Degradation of wood by UV light: A study by EGA-MS and Py-GC/MS with on line irradiation system

Manuscript number JAAP_2018_932

Title Degradation of wood by UV light: a study by EGA-MS and Py-GC/MS with on line irradiation system

Article type Research Paper

Abstract

UV degradation of wood is an important phenomenon that entails loss of aesthetic and mechanic properties. The changes are usually studied with artificial ageing followed by spectroscopy, and focus on colour changes. Analytical pyrolysis coupled with gas chromatography-mass spectrometry (Py-GC/MS) and evolved gas analysis-mass spectrometry (EGA-MS) are powerful tools for wood characterisation, but the change in pyrolytic behaviour of wood after UV irradiation is not well documented. In this work, a new instrumental setup was used to perform UV irradiation on line with EGA-MS and Py-GC/MS with in situ derivatisation of fir and chestnut wood. The effect of UV exposure was evaluated in terms of thermal stability and composition of the pyrolysate. The results showed that UV degradation of wood is strongly related to its lignin content. Fir wood, with higher lignin content, showed extensive degradation after 4 hours of irradiation, while chestnut wood, with lower lignin content, showed very small changes. Holocellulose to lignin ratios (H/L) were calculated, and principal component analysis was performed on the results of Py-GC/MS, revealing that this technique could be used as a fast monitoring tool to assess the UV degradation of wood.

Keywords analytical pyrolysis; evolved gas analysis; UV degradation; biomass; wood; lignin

Manuscript category Analytical - archaeometry, cultural heritage, environmental, biomass, food Corresponding Author Marco Mattonai

Corresponding Author's Institution

Department of Chemistry and Industrial Chemistry, University of Pisa Order of Authors Marco Mattonai, Atsushi Watanabe, Ai Shiono, Erika Ribechini Suggested reviewers Cristian Torri, Dietrich Meier, Kaige Wang, Taina Ohra-aho

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UNIVERSITÀ DI PISA

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INDUSTRIALE

Dear Editor,

I, as corresponding author, submit the manuscript “Degradation of wood by UV light: a study by EGA-MS and Py-GC/MS with on line irradiation system” by Marco Mattonai, Atsushi Watanabe, Ai Shiono, and Erika Ribechini for publication in Journal of Analytical and Applied Pyrolysis as a full research article.

The manuscript describes the effect of UV light on the pyrolytic behaviour of wood using a recently developed instrumentation. Both a softwood and a hardwood samples were tested, and two different instrumental setups were used. First, UV irradiation followed by evolved gas analysis was used to establish the effect of irradiation on the thermal stability of the samples. Softwood was found to be significantly more affected than hardwood. Second, UV irradiation followed by analytical pyrolysis–GC/MS with in situ derivatisation was used o obtain a detailed screening of the composition of the pyrolysates as a function of irradiation time. The data obtained were processed with principal component analysis to outline trends in the yields of the various pyrolysis products.

We really hope that our paper could be considered for the publication in Journal of Analytical and Applied Pyrolysis.

I declare that the work presented in the manuscript is original and not under consideration elsewhere. All the authors approved the manuscript and agree that it should be submitted to Journal of Analytical and Applied Pyrolysis.

Best regards, Marco Mattonai

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1

Degradation of wood by UV light: a study by EGA-MS and Py-GC/MS with on

line irradiation system

3

4

Marco Mattonai

_{, Atsushi Watanabe}

_{, Ai Shiono}

_{, Erika Ribechini}

5 a _{Department of Chemistry and Industrial Chemistry, University of Pisa, Via Moruzzi 13, 56124 Pisa, Italy}

6 b _{Frontier Laboratories Ltd., 4-16-20 Saikon, Koriyama, Fukushima, 963-8862, Japan}

7

8 Abstract

9 UV degradation of wood is an important phenomenon that entails loss of aesthetic and mechanic 10 properties. The changes are usually studied with artificial ageing followed by spectroscopy, and focus 11 on colour changes. Analytical pyrolysis coupled with gas chromatography-mass spectrometry (Py-12 GC/MS) and evolved gas analysis-mass spectrometry (EGA-MS) are powerful tools for wood 13 characterisation, but the change in pyrolytic behaviour of wood after UV irradiation is not well 14 documented. In this work, a new instrumental setup was used to perform UV irradiation on line with 15 EGA-MS and Py-GC/MS with in situ derivatisation of fir and chestnut wood. The effect of UV exposure 16 was evaluated in terms of thermal stability and composition of the pyrolysate. The results showed 17 that UV degradation of wood is strongly related to its lignin content. Fir wood, with higher lignin 18 content, showed extensive degradation after 4 hours of irradiation, while chestnut wood, with lower 19 lignin content, showed very small changes. Holocellulose to lignin ratios (H/L) were calculated, and 20 principal component analysis was performed on the results of Py-GC/MS, revealing that this technique 21 could be used as a fast monitoring tool to assess the UV degradation of wood.

22

23 Keywords: Analytical pyrolysis; Evolved gas analysis; UV degradation; Biomass; Wood; Lignin

24 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

25 INTRODUCTION

26 Light exposure is a major cause for wood degradation, leading to colour change and loss in mechanical 27 properties [1-5]. The study of changes in properties of wood after UV degradation are of paramount 28 importance in many fields, including materials science, construction, biomass and art. 29 Photodegradation of wood is known to be influenced by many factors, the most important ones being 30 wood species, temperature and humidity [6-9]. UV light absorption of wood is mainly due to the 31 aromatic structure of lignin, and leads to the formation of free radicals [10,11]. The colour change can 32 be attributed to the oxidation of the hydroxyl groups of lignin to carbonyl groups [5,12].

33 Since natural weathering is not reproducible, photodegradation of lignocellulose is usually studied by 34 artificial ageing using UV lamps, and the results of degradation are typically observed with 35 spectroscopic techniques such as IR, UV-Vis and fluorescence [12-14]. Baur and co-workers [11] used 36 electron paramagnetic resonance spectroscopy (EPR) to evaluate the formation of free radicals in 37 wood after UV irradiation. The main disadvantage of these techniques is that they provide information 38 on the whole sample, with only few structural insights on the components of lignocellulose. Moreover, 39 as noted by Pandey and co-workers [15], the presence of additional species such as extractives in 40 wood can lead to misinterpretation of the results.

41 Analytical pyrolysis coupled with GC/MS is the most widely used and flexible techniques for 42 lignocellulose characterisation [16-22]. Py-GC/MS experiments require very small sample quantities 43 and provide short time of analysis with good reproducibility. The chromatographic quality can also be 44 improved using in situ derivatisation, which prevents tailing of compounds bearing highly polar 45 functional groups [23-25].

46 Modern instrumentation for analytical pyrolysis is constantly evolving, unlocking new possibilities for 47 detailed analyses of many types of samples. In recent years, the combination of UV-light irradiation 48 and analytical pyrolysis was made possible and was successfully applied to study the ageing of 49 synthetic polymers [26-29]. The results of these studies showed that UV/Py-GC/MS and UV/EGA-MS 50 can be used as reliable techniques to gain information on the effect of UV irradiation on both the 51 generation of volatile molecules, and the structure of the original material.

52 In this work, UV/EGA-MS and UV/Py-GC/MS with in situ derivatisation were performed on wood 53 samples, to obtain insight on the effect of irradiation on the thermal stability and composition of 54 lignocellulose. Both a softwood and a hardwood samples were used, and the different behaviours 55 were compared. To the best of our knowledge, this is the first work that describes the effect of light 56 irradiation on wood using analytical pyrolysis as a monitoring technique.

57

58 MATERIALS AND METHODS

62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115

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59 Materials: Hexamethyldisilazane (HMDS, Sigma-Aldrich, USA) was used as a derivatising agent in all

60 UV/Py(HMDS)-GC/MS experiments. Fir (Abies alba) and chestnut (Castanea sativa) wood were 61 acquired as slabs from a local provider in Pisa, Italy. The slabs were cut into small pieces (approx. 5 cm 62 length) and milled with a Pulverisette 23 ball mill (Fritsch, Germany) until a fine and homogeneous 63 powder was obtained. Before analysis, the powder samples were also dried in oven at 50 °C for 8 64 hours.

65 Apparatus: In situ irradiation and pyrolysis were performed using an EGA/PY-3030D micro-furnace

66 pyrolyser, equipped with an UV-1047Xe micro UV-irradiator and a QSP-1046E quick-stabilising 67 pressure flow switch (Frontier Laboratories Ltd., Japan). This instrument has been described in 68 previous publications [26,27,29]. The micro-UV irradiator is equipped with a Xe arc lamp, with 69 emission in the range 280-450 nm. The intensity of emitted light was approximately 40 W/m2_. 70 Irradiation took place directly inside the pyrolysis furnace at a temperature of 60 °C. To simulate 71 natural conditions, during irradiation the pyrolysis furnace was filled with air instead of helium. When 72 the irradiation was complete, the whole system was purged with helium for 15 min before analysis. 73 The irradiation times were 1, 2 and 4 h. To ensure that the observed changes in pyrolytic behaviour 74 were only caused by UV irradiation, the samples were also analysed after being kept in the pyrolysis 75 furnace under air at 60 °C for 4 hours with no irradiation. GC/MS analyses in all experiments were 76 performed with a 7890 gas chromatograph (Agilent Technologies, USA) coupled to a 5975 Mass 77 Selective Detector (MSD, Agilent Technologies, USA). All analyses were carried out using helium as 78 carrier gas (1 mL min-1_{). The MSD was operated in EI positive mode (70 eV). The ion source was kept} 79 at 230 °C, while the quadrupole analyser was kept at 150 °C.

80 UV/EGA-MS setup: During UV/EGA-MS analyses, the pyrolysis furnace was heated from 60 °C to

81 700 °C at 10 °C min-1_{. The pyrolysis interface was kept at 100 °C higher than the furnace temperature,} 82 up to a maximum of 300 °C. The GC inlet was at 280 °C, and operated in split mode with a 10:1 ratio. 83 All analyses were performed using an UADTM-2.5N deactivated metal capillary tube (3 m x 0.15 mm, 84 Frontier Laboratories Ltd., Japan), directly connecting the pyrolysis furnace to the MSD. The GC oven 85 was kept at 60 °C during irradiation, and was raised to 300 °C during the analyses. The transfer-line 86 was kept at 300 °C. Mass spectra were recorded in the m/z range 50-600. The sample amount was 87 approximately 100 μg.

88 UV/Py(HMDS)-GC/MS setup: All experiments were performed with a pyrolysis temperature of 550 °C

89 and a pyrolysis time of 0.2 min. The pyrolysis interface temperature was 280 °C. The GC inlet was at 90 280 °C, and operated in split mode with a 20:1 ratio. All analyses were performed with an Ultra ALLOY+ -91 1 capillary column (30m x 0.25 mm, film thickness 0.5 μm, Frontier Laboratories, Japan). During 92 irradiation, the GC oven was kept at 40 °C to prevent column damage. During analysis, the following 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175

93 temperature program was used: 50 °C isothermal for 1 min; 10 °C min-1 _{up to 100 °C, then isothermal} 94 for 2 min; 4 °C min-1 _{up to 190 °C, then isothermal for 1 min; 30 °C min}-1 _{up to 280 °C, then isothermal} 95 for 20 min. The transfer-line was kept at 280 °C. Mass spectra were recorded in the m/z range 50-600. 96 The sample amount was approximately 100 μg, and derivatisation was performed by adding 5 μL of 97 HMDS immediately before the analysis.

98 Data processing: Both EGA-MS and Py-GC/MS data were processed with Origin Pro 8 SR0 (v8.0724,

99 OriginLab Corporation, USA). Py-GC/MS data were also processed using the Automated Mass spectra 100 Deconvolution and Identification Software (AMDIS, version 2.71, NIST, USA). Signals in the mass 101 spectra of EGA thermograms were identified based on the literature [30-33]. Compounds in the 102 pyrograms were identified based on their mass spectra, using literature references [18,23,24], Wiley 103 and NIST-EPA-NIH reference libraries. Semi-quantitative calculations were performed by integrating 104 the peaks of identified compounds, and then expressing the peak areas as percentages. Principal 105 component analysis (PCA) was performed with R (version 3.10, R foundation), using percentage areas 106 as database. Replicates of the same sample were performed to evaluate reproducibility of both 107 UV/EGA-MS and UV/Py(HMDS)-GC/MS analyses. Relative standard deviations were 5% for EGA and 108 10% for Py-GC/MS on average.

109

110 RESULTS AND DISCUSSION

111 UV/EGA-MS: The total ion thermograms (TIT) for all samples at all irradiation times are shown in

112 Figure 1. Each thermogram was normalised using its peak signal.

113 The TIT of non-irradiated fir wood showed three thermal degradation regions. The first region is 114 between 200 and 350 °C, the second is between 350 and 400 °C, and the third is between 400 and 500 115 °C. After irradiation, several changes in the TIT were observed. First, there was a shift to lower 116 temperatures of the signal peak. After 4 hours of irradiation, the shift was greater than 40 °C. The shift 117 in the signal peak is due to a broadening of the EGA profile, which suggests that degradation of the 118 sample was only partial. This is probably because UV light has a low penetrating power, and the 119 internal portion of the sample remained unaffected by the irradiation. Finally, a remarkable change in 120 the shape of the TIT was observed, and only two regions of thermal degradation could be outlined. 121 The average mass spectra for the non-irradiated and 4-hour irradiated samples are also shown in 122 Figure 1. In the average mass spectrum of the non-irradiated fir sample, the characteristic peaks of 123 softwood were observed. The signals at m/z 55, 57, 60, 69, 73, 85, 98, 114 and 126 are characteristic 124 of holocellulose, while the signals at m/z 91, 137, 151, 164, 178 and 272 are typical of guaiacyl-lignin 125 [30-32]. 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233

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126 Significant changes were observed after 4 hours of irradiation. All m/z signals of lignin were either 127 absent or their intensity was considerably reduced, suggesting that lignin underwent an extensive 128 degradation after UV irradiation. This result agrees with the literature [10,11]. While the m/z signals 129 of holocellulose were still present in the mass spectrum after degradation, their relative intensities 130 were different. There was an increase in the signals at m/z 60 and 73, which are characteristic of 131 levoglucosan, the main pyrolysis product of cellulose [31]. On the contrary, other holocellulose signals, 132 such as m/z 85 and 114, decreased. This suggests that irradiation also caused a partial degradation of 133 holocellulose.

134 The main m/z signals corresponding to both holocellulose and lignin were extracted from the 135 thermograms of the non-irradiated and 4-hour irradiated samples of both wood species, to obtain 136 information about the thermal degradation processes occurring in each thermal degradation region: 137 m/z 60 and 73 for cellulose, m/z 85 for holocellulose, m/z 91 and 137 for lignin. The ion profiles are 138 shown in Figure 2.

139 In the non-irradiated fir sample, the main signals of holocellulose appeared in the first and second 140 region, while the main signals of lignin are present throughout the whole thermogram. The signals at 141 m/z 60 and 73 show a peak in the second region, suggesting that cellulose pyrolysis is taking place in 142 this portion of the thermogram. A comparison of the profiles of m/z 137 and m/z 91 also suggests that 143 in the first region of the thermogram (peak of m/z 137) lignin undergoes the first thermal degradation, 144 while in the second and third regions (peak of m/z 91) secondary pyrolysis takes place. These results 145 agree with EGA-MS studies of wood [31].

146 After 4 hours of irradiation, the ion profiles of fir changed drastically. The signals of lignin showed very 147 low intensities, indicating an extensive degradation. On the other hand, the signals of cellulose 148 became dominant in the whole thermogram, while m/z 85 decreased. This confirms that 149 hemicellulose was more degraded than cellulose. The broadening of the ion profiles of m/z 60 and 73, 150 however, suggests that partial degradation of cellulose also took place.

151 The TIT profiles of chestnut samples are similar to those of fir samples, with three thermal degradation 152 regions. However, the thermogram of chestnut is considerably narrower than the one obtained for fir. 153 A likely explanation for this is that chestnut wood has a higher cellulose content than fir.

154 Contrary to fir wood, the thermograms of chestnut showed little to no variation after irradiation. A 155 shift in the peak of signal was observed, although it was much less evident, and amounted to only 8 °C 156 after 4 hours of irradiation. Broadening of the thermogram was also observed, but this also was less 157 evident than the one observed for fir wood. These results suggest that UV irradiation of chestnut wood 158 was much less effective than fir wood.

239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293

159 The main signals in the average mass spectra of chestnut wood were the same observed for fir wood, 160 with some additional signals that can be attributed to syringyl-lignin: m/z 167, 181 and 208 [30,32,33]. 161 In both chestnut average mass spectra, the signals of holocellulose were much higher than those of 162 lignin. The signals at m/z 60 and 73, which are characteristic of cellulose, were very high even before 163 irradiation.

164 In agreement with the EGA profiles, there was very little change in the average mass spectrum of 165 chestnut after irradiation. The intensity of the lignin signals decreased slightly, but the signals of 166 holocellulose did not show any meaningful change. The fact that the relative intensities of 167 holocellulose signals were not affected by irradiation suggests that, despite the high abundance, the 168 polysaccharide fraction was not significantly degraded by UV light. A possible explanation of this is 169 that the degradation of the polysaccharide fraction is not directly induced by irradiation, but rather 170 by the free radicals generated from UV absorption by lignin. This hypothesis has already been 171 discussed in the literature [10,11]. It is likely that the low content of lignin, which is responsible for 172 the absorption of UV light, does not generate enough free radicals, and therefore holocellulose 173 remained mostly unaltered by irradiation. The case of chestnut is therefore opposite to the one of fir, 174 in which the lignin content is sufficiently high to generate a considerable number of free radicals and 175 cause a partial degradation of holocellulose at long irradiation times.

176 The ions extracted for fir wood were the same as those extracted from the EGA profiles of chestnut, 177 with the addition of m/z 167 which is characteristic of syringyl-lignin. The ion profiles, which are shown 178 in Figure 2, confirmed that cellulose is the main component of chestnut wood, and there is a low 179 amount of lignin. The signals of holocellulose (m/z 60, 73 and 85) showed a peak in the first region, 180 while the signals of lignin (m/z 91, 137 and 167) showed a peak in the second region. It is worth noting 181 that the extracted ion profiles of the lignin signals in chestnut wood showed peaks at lower 182 temperatures than in fir wood. This is because hardwood lignin has a lower C-C and a higher C-O-C 183 bond content than softwood lignin, resulting in less thermal stability [34]. In agreement with the TIT 184 profiles, there was very little change in the ion profiles after irradiation.

185

186 UV/Py(HMDS)-GC/MS: The pyrograms of non-irradiated and 4-hour irradiated fir and chestnut

187 samples are shown in Figure 3, and all identified compounds are listed in Table 2. Some compounds 188 that were not available in the literature or in reference mass spectra libraries were identified by 189 comparison with known compounds showing similar mass spectra.

190 Pyrolysis of wood takes place with many competitive and parallel processes, and therefore many 191 different products are obtained. Small molecules (#1, 3, 5, 7, 13, 25, 33, 39) and hydroxybenzenes (#21, 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351

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192 36, 43 ,59, 69) can be generated both by holocellulose and lignin following various pyrolysis 193 mechanisms.

194 Anhydrosugars (#37, 44, 51, 52, 62, 63, 66, 70, 75, 78, 79, 82) are obtained from depolymerisation of 195 holocellulose, which is the primary pyrolysis process. Secondary pyrolysis of holocellulose consists 196 mainly of multiple dehydration reactions, and generates cyclopentenones (#8, 10, 16, 18, 22, 23, 27, 197 38, 46, 47, 49), furans (#2, 9, 29, 30, 32) and pyrans (#15, 19, 20, 26, 28, 35, 55, 65, 77). Uronic acids 198 (#72, 84, 90, 91) are components of hemicellulose. Their presence as whole monosaccharides is likely 199 due to a high reactivity of the carboxylic moiety towards the derivatising agent, which prevents further 200 degradation processes. Uronic acids or their isomers could also be obtained from oxidation of 201 monosaccharides during the irradiation of the sample.

202 Lignin pyrolysis starts with depolymerisation and the formation of whole monomers (#86, 87, 92, 95, 203 98, 99). Secondary pyrolysis of lignin mainly involves the cleavage of the alkyl chain or the loss of the 204 aliphatic hydroxyl group, leading to the formation of shortened chain compounds (#6, 17, 24, 31, 34, 205 40, 41, 45, 48, 50, 54, 60, 64, 68, 76, 88, 94). Loss of methyl or methoxyl groups from the aromatic ring 206 can also take place, generating demethylated products (#42, 53, 58, 83, 97, 100). If both the cleavage 207 of the alkyl chain and the demethylation take place, simple phenolic compounds are obtained (#4, 11, 208 12, 14). Some oxidised monomers, and fragments thereof, were also found in the pyrograms (#56, 57, 209 61, 67, 71, 73, 74, 80, 81, 85, 89, 93). The presence of these compounds could be due to both 210 rearrangements reactions during lignin depolymerisation, and to oxidation reactions that took place 211 during the irradiation.

212 The pyrograms of non-irradiated and 4 hours irradiated fir and chestnut samples are shown in Figure 213 3. The pyrogram of non-irradiated fir was dominated by the two peaks of guaiacyl alcohol isomers 214 (#86, 87, 92). Other intense peaks were observed for some furans and pyrans (#32, 46 and 65). Small 215 molecules generated very low peaks.

216 After irradiation, the complexity of the pyrogram increased. The peak height of the two lignin 217 monomers decreased considerably, while the height of many of the other peaks increased. The most 218 evident increase was observed for some small molecules (#5, 7 and 25) and some anhydrosugars (#63, 219 70, 78). The increase in small molecules after irradiation is an indication of a lower thermal stability of 220 fir wood, which is consistent with the EGA results. The increase in the yield of anhydrosugars suggests 221 that depolymerisation is the most likely degradation process of holocellulose in this case. In fact, it has 222 been shown that the yield of anhydrosugars increases with the decrease of polymerisation degree 223 during the pyrolysis of carbohydrates in the presence of hexamethyldisilazane [24].

224 The pyrogram of non-irradiated chestnut was considerably different from that of fir. The increased 225 number of peaks is due to the more complex structure of hardwood lignin, in which both guaiacyl 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411

226 alcohols (#86, 87, 92) and synapyl alcohols (#95, 98 and 99) are present. The yield of anhydrosugars 227 in chestnut wood is very high, which is consistent with the EGA results suggesting a high holocellulose 228 content. It is interesting to notice that mono-derivatised anhydrosugars (#51 and 52) showed very 229 high peaks compared to non-irradiated fir, meaning that derivatisation of holocellulose is less efficient 230 for chestnut. This could be due to different arrangements of holocellulose and lignin in the two wood 231 species.

232 Another interesting feature of the pyrograms of chestnut is the presence of gallic acid (#96). This 233 compound is a well-known antioxidant, and could be partially responsible for the resistance of 234 chestnut to UV degradation (alone or with other polyphenols that were not detected by this analysis). 235 The role of tannins in increasing the resistance to photo-degradation of wood has already been 236 observed in the literature [15,35].

237 As expected from the EGA results, there was very little change in the pyrogram of chestnut even after 238 4 hours of irradiation. As observed for fir wood, there was a significant decrease in the peaks of lignin 239 monomers, confirming the UV degradation of lignin. The peak of gallic acid also decreased, suggesting 240 that it could have acted as radical scavenger and reduced UV degradation. Moreover, the peak height 241 ratio between mono-derivatised anhydrosugars (#51, 52) and bi- and tri-derivatised anhydrosugars 242 (#62, 63, 66, 70, 75, 78, 79, 82) decreased considerably after irradiation. This means that derivatisation 243 was more efficient for the irradiated chestnut sample than for the non-irradiated one. This could be 244 due to an alteration of the spatial arrangement of the polysaccharide chains after UV exposure. For 245 instance, the degradation of the lignin fraction could have caused an increase in the disorder of the 246 cellulose strands, with a reduction of the crystallinity index. Since the crystalline structure of cellulose 247 is due to the hydrogen bonds between hydroxyl groups, an amorphization process could have 248 increased the reactivity of these groups towards derivatisation.

249 To evaluate the changes in composition of the pyrolysate, the holocellulose-to-lignin (H/L) ratios were 250 calculated for all samples by dividing the total area of holocellulose pyrolysis products by the total 251 area of lignin pyrolysis products. The results are summarised in Table 1. Starting H/L values were 1.2 252 for fir wood and 5.6 for chestnut wood. After irradiation, an increase in H/L was observed for both 253 samples, consistently with the degradation of lignin. However, different trends were observed for the 254 two wood species. After 2 hours, the H/L increased by 25% (from 5.6 to 7.1) for chestnut, and by over 255 300% (from 1.2 to 5.3) for fir. After 4 hours of irradiation, the two species showed different trends: 256 the H/L value of chestnut increased again, while the value for fir decreased. This is most likely due to 257 a partial degradation of holocellulose in fir at long irradiation times.

258 To evaluate the change in pyrolytic behaviour of the samples after irradiation, percentage yields were 259 calculated for all pyrolysis product categories. Calculations were performed by imposing the total area 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469

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260 for both holocellulose and lignin-exclusive pyrolysis products equal to 100. The yield of small 261 molecules and hydroxybenzenes, which were not considered as exclusive of holocellulose or lignin, 262 were calculated by dividing their total area for the area of all peaks. In this way, the observed yield 263 changes were independent from the total change in composition of the pyrolysate. The results are 264 summarised in Table 1.

265 The two wood species showed similar trends. When irradiation time was increased, the yield of small 266 molecules increased considerably for fir wood, and to a lesser extent for chestnut wood. This is 267 consistent with the lower thermal stability of samples after irradiation. The yield of hydroxybenzenes 268 did not show any meaningful trend. Regarding the pyrolysis products of holocellulose, irradiation 269 caused an increase in the yield of anhydrosugars at the expense of furans and pyrans. As already 270 mentioned, this increase could be due to depolymerisation of the polysaccharide chains. The yield of 271 uronic acids increased as well for fir wood, indicating that UV light caused partial oxidation of 272 holocellulose in this species. The yield of uronic acids for chestnut increased only slightly within the 273 first hour of irradiation, and remained constant at longer irradiation times.

274 Regarding the pyrolysis products of lignin, irradiation caused an increase in the phenols and oxidised 275 compounds, at the expense of short chain products and whole lignin monomers. This result suggests 276 that the effect of UV on lignin is the cleavage and oxidation of the alkyl side chain of guaiacyl and 277 syringyl units, generating carbonyl and carboxyl groups.

278 To highlight the differences in the pyrolytic behaviour of the samples, principal component analysis 279 (PCA) was performed using the percentage category yields as database. The loading and score plots 280 for the first two principal components, which accounted for more than 88% of the total variance, are 281 shown in Figure 4. The first principal component showed main contributions from most of the pyrolysis 282 products of both holocellulose and lignin. This first component is most likely highlighting the 283 differences between fir and chestnut. The second principal component showed contributions from all 284 the pyrolysis products whose yield increased when irradiation time was increased: small molecules, 285 oxidised compounds, phenols and uronic acids. This component can therefore be related to the photo-286 degradation and oxidation of wood after irradiation. This is also confirmed by the fact that the sample 287 loadings shift to higher values of the second principal component as irradiation time is increased. 288

289 CONCLUSIONS

290 A new pyrolysis system equipped with on line UV irradiation was used to study the degradation of fir 291 and chestnut after exposure to UV light, using two different instrumental setups.

292 The UV/EGA-MS setup allowed us to estimate the effect of UV light on the thermal stability of wood 293 and its components. Fir wood showed extensive degradation, affecting mainly lignin and, to a lesser 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529

294 extent, holocellulose. The shape of the thermogram of fir wood was deeply altered by UV irradiation, 295 showing broadening and peak shift. On the other hand, chestnut wood showed little to no change in 296 the thermograms, most likely due to the lower lignin content.

297 The UV/Py-GC/MS with in situ silylation setup was used to evaluate the changes in composition and 298 the pyrolytic behaviour of the two wood species after UV irradiation. In agreement with the results of 299 UV/EGA-MS analyses, fir wood showed significant changes after UV exposure, including an increase in 300 the H/L ratio, and higher yields of both small molecules and anhydrosugars. Similar changes were 301 observed for chestnut wood, although to a much lesser extent.

302 Principal component analysis performed on percentage category yields could distinguish softwood 303 and hardwood along the first principal component, and the photo-degradation of the samples along 304 the second principal component.

305 The results of this work prove that analytical pyrolysis with in situ derivatisation can be used as a 306 screening method to assess the degradation of wood by UV light, providing the advantage of short 307 times, low sample amounts and high reproducibility. Moreover, the changes in the pyrolytic behaviour 308 due to UV light exposure could be exploited in the field of biomass pyrolysis, to favour the formation 309 of specific compounds and obtain pyrolysis mixtures with more desirable characteristics.

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368 33.

369 [35] E. Tomak and O. Gonultas, Journal of Wood Chemistry and Technology, 38, (2018) 183. 370 371 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705

13

372 Table 1: Temperature values for the degradation range, temperature of signal peaks from

UV/EGA-373 MS analyses, H/L and percentage category yields for all samples. The degradation ranges were 374 determined using the temperatures at which the signal intensity reaches 5% of the maximum 375 intensity. Smo = small molecules, Hyb = hydroxybenzenes, Cyp = cyclopentenones, Fur = furans, Pyr 376 = pyrans, Ahs = anhydrosugars, Ura = uronic acids, Phe = phenols, Cha = shortened chain, Dem = 377 demethylated, Oxd = oxidised, Mon = lignin monomers.

Fir wood Chestnut wood

Irrad. time (h) 0 1 2 4 0 1 2 4 Deg. range (°C) 243-453 ₍₂₁₀₎ 216-464 ₍₂₄₈₎ 203-474 ₍₂₇₁₎ 193-469 ₍₂₇₆₎ 235-397 ₍₁₆₂₎ 225-409 ₍₁₈₄₎ 222-400 ₍₁₇₈₎ 223-409 ₍₁₈₆₎ EGA peak (°C) 370 359 337 326 296 293 286 288 H/L 1.2 3.2 5.3 4.7 5.6 6.4 7.1 9.1 Smo 6.6 10.4 11.8 12.0 3.1 4.3 5.5 5.6 Hyb 4.7 6.2 4.9 5.6 3.4 3.0 3.8 2.7 Cyp 41.5 27.4 22.3 21.0 16.8 14.8 14.5 13.4 Fur 13.5 5.5 4.8 5.3 3.3 3.8 3.6 3.1 Pyr 34.9 27.2 23.5 24.8 21.4 22.1 21.2 18.1 Ahs 10.0 38.2 46.8 46.1 58.4 58.7 60.3 64.9 Ura 0.0 1.6 2.5 2.7 0.1 0.5 0.4 0.4 Phe 0.8 2.3 3.3 2.7 0.6 0.8 1.7 1.4 Cha 30.0 30.5 29.3 29.6 55.2 51.8 48.8 45.3 Dem 1.3 3.1 3.6 2.6 4.5 1.7 2.7 1.9 Oxd 12.4 20.1 23.6 23.5 7.3 15.1 18.8 20.4 Mon 55.6 44.0 40.2 41.6 32.4 30.6 28.0 31.1 378 379 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765

380 Table 2: Identified compounds from the UV/Py(HMDS)-GC/MS analyses of chestnut and fir wood. For

381 each compound, the original wood component (Orig.), category (Cat.) and main m/z signals are shown. 382 H = holocellulose, L = lignin; Smo = small molecules, Hyb = hydroxybenzenes, Fur = furans, Pyr = pyrans, 383 Ahs = anhydrosugars, Cyp = cyclopentenones, Cha = shortened chain, Dem = demethylated, Phe = 384 phenols, Mon = monomers, Oxd = oxidised. Compounds labelled with [F] or [C] were found only in the 385 pyrograms of fir or chestnut wood, respectively.

# Compound Orig. Cat. m/z

1 1,2-dihydroxyethene (2TMS) - Smo 73, 189, 204

2 2-hydroxymethylfuran (TMS) H Fur 73, 81, 111, 125, 142, 155, 170

3 1,2-dihydroxyethane (2TMS) - Smo 73, 103, 147, 191

4 phenol (TMS) L Phe 75, 151, 166

5 2-hydroxypropanoic acid (2TMS) - Smo 73, 117, 147, 190

6 guaiacol L Cha 81, 109, 124

7 2-hydroxyacetic acid (2TMS) - Smo 73, 147, 177, 205

8 1-hydroxy-1-cyclopenten-3-one (TMS) H Cyp 73, 81, 101, 111, 127, 155, 169

9 2-furancarboxylic acid (TMS) H Fur 73, 95, 125, 169, 184

10 2-hydroxy-1-cyclopenten-3-one (TMS) H Cyp 73, 81, 101, 111, 127, 155, 170

11 o-cresol (TMS) L Phe 73, 91, 135, 149, 165, 180

12 m-cresol (TMS) L Phe 73, 91, 165, 180

13 3-hydroxypropanoic acid (2TMS) - Smo 73, 75, 147, 177, 219

14 p-cresol (TMS) L Phe 73, 91, 165, 180 15 3-hydroxy-(2H)-pyran-2-one (TMS) H Pyr 75, 95, 125, 151, 169, 184 16 3-hydroxycyclopenta-1,2-dione (TMS) H Cyp 73, 115, 143, 171, 186 17 4-methylguaiacol L Cha 95, 123, 138 18 2-hydroxycyclopenta-2,3-dione (TMS) H Cyp 73, 75, 101, 143, 171 19 5-hydroxy-2H-pyran-4(3H)-one (TMS) H Pyr 73, 75, 101, 129, 143, 171, 186 20 3-hydroxy-(4H)-pyran-4-one (TMS) H Pyr 95, 169 21 1,2-dihydroxybenzene (TMS) H Hyb 75, 91, 136, 151, 166, 167, 182 22 1-hydroxy-2-methylcyclopent-1-en-3-one (TMS) H Cyp 139, 169 23 1-methy-2-hydroxycyclopent-1-en-3-one (TMS) H Cyp 73, 169, 184 24 guaiacol (TMS) L Cha 136, 151, 166, 181, 196 25 dihydroxyacetone (2TMS) H Smo 73, 103, 147, 189, 219 26 2-methyl-3-hydroxy-(4H)-pyran-4-one (TMS) H Pyr 153, 183 27 2-methyl-3-hydroxymethyl-2-cyclopentenone (TMS) H Cyp 183, 198 28 3-hydroxy-6-methyl-(2H)-pyran-2-one (TMS) [C] H Pyr 75, 168, 183, 198 29 2-furyl-hydroxymethylketone (TMS) H Fur 73, 95, 103, 125, 183 30 2,3-dihydrofuran-2,3-diol (2TMS) H Fur 73, 133, 147, 157, 2321, 246 31 4-vinylguaiacol L Cha 107, 135, 150 32 5-hydroxymethyl-2-furaldehyde (TMS) H Fur 73, 81, 109, 111, 139, 169, 183 33 glycerol (3TMS) H Smo 73, 103, 117, 133, 147, 205, 218 34 4-methylguaiacol (TMS) L Cha 180, 195, 210 35 2-hydroxymethyl-2,3-dihydropyran-4-one (TMS) H Pyr 170, 185, 200 36 1,2-dihydroxybenzene (2TMS) H Hyb 73, 136, 151, 166, 239, 254 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823

15

37 1,4:3,6-dianhydro-a-D-glucopyranose (TMS) H Ahs 73, 81, 103, 129, 145, 155, 170

38 1-hydroxycyclopenta-2,3-dione, enolic form (2TMS) H Cyp 73, 147, 169, 230, 243, 258

39 2,3-dihydroxypropanoic acid (3TMS) - Smo 73, 103, 117, 133, 147, 189, 205

40 syringol (TMS) [C] L Cha 153, 181, 196, 211, 226 41 4-ethylguaiacol (TMS) L Cha 179, 194, 209, 224 42 4-methylcatechol (2TMS) L Dem 73, 180, 253, 268 43 1,4-dihydroxybenzene (2TMS) H Hyb 239, 254 44 arabinofuranose (4TMS) H Ahs 73, 143, 147, 217, 230 45 4-vinylguaiacol (TMS) L Cha 162, 177, 192, 207, 222

46 2-hydroxycyclopenta-1,3-dione, enolic form (2TMS) H Cyp 73, 243, 258

47 3-hydroxy-2-hydroxymethyl-2-cyclopentenone (2TMS) H Cyp 257, 272

48 eugenol (TMS) L Cha 179, 206, 221, 236

49 3-hydroxy-2-(hydroxymethyl)cyclopenta-2,4-dienone (2TMS) H Cyp 73, 255. 270

50 4-methylsyringol (TMS) [C] L Cha 167, 195, 210, 225, 40

51 1,6-anydro-beta-D-glucopyranose (TMS at position 4) H Ahs 73, 75, 129, 145, 155

52 1,6-anydro-beta-D-glucopyranose (TMS at position 2) H Ahs 73, 75, 101, 116, 132, 145, 155

53 3-(methoxy)benzene-1,2-diol (2TMS) L Dem 153, 239, 254, 269, 284 54 Z-isoeugenol (TMS) [F] L Cha 73, 206, 221, 236 55 3,5-dihydroxy-2-methyl-(4H)-pyran-4-one (2TMS) H Pyr 73, 147, 271 56 vanillin (TMS) L Oxd 194, 209, 224, 239 57 2-(methoxy)benzene-1,4-diol (2 TMS) L Oxd 254, 269, 284 58 5-methyl-3-(methoxy)benzene-1,2-diol (2TMS) [C] L Dem 73, 210, 253, 268, 283, 298 59 1,2,3-trihydroxybenzene (3TMS) H Hyb 73, 133, 239, 327, 342 60 Z-isoeugenol (TMS) L Cha 206, 221, 236

61 4-hydroxybenzoic acid (2TMS) L Oxd 73, 193, 223, 267, 282

62 1,6-anydro-D-galactopyranose (2TMS) H Ahs 73, 142, 145, 161, 189, 204 63 1,4-anydro-D-galactopyranose (2TMS) H Ahs 73, 101, 116, 129, 145, 155, 217 64 4-vinylsyringol (TMS) [C] L Cha 179, 222, 237, 252 65 2-hydroxymethyl-5-hydroxy-2,3-dihydro-(4H)-pyran-4-one (2TMS) H Pyr 73, 129, 147, 155, 183, 273, 288 66 1,4-anydro-D-glucopyranose (2TMS) [C] H Ahs 73, 101, 116, 129, 155 67 acetovanillone (TMS) L Oxd 193, 208, 223, 238 68 Z-propenylsyringol (TMS) [C] L Cha 205, 221, 236, 251, 266 69 1,2,4-trihydroxybenzene (3TMS) H Hyb 73, 239, 327, 342 70 1,6-anydro-beta-D-glucopyranose (2TMS) H Ahs 73, 75, 116, 129, 191, 204, 217

71 vanillic acid methyl ester (TMS) [F] L Oxd 73, 193, 224, 239, 254

72 arabinonic acid-gamma lactone (3TMS) H Ura 73, 117, 147, 189, 204, 217, 244

73 4-hydroxy-3,5-(dimethoxy)cinnamic acid methyl ester (TMS) [C] L Oxd 280, 295, 310

74 syringaldehyde (TMS) [C] L Oxd 224, 239, 254 75 1,4-anydro-D-galactopyranose (3TMS) H Ahs 73, 147, 157, 191, 217, 243 76 E-propenylsyringol (TMS) [C] L Cha 205, 236, 251, 266 77 2,3,5-trihydroxy-4H-pyran-4-one (3TMS) H Pyr 73, 147, 255, 330, 345, 360 78 1,6-anydro-beta-D-glucopyranose (3TMS) H Ahs 73, 129, 147, 204, 217, 333 79 1,4-anhydro-D-glucopyranose (3TMS) H Ahs 73, 147, 157, 191, 217, 332 80 acetosyringone (TMS) [C] L Oxd 223, 238, 253, 268 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883

81 vanillic acid (2TMS) L Oxd 126, 226, 253, 267, 282, 297, 312

82 1,6-anydro-beta-D-glucofuranose (3TMS) H Ahs 73, 101, 116, 129, 191, 217

83 p-coumaryl alcohol (2 TMS) L Dem 236, 294, 309, 324, 331

84 3-deoxy-D-arabino hexonic acid gamma lactone [F] H Ura 73, 103, 129, 127, 145, 205, 246

85 coniferylaldehyde (TMS) [F] L Oxd 73, 177, 192, 220, 235, 250

86 Z-coniferyl alcohol (TMS) L Mon 73, 91, 103, 131, 162, 204, 252

87 Z-coniferyl alcohol (2TMS) L Mon 73, 91, 103, 131, 162, 204, 252

88 3-vanillylpropanol (2TMS) L Cha 179, 206, 221, 236, 311, 326

89 syringic acid (2TMS) [C] L Oxd 223, 253, 283, 297, 312, 327, 342

90 gluconic acid lactone (4TMS) [F] H Ura 73, 103, 129, 147, 220, 319, 333

91 altronic acid lactone (4TMS) H Ura 73, 147, 189, 204, 217

92 E-coniferyl alcohol (2 TMS) L Mon 73, 204, 219, 235, 293, 309, 324

93 3,4-dihydroxy-5-methoxy benzoic acid (3TMS) [C] L Oxd 73, 223, 385, 400

94 3-syringylpropanol (2TMS) [C] L Cha 210, 225, 236, 240, 326, 341, 356

95 Z-synapyl alcohol (2TMS) [C] L Mon 73, 204, 234, 323, 339, 354

96 gallic acid (4TMS) [C] - - 73, 147, 179, 281, 443, 458

97 3,4-dihydroxy cinnamyl alcohol (3TMS) L Dem 179, 205, 293, 355, 382

98 E-synapyl alcohol (TMS) [C] L Mon 73, 234, 251, 267, 282

99 E-synapyl alcohol (2TMS) [C] L Mon 73, 234, 265, 293, 323, 339, 354

100 E-2-methoxy-3,4-dihydroxy cinnamic alcohol (3TMS) L Dem 73, 204, 235, 323, 381, 397, 412 386 387 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941

17

388 Figure 1: Top – normalised total ion thermograms for all samples of fir (left) and chestnut (right).

389 Centre – average mass spectra for non-irradiated fir (left) and chestnut (right). Bottom – average mass 390 spectra for 4-hour irradiated fir (left) and chestnut (right).

391

392 Figure 2: Extracted ion profiles from the non-irradiated and 4-hour irradiated sample thermograms of

393 fir (top) and chestnut (bottom). 394

395 Figure 3: Pyrograms for non-irradiated and 4-hour irradiated fir and chestnut wood samples. Peaks

396 are numbered according to Table 1. 397

398 Figure 4: Loading plot and score plot for the PCA of all wood samples using percentage category yields

399 as database. The samples in the score plot are labelled with the wood species and number of hours of 400 irradiation. The arrows highlight the shift in the scores of the samples after UV irradiation.

401 402 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001