Analysis of Solid Wood Surfaces by Internal Reflection Fourier Transform Infrared Spectroscopy (FTIR-IRS)

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Abstract
The refractive index of white fir wood in the infrared region of the spectrum was determined by a measurement of the critical angle in FTIR-IRS to be 1.43. The depth of light penetration into wood (decrease in the original light intensity to 1/e) was calculated to be 0.13–2.15 μm depending upon the wavelength of light, angle of incidence, and the refractive index of the prism used. It was found that the signal-to-noise ratio in FTIR-IRS of wood could be greatly increased by either pressing the wood for 10 min. at 2,500 psi or sanding it against a flat surface with fine (600 grit) sandpaper. Microtoming alone was not satisfactory in producing a smooth surface and a high signal/noise ratio. The FTIR-IRS spectrum of oxygen plasma-treated wood indicated a substantial absorbance increase in the 1648–1633 cm−1 region due to formation of carbonyl groups.