A Review of Band Assignments in near Infrared Spectra of Wood and Wood Components

Abstract

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 sampleexceeding 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.

Thermo-Vacuum Modification of Poplar Veneers and its Quality Control

“…Bruker Optics OPUS 7.0 software was used for spectra collection, pre-processing, and data mining. Spectra interpretation was performed according to Schwanninger et al (2011). The list of assigned bands is summarized in Table 2.…”

Surface Characteristics of Poplar Wood with High-Temperature Heat Treatment: Wettability and Surface Brittleness

“…Near-infrared spectroscopy and X-ray photoelectron spectroscopy have been widely used for the investigation of the surface chemical composition, providing information regarding the chemical composition and molecular structure of wood (Schwanninger et al 2011;Muñiz et al 2013). Mitsui et al (2008) studied the variation of the molecular structure of heat-treated wood using NIR spectroscopy and found that the hydroxyl groups in the cellulose became degraded in the following order: amorphous, semi-crystalline, and crystalline regions.…”

Detection of Knot Defects on Coniferous Wood Surface Using Near Infrared Spectroscopy and Chemometrics

“…All spectra showed several absorption band peaks in the wavelength region of 1000 nm to 1650 nm, including 1195 nm and 1462 nm. The absorbance near 1195 nm was bound up with the second overtone of C-H stretching vibration from lignin or cellulose (Schwanninger et al 2011). The strong peak at approximately 1462 nm was primarily attributed to the O-H stretching from water or phenolic groups of lignin (Schwanninger et al 2011;Yang et al 2015).…”

“…The peak at 1409 nm is due to the first overtone of O-H stretching vibration, which is attributed to phenolic groups of lignin, while that at 1586 nm is bound up with the first overtone of O-H stretching vibration from cellulose (Schwanninger et al 2011). Knots generally contain a high percentage of compression wood, which has a relatively high content of lignin and low content of cellulose .…”