Sensors 2026 , 26 , 2049
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The volatile compounds emitted by office papers are related to wood and to the in- dustrial paper-making process. This means that they should reflect the presence (or not) and relative content of rosin, lignin, cellulose degree of polymerization, carbonyl group content, and paper acidity [19]. The hemicellulose content is higher in hardwood than in softwood. Acetic acid is derived from the degradation of acetylated hemicelluloses from wood through the hydrolysis of acetyl group esters during paper-making technology. If hemicellulose is not acetylated, acetic acid is not a significant volatile product [19–21]. Dur- ing the (paper-making) process, cellulose, the main chemical constituent of wood, degrades into low-molecular-weight organic acids through hydrolytic and oxidative reactions. Acid hydrolysis of cellulose and hemicellulose leads to the formation of reducing alcohol groups, while oxidation leads to the formation of carboxylic acids [19,22,23]. Formic acid, on the other hand, is formed during the microbial fermentation of the sugars present in the wood. Furfural and vanillin are degradation products of lignin and can be considered as its main markers. The presence of alkanes and aldehydes indicates lipid oxidation. The various chemicals involved in paper production may also produce volatile compounds, including alcohols, aldehydes, ketones, carboxylic acids, and hydrocarbons. As shown in Figure 3, Portuguese paper samples from E. globulus (P8, P9, P15, and P16), produced by the same manufacturer (The Navigator Company, Lisbon, Portugal) but from two different plants (ATF and FIG), exhibited similar volatile composition with predominance of an ester and an aldehyde: dodecanoic acid 1-methylethyl ester (isopropyl dodecanoate) and nonanal. When comparing Portuguese paper profiles with those from E. globulus papers from Spain (samples P13 and P14), differences were observed: a strong signal for 3-hydroxybutanone, a ketone, occurred in the Spanish samples. Spanish paper sample P13 showed very low levels of isopropyl dodecanoate and nonanal compared to the Portuguese papers, compounds that were not detected in the second Spanish paper sample, P14, which instead showed the presence of propanoic acid. Because pulps were also obtained from E. globulus trees, the differences in volatiles may be attributed to differences in the industrial manufacturing processes in Portugal and Spain. Samples from Italy (P6 and P20) exhibited a volatile profile similar to that of Portuguese papers, but acetic acid was more abundant. Comparing volatile profiles of papers from France (samples P2 and P19), the USA (sample P7, origin not fully confirmed), Thailand/France (sample P18), Russia (samples P3 and P4, origin not fully confirmed), and Indonesia (samples P10 and P11), all of them have close VOC profiles. As shown in Table 1, the Indonesian papers (samples P10 and P11) are from acacia trees and also show the predominance of isopropyl dodecanoate and nonanal. Both eucalyptus and acacia are sources of hardwood fibers. It is well known that acacia is a strong competitor of eucalyptus in Asia, due to propitious ecological conditions. So, it seems that the wood composition of acacia is similar to that of eucalyptus, and the paper-making technology used in Indonesia is the same as that in Portugal. Samples P5 and P12, both from Nordic birch pulp, show differences that could arise from differences in paper production by different manufacturers (UPM for P5 and STORAENSO for P12). The paper sample from India (sample P21) was confirmed to be manufactured from recycled paper. It showed a very different profile, with a prominent amount of acetic acid. Recycled paper is an eco-friendly option for manufacturing paper products, whose production process involves different types of raw materials: pre-consumer waste, post- consumer waste, and sawdust. The increased presence of acetic acid is likely due to both the treatment of the raw material and the production process for recycled paper [23]. Not surprisingly, furfural was not detected in the paper compositions studied, all of which had an alkaline pH.
https://doi.org/10.3390/s26072049
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