Structural information on lignins could be obtained by wet-chemical and spectroscopic methods using the approach for analysis of wood given in Figure 17. Here, 5 mL 20% AcBr in pure acetic acid is added to ca. 1-10 mg of wood followed by the addition of 0.1 mL perchloric acid (70%) and keeping the mixture for 3 hours at 50 °C, with subsequent neutralization with NaOH, dilution and UV-Vis analysis at 280 nm.

Direct analysis of lignin in wood can be performed by selective / spe­cific degradation followed by GC analysis. Among degradation methods acidolysis, thioacidolysis, permanganate oxidation and pyrolysis can be mentioned. Acidolysis (Figure 18) cleaves predominantly p-O-4-ether bonds by acid hydrolysis and gives many degradation products with a rather low yield of ca. 60%.

Thioacidolysis (Figure 19) gives selective cleavage of p-O-4-ether bonds and results in less complex mixtures than acidolysis and also gives higher yields (> 80%) being able to quantify units with p-O-4-ether bonds and a free hydroxyl. This reaction is performed in dioxane-ethanethiol with boron trifluoride etherate. The degradation products are silylated prior to analysis by GC.

Oxidation by permanganate (KMnO4 — NaIO4 at 82 °C for 6 hours) in acidic solution of ethylated (at pH = 11 and 25 °C with diazoethane for 25 hours) samples with further oxidation by alkaline peroxide for 10 min at 50 °C and final methylation results in samples which can be analyzed by GC (Figure 7.20). This method gives information only on aromatic units with a free hydroxyl, comprising about 10% of lignin in wood and ca. 70% of lignin after kraft cooking.

Another method for the analysis of lignin is pyrolysis combined with GC-MS (Py GC-MS) allowing even a simultaneous determination of lig­nin and carbohydrates. Py GC-MS can be combined with advanced che — mometric methods such as principal component analysis to enable a more complete identification of various lignin fragments. In summary, it can be stated that because of the heterogeneity of lignin there is no universal degradation method giving all desired information on the lignin structure, however, by combination of several methods the structure of lignin can be described fairly well.

FIGURE 22: Analytical procedures for wood extractives