risks, requiring dilution of the digests before analysis and generating significant volumes of acidic waste.

References

[1] E.I. Muller et al. Microchemical Journal, 2017, v. 134, p. 257-261.

[2] ISO Guide 80:2014. Guidance for the preparation of quality control materials, 2014, Switzerland.