Proceedings e report - Firenze University Press

NIR SPECTROSCOPIC MONITORING OF WATER ADSORPTION/DESORPTION
PROCESS IN MODERN AND ARCHAEOLOGICAL WOOD
Tetsuya Inagaki 1 *, Hitoshi Yonenobu 2 , Satoru Tsuchikawa 1
1 Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan
2 Naruto University of Education, Naruto 772-8502, Japan
Abstract
We investigated the adsorption/desorption mechanism of water for modern and archaeological wood using nearinfrared
(NIR) spectroscopy. A mixture model of water was used to decompose the near-infrared difference
spectra into three components (free water molecules (S0), those with one OH group engaged in hydrogen
bonding (S1) and those with two OH groups engaged (S2)). The variations of each water component with relative
humidity could be explained by proposing a model that describes water absorption in three stages. It was
concluded that the ageing phenomenon in wood leads to the decrease of adsorption sites on hemicellulose and
amorphous cellulose [1]
1. Introduction
The physicochemical condition of the hydroxyl groups plays a key role in water adsorption/desorption
process in wood. However, for lack of an effective analytical technique, the behavior of water in wood
is not well understood at a molecular level. Near-infrared (NIR) spectroscopy is useful in a
comprehensive material assessment as a means of nondestructive measurement for biological
materials (e.g. forest and agricultural products, textiles and so on) [2]. NIR spectroscopic information
on biological materials is mostly related to water, which has specific absorption bands at 5200cm -1
(combination of stretching and deformation vibrations for OH) and 6900cm -1 (first overtone of the OH
stretching vibration) [3].
In this study, we report on the changes of water condition in modern and archaeological wood as
inferred from NIR spectra. The objective was to investigate the water adsorption mechanism in wood
using a structure model of water. NIR spectra were decomposed into three components of water. The
change in water adsorption mechanism with ageing was examined by comparing the analytical results
between modern and archaeological wood.
2. Experimental Section
We used modern and archaeological wood samples of Hinoki cypress (Chamaecyparis obtusa). The
modern wood sample was taken from a living tree. The archaeological sample was collected from an
upright pillar of a historical building in Japan of which construction date was estimated to be the early
7th century by the documentary analysis. Archaeological wood and modern wood plates were
10×20×0.5 mm and 50×50×2 mm in tangential, radial and longitudinal directions of the samples,
respectively.
The wood plates were humidified gradually from an oven-dried to the fiber saturation state in a sealed
desiccator, in which the internal temperature was maintained at 20°C. Subsequently, the plates were
dehumidified to an oven-dried state.
Diffuse reflection NIR spectra were measured on a FT-NIR spectrophotometer (Bruker MATRIX-F)
with a fiber optic probe. 128 scans were signal-averaged at a spectral resolution of 8 cm -1 .
* E-mail: inagaki.tetsuya@e.mbox.nagoya-u.ac.jp
Joseph Gril (edited by), Wood Science for Conservation of Cultural Heritage –Braga 2008: Proceedings of the International
Conference held by COST Action IE0601 (Braga - Portugal, 5-7 November 2008, ISBN 978-88-6453-157-1 (print)
ISBN 978-88-6453-165-6 (online) © 2010 Firenze University Press