Proton (1H) NMR Spectroscopy

1H NMR spectroscopy can be used for the characterization and classification of lignins and determination of lignin structure. Ludwig (1971) reviewed the literature dealing with the 1H NMR spectroscopy of lignins through 1968, and a comprehensive 1H NMR study of lignins and lignin model compounds was published in 1964 by Ludwig et al. (1964a,b). In this latter instance, the lignins were analyzed as acetate derivatives using deuteriochloroform as solvent. The methodology developed by Ludwig et al. (1964) has been applied in numerous studies (e.g., Šutý and Golis 1972, Košiková et al. 1973, Faix and Schweers 1974, Simionescu et al. 1981). During the 1970s and 1980s, important developments relating to the techniques used to record NMR spectra have taken place. For example, modern high-frequency instruments of the Fourier transform-type provide comparatively well-resolved 1H NMR spectra. With such instruments, high signal/noise ratios and very accurate NMR data are obtained. Owing to these improvements, it is possible to enlarge regions of the spectra for closer examination and to determine trace amounts (~0.1%) of certain types of lignin units (e.g., benzaldehyde units).