Resonance Raman spectroscopy of highly fluorescing lignin containing chemical pulps: Suppression of fluorescence with an optical Kerr gate

Abstract

Raman spectroscopy using 400 nm excitation was successfully applied to chemical pulp samples and the fluorescence background that usually limits the application of this method to such samples was effectively suppressed. This enabled the detection of much weaker Raman bands from the pulps. The rejection ratio of the fluorescence background to Raman scattering was estimated to be about 250. The resonance Raman spectra of peroxide bleached chemical pulps had chromophoric lignin bands at 1605 and 1655 cm y1 , whereas the chlorine dioxide bleached pulps had only the aromatic band at 1605 cm y1 . The square root of the aromatic chromophore band relative to cellulose band correlated linearly with the brightness which is in accordance with the Kubelka-Munk theory. This correlation indicated that the resonance enhanced Raman bands were mainly due to chromophoric lignin structures. Chlorine dioxide and peroxide bleached pulps gave different correlations to brightness, which was an indication of different kinds of chromophores in these pulps. The intensity of the aromatic band relative to the cellulose band was about 20 times higher with the ultraviolet (257 nm) than with the visible (400 nm) excitation. This clearly illustrated the importance of working with different excitation wavelengths. On one hand the UV excitation is more sensitive towards aromatic residual lignin in pulp, and on the other hand the visible excitation enables the selective detection of chromophoric lignin structures.

Microspectroscopy as applied to the study of wood molecular structure

“…The fluorescence problem can also be circumvented by probing well below the fluorescence emission in the ultraviolet (UV) (Halttunen et al 2001), an approach that offers also the benefit of a higher signal strength compared to NIR excitation. The UV resonance Raman (UVRR) technique was successfully applied to pulp and paper samples (Halttunen et al 2001;Jääskeläinen et al 2006;Saariaho et al 2003Saariaho et al , 2004Saariaho et al , 2005, heat treated wood (Nuopponen et al 2004) and for investigations on the photodegradation of lignin (Pandey and Vuorinen 2008).…”

“…The two signals are thus effectively separated. By use of the Kerr gate, the fluorescence background of chemical pulp samples excited with a 400-nm excitation line was suppressed effectively (Saariaho et al 2004). Kerr gated resonance Raman spectroscopy enabled (Barsberg et al 2005) to elucidate lignin structures with unique sensitivity and selectivity and for the first time to study lignin radicals as crucial intermediates for lignin biosynthesis in wood cell walls (Barsberg et al 2006).…”

Raman Spectroscopic Characterization of Wood and Pulp Fibers

“…Considering that the Raman spectra of lignins (syringyl, guaiacyl, and coumaryl) are complex, significant advances have been made to interpret them. This has come about as a result of studying, with a number of approaches, not only lignins (native [18], milled wood [18,42,43], and residual lignins [37,[44][45][46][47]) and their models (deuterated and normal dehydrogenation polymer [DHP] lignins [48] and a large number of lignin model compounds [38,49]) but also chemic ally modified (e.g., bleaching, hydrogenation, and acetylation) lignins and lignocellulosics [10,39,43,50,51]. An additional tool in the aid to interpretation has been the theoretical cal culations on lignin models wherein some degree of preliminary work has been done [52,53], but a lot more remains.…”

“…UVRR spectroscopy was used [37,46,47,65] not only for determination of lignin in unbleached and fully bleached chemical pulps, but also to detect bleaching-related changes in residual lignin. Both lignin and hexenuronic acid could be investigated.…”

“…In another application of resonance Raman spectroscopy to detect highly fluorescent residual lignin in chemical pulps, optical Kerr gate approach was successfully used [47]. Sample fluorescence was effectively suppressed and much weaker bands of lignin could be observed and information on chromophore groups in pulps could be obtained.…”