FT Raman Spectroscopy: What It Is and What It Can Do for Research on Lignocellulosic Materials

THE 8TH INTERNATIONAL SYMPOSIUM ON WOOD AND PULPING CHEMISTRY CONTENTS Poster Page Number Number

5 Photochemistry of lignocellulosic materials 5 - 1 Colour as the complex feature of photochemical changes of wood. 1 K. Németh and V. Vanó 5 - 2 Photoyellowing of high yield pulps - looking for oxygen radicals. 5 M. Beyer, H. Koch and K. Fischer 5- 3 Temperature and humidity effects on the light-induced changes in lignocellulosic pulps. 9 I. Forsskåhl 5 - 4 Fluorescence spectroscopy of mechanical pulps. III: Effect of chlorite delignification. 15 J. A. Olmstead, J. H. Zhu and D. G. Gray 5 - 5 Spectral-luminescence study of lignin chromophores of the mechanical pulp. 21 E. Chupka, A. Chupka, G. Ljalin, A. Mikhalevkin and E. Artamonova 5 - 6 The photodegradation of milled wood lignin. Part III. The effect of solvent and light intensity. 27 J. Wang, C. Heitner and R. St. J. Manley 5 - 7 The photochemical formation of lignin - carbohydrate complexes. 33 V. Heller, H. Koch, M. Beyer and K. Fischer 5 - 8 Thermochemical description of light-induced reactions of lignin model compounds. 39 D. A. Ponomarev 5 - 9 The role of xylan and glucomannan in yellowing of kraft pulps. 43 J. Buchert, E. Bergnor, G. Lindblad, L. Viikari and M. Ek

5 - 10 Metal interactions during H2O2 bleaching and irradiation of pulp chromophores. 49 H. Tylli, I. Forsskåhl and C. Olkkonen 5 - 11 Studies on bleaching of Eucalyptus urophylla SCMP and yellowing control. 55 W.-Z. Liang and K. Li 5 - 12 Photostabilization of paper made from high-yield pulps by acetylation. 61 M. Paulsson, R. Simonson and U. Westermark

6 Analytical methods in wood, pulping and bleaching chemistry 6 - 1 FT Raman spectroscopy: What it is and what it can do for research on lignocellulosic materials. 67 U. P. Agarwal and R. H. Atalla 67 FT RAMAN SPECTROSCOPY: WHAT IT IS AND WHAT IT CAN DO FOR RESEARCH ON LIGNOCELLULOSIC MATERIALS Umesh P. Agarwal and Rajai H. Atalla

USDA Forest Service - Forest Products Laboratory, One Gifford Pinchot Drive, Madison WI 53705, USA

Abstract. Near-IR excited FT Raman spectroscopy was developed only recently (1986). Within a short time, however, it has become a very useful technique and is well suited to studies of research and industry samples. The reason why FT Raman is so successful compared to conventional Raman spectroscopy (visible laser excitation) is that: (1) for most samples, spectra are free of fluorescence, so that it’s applicable to many samples that could not be examined by conventional Raman spectroscopy, (2) spectra can be acquired rapidly, and (3) spectral subtraction is accurate. In the near-ill excited FT Raman, sample fluorescence is suppressed (or eliminated) due to sample excitation at 1064 nm where most materials do not absorb. Spectra are obtained rapidly due to the well known signal-to-noise advantages associated with FT instruments.

In the field of lignocellulosic materials, FT Raman spectroscopy has not been applied very much. In our laboratory, a number of topics have been investigated using FT Raman. These included photoyellowing of mechanical pulps, hydrogenation of milled-wood lignin and thermomechanical pulp, measurement of residual lignin in kraft pulps, study of coated commercial-papers, and studies of Zinnia plant cells. In this paper, examples of FT Raman applications from these areas of research are presented.

Keywords. FT Raman spectroscopy, Llgnocellulosics, Wood, Thermomechanical pulp, Lignin, Cellulose, Zinnia cells, Photoyellowlng, Hydrogenation, Coated Paper.

INTRODUCTION

Although conventional Raman spectroscopic techniques have been used to study wood, pulp, and lignin [1-3], in such work, special sampling methods were needed to deal with the problem of laser-induced fluorescence (LIF). LIF was the single most important reason why Raman spectroscopy was under utilized as an analytical technique. For example, in the case of wood samples, when a spectrum was obtained using a scanning Raman spectrometer not only spectral acquisition took several hours, but the signal-to-noise ratio of the obtained spectrum was not good [1]. Moreover, weak features in the spectrum could not be easily detected. Availability of multichannel detectors reduced the time requirement per spectrum but the problem of LIF remained [4]. For lignin containing materials, the methods that were used to limit the the contribution of LIF were only partly effective. Two of these, namely sampling under oxygen [5] and sampling under water [6], were used in most studies. The third method was based on the fact that the process of Raman scattering occurs at a time scale (pico-seconds) that is shorter than the fluorescence life-time (nano-seconds). And therefore, a gated detector, was used to discriminate against LIF [4]. However, the gated-detection method was not used because of the overall complexity of a time-resolved Raman system.Wood and pulp samples were mostly 68 analyzed using the first two sampling methods. Two of the important contributions of conventional Raman spectroscopy were microprobe evidence for lignin orientation [7] and evidence indicating degradation of coniferaldehyde and coniferyl alcohol structures upon photoyellowing of mechanical pulps [8]. Another major contribution was the detection of different secondary structures in cellulose I and II, and different hydrogen-bonding patterns in cellulose I a and I b [9]. Nonetheless, the technique was not used widely. Some of the reasons for this situation, in addition to LIF, were lack of familiarity with the technique and the cost of Raman systems. Another factor that might have contributed to its less than anticipated use was the impression that IR and Raman spectroscopes provide the same information. In reality, the two techniques are complementary.

With the availability of the near-IR excited FT Raman spectroscopy, LIF is no longer a problem [10]. This is evident from the fact that even unbleached kraft pulp, which is extremely fluorescent in conventional Raman, has been analyzed using FT Raman spectroscopy [11]. Indeed, the problem of LIF was one of the primary reasons why FT Raman was developed. Another major advantage with the FT method is that the time required per spectrum has been drastically reduced. Raman spectra of samples that took hours can now be obtained in minutes. Furthermore, considering that heterogeneous samples can be better analyzed using Raman than IR, it is clear that the former technique is much more well-suited for applications in the field of lignocellulosics.

FT RAMAN SPECTROSCOPY

Both conventional and FT Raman spectroscopies are based on the same principle [12]. FT Raman differs from conventional Raman in two important ways; (1) the laser wavelength used to excite samples lies in the near-IR and (2) instead of using dispersive gratings a Michelson interferometer is used to analyze scattered light [12]. An FT Raman instrument consists of the following components; (1) a laser (most often a 1064 nm Nd:YAG laser) for sample excitation, (2) one or more filters to effectively block the Rayleigh scattering, (3) an efficient interferometer, (4) a highly sensitive detector, and (5) a capability to do a fast Fourier-transform on an acquired interferogram. The laser chosen is usually a near-IR laser to avoid any sample fluorescence that might arise. An FT instrument is built around an interferometer. Such an instrument has several advantages. Two of the most important advantages associated with the FT approach are Jacquinot and Felgett (also known as “multiplex”) advantages [12]. The latter advantage allows simultaneous detection of all the wavelengths of light and is the primary reason why an FT instrument records a spectrum in a shorter time than a grating instrument. The high throughput advantage of the inteferometer is called the Jacquinot advantage. The twin advantages more than offset the loss in scattering efficiency as a result of longer wavelength excitation (compared to visible). An additional advantage of FT Raman 69 spectroscopy is the accuracy of the wavenumber values in a spectrum. This is important when spectra are to be subtracted.

In Fig. 1, spectra of a sample of black spruce obtained on conventional and FT Raman systems are shown. From the conventional Raman spectrum, the LIF background has been subtracted and the subtracted spectrum has been smoothed 7 times to reduce noise (Fig. 1a). As can be noted, spectrum (a) is of poor quality. This spectrum took 15 hours to acquire (average of 15 scans) and was sampled under an atmosphere of molecular oxygen. In contrast, spectrum of spruce (Fig. 1b) was obtained in less than 15 minutes under ambient conditions.

Fig. 1 Conventional (a) and FT (b) Raman spectra of black spruce. (a) was obtained after removing laser-induced fluorescence background and smoothing the resultant spectrum.

EXPERIMENTAL

Because of the nature of this report, it is not possible to provide details of the experimental work associated with each of the examples presented (Results and Discussion section). However, original references cited in appropriate places provide references to such details. All FT Raman spectra were obtained on a Bruker RFS- 100 instrument. It is equipped with an ND:YAG laser (1064 nm line) and the laser power can be controlled using the OPUS software. The main instrument is connected to a microscope, using which small areas of samples can be analyzed. In the analysis of Zinnia plant cells, this Raman microscope was used. All other samples were analyzed in the macro mode, using the back scattering geometry.

RESULTS AND DISCUSSION

Lignocellulosic materials. Using FT Raman spectroscopy, a number of naturally occurring lignocellulosics have been analyzed. Spectra are shown in Fig. 2. Nearly all spectra were free of LIF. In the cases where some LIF contribution was detected, sample extraction with toulene:ethanol removed such contributions. Although detailed analyses of these spectra are ongoing, it is clear that the Raman scattering coefficients of various lignin-structural-units are different. This observation is supported by the results of an earlier Raman study of lignin models where it was found that aromatic ring-conjugated model-structures were most sensitive [13]. 70

Fig. 2 FT Raman spectra of (a) Bleached jute, (b) Mango seed fibers, (c) Kenaf, (d) Aspen, (e) Black spruce, (f) Sugarcane bagasse.

Light-induced changes in bleached TMP. Although conventional Raman spectroscopy was previously used to study photoyellowed pulps, the quality of spectra left something to be desired. Recently obtained FT spectra of photoyellowed thermomechanical pulps (TMTs) [14] supponed earlier Raman findings [8] that both coniferaldehyde and coniferyl alcohol structures were degraded when mechanical pulps were exposed to light. From the FT Raman results of a borohydride bleached TMP that was exposed to light for various durations, it was reported that the intensity of the 1654 cm- 1 Raman band decayed with time [14]. Previously, in bleached pulps, this band been assigned to C=C bonds in coniferyl alcohol structures [3].

Hyrogentation of MWL and TMP. In order to monitor the success of the hydrogenation reaction, a method is required that can provide information on the amount of C=C bonds in lignocellulosics. At the concentration at which these bonds are present in TMP and milled-wood lignin (MWL), IR, spectroscopy fails to detect these bonds. Raman, on the other hand, is well suited to this task. Using FT Raman, the hydrogenation reaction (in both TMP and MWL) was monitored to ensure that all C=C bonds were reduced [4].

Residual lignin in kraft pulps. Traditional methods to quantify residual lignin in kraft pulp are tedious and time consuming. Moreover, in some situations, the results may not be reliable. As an alternative, FT Raman spectroscopy was used to determine the amount of residual lignin in kraft pulps. Pulps were analyzed after each stage of a multistage CEDED bleaching sequence. The results are shown in Fig. 3, As can be noted, the value of the regression coefficient (R2) indicated a good linear fit. This suggested that reliable values of the amount of lignin amount can be obtained using FT Raman spectroscopy. Fig. 3 Linear regression between microkappa # and the 1600 cm-1 band intensity (peak ratio relative to cellulose’s 1098 cm-1 band).

Coated paper. In situ analysis of paper coating can be carried out using both conventional and FT Raman spectroscopies. Recently, coating-related Raman features were identified in the Raman spectrum of a coated-paper [15]. These features were aslo detected in the FT spectra [4] and can be used in studies of coated paper.

Zinnia plant cells. Using a Raman microscope, cells from Zinnia elegans tissue cultures were analyzed for the presence of cellulose and lignin [4]. Some of the cells were treated with the cellulose synthesis inhibitor 2,6-dichlorobenzonitrile (DCB) to determine how this treatment affected the biosynthesis of cellulose and lignin [16]. Microprobe spectra were consistent with earlier observations that when DCB was used, the syntheses of both cellulose and lignin were affected.

CONCLUSIONS

On the basis of the examples provided, it is seen that FT Raman spectroscopy has made imponant contributions to various research topics in the field of lignocellulosics. In the future, its use is expected to grow as more and more researchers find that the technique is capable of generating uesful information in their areas of activity..

REFERENCES