Assessing Trees, Wood and Derived Products with near Infrared Spectroscopy: Hints and Tips

Abstract

Wood is a renewable and valuable resource for a variety of end-use application areas. However, rapid and reliable assessments are needed to identify the quality of the tree, timber or wood product at all stages of production and processing. The ideal technology for assessing wood and wood products must provide reliable data, be user-friendly, cost-competitive and provide a rapid analysis. The ultimate application of near infrared (NIR) spectroscopy of wood or wood products is to substitute for costly and time-consuming reference measurements in order to aid process optimisation or determine properties and genetic traits on large numbers of individual samples.Increased interest in the application of NIR spectroscopy in various research fields including wood is observed nowadays. A vast number of publications highlight the potential of NIR spectroscopy for the characterisation of wood in a broad area of uses. The Journal of Near Infrared Spectroscopy has published two special issues on the application of NIR to forestry and wood research in 2010 and 2011 and a recent literature search yielded in excess of 556,000 results which can be easily found by using the search terms "NIR" and "wood". This mass of published data may suggest that the technique of NIR spectroscopy is widely understood and broadly adopted by the timber industry, but even in recent papers it is evident that there is still a need to better understand the fundamental issues regarding sample selection and preparation, instrument choice, correct measurement and spectral interpretation. In this paper we draw on more than 40 years of collective experience and summarise state-of-the-art knowledge regarding instrumentation, spectral acquisition and data mining in regard to wood science and technology. The goal of this tutorial is two-fold: first, to inform early career wood scientists of the critical steps in utilising NIR spectroscopy to assess the quality of wood. Second, to alert managers to the level of operator skill required for the successful adoption of NIR technology. Some basic information is presented here, but due to the limited size of the manuscript, reference to more specific and detailed literature is provided in each section.

Compositional Variability of Lignin in Biomass

“…The NIR region spans the wavelength range 780-2500 nm (12,821-4000 cm −1 ), in which absorption bands correspond mainly to overtones and combinations of fundamental molecular vibrations, especially stretching and bending [70,71]. The NIR spectrum is a superposition of scatter and light absorbance signals [72] and consequently contains information specific to the molecular vibrational aspects and their physical environments. NIRS methods require multivariate calibration algorithms (PCA, SIMCA, PCR or PLSR) usually referred to as chemometric methods to model spectral response to chemical or physical properties of a calibration sample set [71].…”

“…NIRS methods require multivariate calibration algorithms (PCA, SIMCA, PCR or PLSR) usually referred to as chemometric methods to model spectral response to chemical or physical properties of a calibration sample set [71]. Spectra pre-processing is used to eliminate or minimize variability not related to the investigated property [72] and include normalization, derivatives (usually first or second), multiplicative scatter correction (MSC), standard normal variate (SNV), de-trending (DT) or a combination thereof [70]. NIRS has been successfully applied to evaluate different features of lignocellulosic materials [73,74].…”

Development of Low-Cost Portable Spectrometers for Detection of Wood Defects

“…NIR spectrometers are highly suitable for assessment of heterogeneous organic matter, including living trees and wood [3,10,30]. The NIR spectrum contains information regarding both chemical composition and physical state of measured samples.…”

“…Near infrared (NIR) spectroscopy has great potential for wood quality evaluation [4,5], mapping of wood properties [6,7] and mechanical properties estimation [8]. However, its practical application in field/forest is challenging [9][10][11]. Portable NIR instruments were successfully used in the field for tree breeding [12][13][14], prediction of tracheid length [15], assessment of wood and fiber properties in standing mountain pine beetle-attacked trees [16], wood species recognition [17], wood moisture content prediction [18] and estimation of leaf quality [19].…”

Estimation of phosphorus-based flame retardant in wood by hyperspectral imaging—a new method

“…NIR spectroscopy is widely used for quality control in numerous fields, e.g., the food industry, 6 and the potential for using NIR spectroscopy in the field of wood science has successfully been demonstrated in previous studies. [7][8][9] NIR hyperspectral imaging has also been applied to wood surfaces to determine wood moisture content, 10 map chemical composition, 11 determine wood extractive content, 12 map weathering by UV radiation, 13 for identification of compression wood 14 and for detecting show-through resin defects on painted lumber. 15 The method proposed in this paper involves mapping the NIR absorbance spectrum of a wood sample, by means of a linear regression model, to its corresponding surface concentration of phosphorus.…”