Analysis of Solid Wood Surfaces by Diffuse Reflectance Infrared Fourier Transform (Drift) Spectroscopy

Comparison of infrared spectra of wood taken by several methods indicated that the DRIFT spectrum of a dispersion of wood powder in KBr, expressed in Kubelka-Munk units, agreed best with the spectrum taken by transmission FTIR of solid wood sections; the DRIFT spectrum of solid wood, expressed in Kubelka-Munk units, was a close second, while the solid-wood DRIFT spectrum expressed in absorbance units differed most. The DRIFT spectrum of solid wood depended to a minor extent on the grain orientation and choice of late- vs early-wood. The spectrum depended strongly, however, on surface roughness with rougher surfaces producing more transmission-like spectra. Depth of penetration at 2242 cm-¹ (90% loss in intensity of CN peak of polyacrylonitrile placed under wood sections) varied between 37 and 138 μm with an inverse linear relationship to wood density. DRIFT spectral analysis of chemically treated wood surfaces indicated an increase in carbonyl functional groups after chlorine or nitrogen dioxide treatment, followed by water extraction, and disappearance of aromatic nuclei after chlorine treatment. The DRIFT spectrum of heat-treated wood suggested formation of furan aromatic structures, while that of wood treated with oxygen radiofrequency plasma showed no discernible changes. The last result was most likely due to wood ablation predominantly by physical sputtering rather then chemical oxidation, coupled with the shallowness of plasma-induced effects compared with the depth of infrared light penetration.