Karin Fackler

A review of band assignments in near infrared spectra of wood and wood components

Near infrared (NIR) spectra of wood and wood products contain information regarding their chemical composition and molecular structure. Both influence physical properties and performance, however, at present, this information is under-utilised in research and industry. Presently NIR spectroscopy is mainly used following the explorative approach, by which the contents of chemical components and physico–chemical as well as mechanical properties of the samples of interest are determined by applying multivariate statistical methods on the spectral data. Concrete hypotheses or prior knowledge on the chemistry and structure of the sample—exceeding that of reference data—are not necessary to build such multivariate models. However, to understand the underlying chemistry, knowledge on the chemical/functional groups that absorb at distinct wavelengths is indispensable and the assignment of NIR bands is necessary. Band assignment is an interesting and important part of spectroscopy that allows conclusions to be drawn on the chemistry and physico–chemical properties of samples. To summarise current knowledge on this topic, 70 years of NIR band assignment literature for wood and wood components were reviewed. In addition, preliminary results of ongoing investigations that also led to new assignments were included for discussion. Furthermore, some basic considerations on the interactions of NIR radiation with the inhomogeneous, anisotropic and porous structure of wood, and what impact this structure has on information contained in the spectra, are presented. In addition, the influence of common data (pre)-processing methods on the position of NIR bands is discussed. For more conclusive band assignments, it is recommended that wood is separated into its components. However, this approach may lead to misinterpretations when evaluation methods other than direct comparison of spectra are used, because isolation and purification of wood components is difficult and may lead to chemical and structural alterations when compared to the native state. Furthermore, “pure” components have more distinct and symmetric bands that influence the shape of the spectra. This extended review provides the reader with a comprehensive summary of NIR bands, as well as some practical considerations important for the application of NIR to wood.

Examination of spruce wood biodegraded by Ceriporiopsis subvermispora using near and mid infrared spectroscopy

The wood colonising white-rot basidiomycete Ceriporiopsis subvermispora is able to degrade lignin in preference to cellulose. To differentiate between fungal strains and to estimate their delignification behaviour, both in an early stage of degradation and over a specific period, is important for the wood industry. Mid infrared (MIR) and near infrared (NIR) spectra were taken from 60 milled spruce wood samples and their total lignin content was determined by wet laboratory methods. Good correlations were found between the MIR band–height ratio (H1510 cm−1 / H897 cm−1) and the lignin content (r = 0.965) and between the NIR band height at 5978 cm−1 (1673 nm) taken from spectra in the second derivative mode and the lignin content (r = 0.956). Furthermore, good linear correlations between the band–height ratios calculated from the MIR spectra and the amplitudes of the band around 5978 cm−1 (1673 nm) of NIR spectra in the second derivative mode were found for the calibration samples (r = 0.934) and for the fungal-treated samples (r = 0.984). The good correlation found between the MIR band–height ratio and the band height from NIR spectra in the second derivative mode could be interesting if calibrations exist for MIR (or NIR) to predict samples measured in the NIR (or MIR). MIR and NIR spectra recorded from milled spruce wood shavings that had been subjected to fungal treatment with three strains of Ceriporiopsis subvermispora (CBS 347.63, FPL 90.031 and FPL 105.752), for a period of up to 14 days, were investigated to see if these spectroscopic techniques could replace chemical methods. It is shown that the relative degree of delignification can be obtained directly from NIR spectra in the second derivative mode measuring the amplitude of a distinct band and from MIR spectra normalised with respect to the band at 897 cm−1. Subjecting the spectra to principal component analysis (PCA) made it possible to study the time course along a PC axis. The use of an appropriate NIR wavenumber range subjected to PCA led to a scores plot that made it possible to differentiate between the three strains of C. subvermispora along one axis. It was also possible to give a time course and an indication of the relative degree of delignification along the second axis. In both cases, 99% of the data variance was explained with the first two PCs. A similar time course was obtained from MIR spectra, but the strains could not be separated well. Besides strain differentiation and examination of delignification, some practical applications (for example, in the pulp and paper industries, fungi-screening, evaluation of wood preservatives) are discussed. The results clearly demonstrate that it is possible to compare and differentiate between the strains without applying time-consuming chemical methods. The examination of NIR spectra is sufficient.

Fungal decay of spruce and beech wood assessed by near-infrared spectroscopy in combination with uni- and multivariate data analysis

Abstract Wood is colonised and degraded by a variety of micro-organisms, the most efficient ones are wood-rotting basidiomycetes. Microbial decay processes cause damage to wooden constructions, but also have great potential as biotechnological tools to change the properties of wood surfaces and of sound wood. Standard methods to evaluate changes in infected wood, e.g., EN350-1 1994, are time-consuming. Rapid FT-NIR spectroscopic methods are also suitable for this purpose. In this paper, degradation experiments on surfaces of spruce (Picea abies L. Karst) and beech (Fagus silvatica L.) were carried out with white rot basidiomycetes or the ascomycete Hypoxylon fragiforme. Experiments with brown rot or soft rot caused by Chaetomium globosum were also performed. FT-NIR spectra collected from the degraded wood were subjected to principal component analysis. The lignin content and mass loss of the specimens were estimated based on univariate or multivariate data analysis (partial least squares regression).

Laccase-catalyzed functionalization with 4-hydroxy-3-methoxybenzylurea significantly improves internal bond of particle boards

Abstract Enzymatic functionalization is an attractive tool to provide a reactive interface for further processing of lignocellulosic materials, such as wood particles and fibers. Here, spruce wood particles have been functionalized by fungal laccase combined with 4-hydroxy-3-methoxy-benzylamine (HMBA) or 4-hydroxy-3-methoxybenzylurea (HMBU). The expectation was crosslinking with resins in subsequent glueing processes, which should improve strength properties of particle boards. Essential process parameters, such as liquid to solid mass ratio and treatment time, were optimized on a laboratory scale resulting in HMBA and HMBU binding yields of 90% and above as determined by radiochemical mass balance analysis. We employed a multifactorial experimental design for board production from treated wood particles and urea/formaldehyde resin. Mechanical testing and multivariate data analysis revealed, for the first time, an increase of internal bond (IB) as a result of functionalization with HMBU. HMBA was not successful. Variance analysis of relevant parameters and their interactions demonstrated a highly significant difference (P>99.99%) between boards treated with laccase/HMBU versus untreated wood particles. Due to positive interactions, functionalization was most effective at high bulk density (750 kg m-3) and high resin content (10%) resulting in a calculated IB improvement of 0.12 N m-2 (21%).

How spectroscopy and microspectroscopy of degraded wood contribute to understand fungal wood decay

Nuclear magnetic resonance, mid and near infrared, and ultra violet (UV) spectra of wood contain information on its chemistry and composition. When solid wood samples are analysed, information on the molecular structure of the lignocellulose complex of wood e.g. crystallinity of polysaccharides and the orientation of the polymers in wood cell walls can also be gained. UV and infrared spectroscopy allow also for spatially resolved spectroscopy, and state-of-the-art mapping and imaging systems have been able to provide local information on wood chemistry and structure at the level of wood cells (with IR) or cell wall layers (with UV). During the last decades, these methods have also proven useful to follow alterations of the composition, chemistry and physics of the substrate wood after fungi had grown on it as well as changes of the interactions between the wood polymers within the lignocellulose complex caused by decay fungi. This review provides an overview on how molecular spectroscopic methods could contribute to understand these degradation processes and were able to characterise and localise fungal wood decay in its various stages starting from the incipient and early ones even if the major share of research focussed on advanced decay. Practical issues such as requirements in terms of sample preparation and sample form and present examples of optimised data analysis will also be addressed to be able to detect and characterise the generally highly variable microbial degradation processes within their highly variable substrate wood.

Polysaccharide Degradation and Lignin Modification during Brown Rot of Spruce Wood: A Polarised Fourier Transform near Infrared Study

Radial thin sections of spruce wood that had been degraded by brown-rot fungi (Gloeophyllum trabeum or Poria placenta) exhibiting mass losses up to 16% were investigated by polarised Fourier transform near infrared (FT-NIR) transmission spectroscopy. This method allowed for the analysis of the changes within wood caused by the decay fungi. The main features of brown rot visible in the FT-NIR spectra were the reduction of amorphous wood polysaccharides in preference to crystalline, an overall loss of molecular orientation of the structural polymers, and the formation of phenolic groups in lignin through cleavage of aryl–ether bonds or demethoxylation. Furthermore, polarised FT-NIR spectroscopy allowed us to examine the assignments of the first overtones of O–H stretching vibrations in more detail and to suggest a number of new band assignments in this spectral region. Among them were the bands of the 0(3)–H(3)···O(5) intramolecular hydrogen bond of glucomannan and the strong parallel intramolecular hydrogen bond 0(2)–H(2)···O(6), which is only accessible with polarised NIR spectroscopy because it overlaps with a number of perpendicular bands, among them the perpendicular O(6)–H(6)···O(3)′ intermolecular hydrogen bond of crystalline cellulose. First overtones of strongly H-bonded O–H groups of cellulose Iβ (6340 cm−1) and cellulose Iα (6270 cm−1) were tentatively assigned.

FT-IR imaging microscopy to localise and characterise simultaneous and selective white-rot decay within spruce wood cells

Abstract Spruce wood that had been degraded by white-rot fungi (Trametes versicolor or Ceriporiopsis subvermispora) and suffered mass losses up to 17% was investigated by transmission Fourier transform infrared (FT-IR) imaging microscopy. A significant marker during incipient simultaneous white-rot (T. versicolor) was the cleavage of glycosidic bonds of polysaccharides that preceded their metabolisation. Simultaneous white-rot processes were also characterised by a relative decrease of the overall lignin content and a relative accumulation of wood polysaccharides. No early marker was found for selective white-rot (C. subvermispora) that removes mainly lignin by an oxidative process. This feature was detected only in wood samples exhibiting mass losses higher than 12%. Furthermore, it was shown, that simultaneous and selective white-rot processes were unevenly distributed within the wood samples but quite evenly distributed within single tracheids.

Microspectroscopy as applied to the study of wood molecular structure

Microspectroscopy gives access to spatially resolved information on the molecular structure and chemical composition of a material. For a highly heterogeneous and anisotropic material like wood, such information is essential when assessing structure/property relationships such as moisture-induced dimensional changes, decay resistance or mechanical properties. It is, however, important to choose the right technique for the purpose at hand and to apply it in a suitable way if any new insights are to be gained. This review presents and compares three different microspectroscopic techniques: infrared, Raman and ultraviolet. Issues such as sample preparation, spatial resolution, data acquisition and extraction of knowledge from the spectral data are discussed. Additionally, an overview of applications in wood science is given for each method. Lastly, current trends and challenges within microspectroscopy of wood are discussed.

Accessibility of hydroxyl groups of brown-rot degraded spruce wood to heavy water

The accessibility of alcoholic and phenolic hydroxyl (O–H) groups in non-degraded and brown-rot degraded spruce wood (Piceas abies L. Karst.) was investigated with Fourier transform near infrared (FT-NIR) transmission spectroscopy in combination with deuterium exchange with heavy water (D2O). In the presence of excess D2O, accessible O–H groups were converted into O–D groups and characteristic O–H first overtone bands disappeared in the FT-NIR spectra. Hydroxyl groups of cellulose Iα were more accessible than those of cellulose Iβ. O–H groups of wood degraded by brown-rot fungi showed much lower overall accessibility, and free O–H groups from amorphous matrix polysaccharides, preferentially those located in primary and outer secondary cell walls and middle lamellae were least accessible, whereas H-bonded O–H groups associated with cellulose microfibrils showed residual accessibility in brown-rotted wood of incipient, early and pronounced degradation stages.

Fungal pretreatment of pine wood to reduce the emission of volatile organic compounds

Abstract Sterilized pine wood strands were treated with the ascomycete Ophiostoma piliferum (Cartapip 97™) in an attempt to reduce aldehyde emissions through degradation of aldehyde-forming precursors. Wooden boards were then produced from the treated strands by means of a laboratory press. VOC emissions of these boards were characterized and the concentrations of seven major substances were followed for a period of 28 days in Markes μ-CTE micro-chambers employing Tenax-TDAS/GC/MS analytics. Boards made from treated strands showed a highly significant (P<0.001) reduction in aldehyde emissions by 70% compared to the control boards, while differences in the monoterpene emissions were not significant (P>0.05). Wood extractives from treated and untreated strands were also analyzed by GC/MS after milling and acetone/water extraction. A marked decrease of C18 unsaturated fatty acids was detected in agreement with the reduction of aldehyde emissions, and the monosaccharides as indicators of fungal growth were depleted and stilbenes and lignans were partially degraded.