“Self-Absorption” Phenomenon in Near-Infrared Fourier Transform Raman Spectroscopy of Cellulosic and Lignocellulosic Materials

While cellulosic and lignocellulosic materials have been studied using conventional Raman spectroscopy, availability of near-infrared (NIR) Fourier transform (FT) Raman instrumentation has made studying these materials much more convenient.1 This is especially true because the problem of laser-induced fluorescence can be avoided or minimized in FT-Raman (NIR Raman) spectroscopy. More recently, the method has also been used to generate quantitative results in cellulosic and lignocellulosic materials.2–5 Although linear dependence of Raman intensity on analyte concentration is well established, the occurrence of self-absorption (defined as absorption of Raman scattered photons by the sample itself) is known to cause problems.6,7 The influence of self-absorption in an FT-Raman spectrum is manifested in the spectral region where the sample absorbs as a result of overtoneand combinationvibration bands. The net result is that the Raman band intensity or intensities are diminished. This is also true for samples that have an electronic absorption in the visible region and are being analyzed by visible Raman spectroscopy. For instance, researchers have pointed out the occurrence of self-absorption in resonance Raman spectroscopy.8,9 Compared to visible Raman spectroscopy, where only a small number of samples have strong electronic absorption, in NIR Raman, many more substances are expected to have overtone and combination-