NIR measurement of moisture content in wood under unstable temperature conditions. Part 1. Thermal effects in near infrared spectra of wood

Thermal effects in near infrared (NIR) spectra of solid wood samples of Norway spruce (Picea abies Karst,) were studied between -20 and +25 degrees C, When the temperature increases, the two main hydroxyl absorbance bands at approximately 1450 and 1930 nm (here denoted bands A and B) shift by about 0.4 nm degrees C-1 towards shorter wavelengths. For sapwood, there is a marked peak shift of both peaks of about 25 nm (band A) and 20 nm (band B) when part of the moisture in the wood freezes/thaws at 0 degrees C. For heartwood, which has a much lower moisture content, the shifts are only about 5 and 3 nm, respectively. If ice crystals form on the sample surface, a light scattering effect is also seen in the spectra, causing a multiplicative difference between spectra of samples with and without surface ice. It was demonstrated that it is possible to predict the moisture content even of frozen wood using partial least squares (PLS) regression models. It is, however, necessary to take the temperature into account, especially if the temperature fluctuates around 0 degrees C.