Lignin analytical studies

Our constitutional scheme is based on different kinds of information. The first type of information comes from an analytical study of milled spruce wood lignin prepared according to Bjorkman (3). The analytical data considered comprise the elemental composition of lignin, its content of methoxyl groups and other ether bonds, the types and amount of its different hydroxyl groups, carbonyl, and lactone groups, and the kind and number of its biphenylyl linkages and other bonds in which the benzene nucleus is involved. The work that led to data of this kind has been carried out in various laboratories and has been described previously (9,11). 

A thermo-analytical study of untreated and CCA treated wood samples (more or less cylindrically shaped with diameter less than 2 mm) was performed in order to examine the influence of the presence of CCA on the pyrolytic behaviour of wood samples (TG study), as well as the release of metals (Cu, Cr and As) during the pyrolysis process in the kinetically controlled regime. Since arsenic is the most problematic compound during pyrolysis of CCA treated wood and almost all arsenic is present in the treated wood as CrAs04 (precipitated on cellulose or complexed with lignin) [9], a... 

The most significant information in the gross analytical studies is the fact that lignin, even though altered, is the chemical constituent present in highest amounts in the most degraded woods. 

Van Soest, P.J. (1973) Collaborative study of acid-detergent fibre and lignin. Journal of the Association of Official Analytical Chemists 56, 781-784. 

Jpor several years, studies regarding the behavior of lignin on acidolysis, —i.e., heating with 0.2M hydrogen chloride in dioxane-water 9 1— have been carried out in our laboratory. Analytical examination of the changes incurred by this treatment, and more recently, the isolation of various degradation products helped to elucidate some basic structural features of the lignin molecule. 

The theoretical composition deduced for spruce lignin was confirmed by analytical data for several low sulfonated, high molecular weight ligno-sulfonate fractions. The results of these studies are presented in detail in Paperi Pun 47, 443 (1965). 

Other preliminary experiments on alkali lignin included oxidations by barium peroxide and alkali (5, 6), alkali fusion, and alkali fusions in the presence of calcium peroxide, sodium borate perhydrate, and monopersulfate compound. Ether extractives and water extractives were examined, but in all cases too many of the oxidation products obtained were new and unidentifiable, and it was impossible to evaluate the experiments adequately with the available techniques. Vanillic acid appeared to be the chief oxidation product under conditions which did not demethylate further or destroy the aromatic nature of the oxidation products. Some oxidation conditions yielded p-hydroxybenzyl moieties as products, and some gave no trace of these products whatever. More detailed studies of the ether-insoluble, water-soluble components of the several oxidation mixtures were postponed until adequate procedures were developed for analytical isolation and identification. 

Since there are numerous types of structural units in lignin, it is highly unlikely that a single technique will be sensitive to all of them. In addition to the standard analytical techniques, we have been applying Raman spectroscopy to the studies of lignin in pulps. This report focuses primarily on Raman spectroscopic studies of mechanical pulps. Previously, we applied this technique to the studies of celluloses, chemical pulps, and wood. 

Fengel D, Wegener G, Feckl J (1981) Contribution to characterization of analytical and technical lignins (2) Physical-chemical and electron microscopical studies Holzforschung 35 111-118... 

A complete chemical analysis accounts for all the components of the original wood sample. Thus, if wood is defined as part lignin, part carbohydrate, and part extraneous material, analyses for each of these components should sum to 100%. The procedure becomes more complex as the component parts are defined with greater detail. Summative data are frequently adjusted to 100% by introducing correction factors in the analytical calculations. Wise and coworkers (3) presented an interesting study on the summative analysis of wood and analyses of the carbohydrate fractions. The complete analytical report also includes details of the sample, such as species, age, and location of the tree, how the sample was obtained from the tree, and from what part of the tree. The type of wood analyzed is also important i.e., compression, tension, or normal wood. 

Lignin can be isolated by one of several methods. Acid hydrolysis of wood isolates Klason lignin, which can be quantified (see Analytical Procedures ), but is too severely degraded for use in structural studies. Bjorkman s (21) milled wood lignin procedure yields a lignin that is much less degraded and is, thus, more useful... 

Vibrational spectroscopy is an important tool in modern chemistry. In the past two decades, thanks to signihcant improvements in instrumentation and the development of new interpretive tools, it has become increasingly important for studies of lignin. This chapter presents the three important instrumental methods—Raman spectroscopy, infrared (IR) spectroscopy, and near-infrared (NIR) spectroscopy— and summarizes their contributions to analytical, mechanistic and structural studies of lignin. The conceptual frameworks used to interpret vibrational spectra are first described in the following section. 

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