## 3 Results and Discussion  
### Major Components of Biomass  
Cellulose, hemicellulose, and lignin are three main components in lignocellulosic biomass. Table 1 and Figure 1 present the FTIR band assignments and spectra of poplar and its major components isolated from the same biomass. Prior to the analysis, other extractives in the poplar sample were removed by two-step extraction: 8 h toluene/ethanol Soxhlet-extraction followed by 6 h water extraction. Isolated cellulose, hemicellulose, and lignin were analyzed using FTIR and compared with the extractives-free poplar. The assignment of each band was identified according to the previous studies [23; 24; 25; 26; 27; 28; 29; 30; 31]. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Wavenumber [cm\({}^{-1}\)]** & **Assignment** & **Components** \\ \hline
1 & 3367 & O-H stretching & Cellulose, Hemicellulose, Lignin \\
2 & 2914 & C-H stretching & Cellulose, Hemicellulose, Lignin \\
3 & 1745 & C=O stretching & Hemicellulose, Lignin \\
4 & 1618 & Aromatic skeletal vibration, C=O stretching, adsorbed O-H & Hemicellulose, Lignin \\
5 & 1508 & C=C-C aromatic ring stretching and vibration & Lignin \\
6 & 1457 & C-H deformation (in methyl and methylene) & Lignin \\  & & Symmetric CH\({}_{2}\) bending vibration, symmetric stretching band & Cellulose, Hemicellulose, Lignin \\  & & cf carbonyl group, C-H deformation & Cellulose, Hemicellulose, Lignin \\
8 & 1370 & C-H bending, C-H stretching in CH\({}_{3}\) & Cellulose, Hemicellulose, Lignin \\
9 & 1317 & CH\({}_{2}\) wagging, C-O stretching of C\({}_{5}\) substituted aromatic units & Cellulose, Hemicellulose, Lignin \\
10 & 1235 & C-O stretching of guaiacyl unit & Lignin \\
11 & 1160 & C-O-C stretching & Cellulose, Hemicellulose \\
12 & 1108 & Aromatic C-H in plane deformation & Lignin \\
13 & 1053 & C-OH stretching vibration, C-O deformation & Cellulose, Hemicellulose, Lignin \\
14 & 1032 & C-O stretching, aromatic C-H in plane deformation & Cellulose, Lignin \\
15 & 896 & C-O-C stretching & Cellulose, hemicellulose \\
16 & 846 & Aromatic C-H out of plane bending & Lignin \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Fourier transform infrared (FTIR) band assignments of poplar and its major components: cellulose, hemicellulose, and lignin [23; 24; 25; 26; 27; 28; 29; 30; 31]. The IR spectra of poplar and its components showed strong O-H stretching and C-H stretching absorptions at 3367 and 2914 cm\({}^{-1}\), respectively. These two strong absorptions are because all three major components in biomass (cellulose, hemicellulose, and lignin) have hydroxy groups and many C-H bonds in their structures. The absorption at 1745 cm\({}^{-1}\) was due to C=O stretching in hemicellulose and lignin. The absorption at 1618 cm\({}^{-1}\) represented asymmetric stretching band of the carboxyl group of glucuronic acid in hemicellulose and C=O stretching in conjugated carbonyl of lignin. The band at 1650 cm\({}^{-1}\) in the IR spectrum of cellulose was possibly caused by adsorbed H\({}_{2}\)O. Higher absorption at 3367 cm\({}^{-1}\) was also observed because of the moisture content in the biomass. In addition, the bands due to symmetric CH\({}_{2}\) bending vibration in cellulose, carboxyl vibration in glucuronic acid with xylan, and C-H in plane deformation with aromatic ring stretching in lignin were observed at 1424 cm\({}^{-1}\). The IR absorption bands at 1582 and 1508 cm\({}^{-1}\) assigned to aromatic ring stretching and vibration (C=C-C) in lignin. The band at 1457 cm\({}^{-1}\) was observed in lignin due to its C-H deformation in methyl and methylene. The C-H bending in cellulose, hemicellulose, and lignin (aliphatic C-H stretching in methyl and phenolic alcohol) was observed at 1370 cm\({}^{-1}\). The CH\({}_{2}\) wagging in cellulose and hemicellulose and the C-O stretching of C\({}_{5}\) substituted aromatic units, such as syringyl and condensed guaiacyl units, were assigned at 1317 cm\({}^{-1}\). Similarly, the C-O stretching of guaiacyl unit in lignin was assigned at 1235 cm\({}^{-1}\).