## 3 Results and Discussion
### Commonly Used Pretreatment and Preprocessing Solvents  
Table 2 and Figure S1 show the band assignment for common biomass processing solvents. Water is the most common solution in biomass analysis and the conversion processes. It also exists  
Figure 1: FTIR spectra of poplar and its major components: cellulose, hemicellulose, and lignin. (Note: The assignments of the numbered bands in the figure are described in Table 1). in the air, and a certain amount can be accumulated in biomass during its storing and processing. The existence of water in biomass remarkably increased the bands at 3354 and 1653 cm\({}^{-1}\) because of its O-H stretching and O-H-O scissors bending, respectively [32]. Acetone, ethanol, and methanol are common organic solvents for the diverse pre- and post-processing of biomass, and they are also produced from biomass [33]. Acetone contamination on poplar was observed at 3005, 2908, 1713, 1431, 1364, and 1222 cm\({}^{-1}\) representing its CH\({}_{3}\) degenerated stretching, CH\({}_{3}\) symmetrical stretching, C=O stretching, CH\({}_{3}\) degenerated deformation, CH\({}_{3}\) symmetrical deforming, and C-C stretching, respectively [34]. A decrease in the bands at 3354 and 1653 cm\({}^{-1}\) is possibly due to the displacement of water in biomass by acetone. The spectra of ethanol impurity were shown at 3350, 2980, and 1056 cm\({}^{-1}\) for O-H stretching, C-H stretching, and C-O stretching, and those of methanol were at 3352, 2952, 2879, 1465, 1450, 1336, 1053 and 1026 cm\({}^{-1}\) for O-H stretching, C-H stretching (asymmetric), C-H stretching (symmetric), C-H bending (asymmetric), C-H bending (symmetric), C-H bending (symmetric), O-H bending, CH\({}_{3}\) rocking, and C-O stretching, respectively [34; 35; 36]. Besides these chemicals, tetrahydrofuran (THF), dioxane, toluene, glycerol, pyridine, and chloroform are well-known solvents for diverse biomass pretreatment, isolation/purification, and analyses [1; 37; 19; 3]. In addition, some chemicals, such as toluene, can be produced from biomass components [38]. The assignments of these impurities were assigned according to the previous studies. Contamination of poplar by THF appeared at 2977 and 2875 cm\({}^{-1}\) for its C-H stretching, 1063 cm\({}^{-1}\) for ring deformation, and 912 cm\({}^{-1}\) for CH\({}_{2}\) twisting [39]. The bands of dioxane were observed at 2960, 2890, 1457, 1322, 1255, 1119, 1057, 889 and 872 cm\({}^{-1}\) to show its equatorial (higher frequency) C-H stretching, axial (lower frequency) C-H stretching, symmetric CH\({}_{2}\) deformation, CH\({}_{2}\) wagging, CH\({}_{2}\) twisting, C-O-C symmetric stretching, ring trigonal deformation, C-C stretching, and C-O-C stretching, respectively [40]. The addition of toluene on poplar caused three bands including 3069 cm\({}^{-1}\) for C-H stretching, 1497 cm\({}^{-1}\) for C-C stretching, and 728 cm\({}^{-1}\) for C-H out of plane bending [41]. Glycerol on poplar had the bands for O-H stretching at 3341 cm\({}^{-1}\), C-H stretching at 2948 and 2897 cm\({}^{-1}\), C-H deformation of secondary alcohol at 1333 and 1239 cm\({}^{-1}\), C-O stretching of primary alcohol at 1034 cm\({}^{-1}\), and O-H bending at 923 cm\({}^{-1}\)[42; 43]. Chloroform contaminants also showed at 1220 and 755 cm\({}^{-1}\) for C-H bending and CCl\({}_{3}\) stretching [34]. Pyridine contamination resulted in additional bands for C-H stretching at 3036 cm\({}^{-1}\), C-C bonding at 1583 cm\({}^{-1}\), C-N stretching at1485 cm\({}^{-1}\), C-H in plane wagging at 1438 cm\({}^{-1}\), symmetric C-H wagging at 1203 cm\({}^{-1}\), C-H wagging at 1069 cm\({}^{-1}\), C-C in plane wagging at 1032 cm\({}^{-1}\), C-H out of plane bending at 750 and 693 cm\({}^{-1}\)[44; 45]. The intensities of the bands at 3353 and 1653 cm\({}^{-1}\) decreased with the contaminants that do not contain OH groups such as acetone, THF, dioxane, toluene, chloroform, and pyridine due to the displacement of moisture in biomass by these solvents.