Routine analytical methods, even comprehensive 13C NMR analysis, have failed to reveal the key lignin structures originated by different biomass processes, for example, differences between Alcell OS and kraft Indulin lignins (Tables 18.3 and 18.6). Therefore, this methodology is not sufficient to distinguish typical structural features of different lignins neither in qualitative nor in quantitative analytical mode. This indicates the necessity of advanced analytical methods to describe the key characteristics in the structure of technical lignins.

Two-dimensional NMR methods, specifically the HSQC technique, allow to distinguish specific structural characteristics of various technical lignins (Capanema et al., 2001; Balakshin et al., 2003; Liitia et al., 2003). The most advanced structural characterization has been achieved so far for kraft lignins. The HSQC analysis of OS lignins showed significant amounts of specific struc­tures (Balakshin et al., 2000; Capanema et al., 2001; Berlin et al., 2006), however, their exact signal assignment was not possible due to limited NMR data for specific model compounds. Further studies are required in order to perform the proper assignments in this type of lignins. The first attempts to quantify specific lignin functional­ities in different technical lignins have already been un­dertaken (Capanema et al., 2008). The 2D NMR approach pursued by the authors of the latter article focused on the quantification of lignin moieties, which were not possible to quantify with 13C NMR alone (and other 1D NMR techniques), especially of those structures formed during pulping. The study provided important quantitative information on various structural lignin units, such as condensed lignin moieties, products of b­O-4 bond cleavage, vinyl, and alkyl-aryl structures, saturated aliphatic moieties and others, as well as lignin-carbohydrate linkages (Table 18.7).

Another useful advanced NMR analytical method is DEPT 13C NMR that allows for quantification of specific lignin functionalities overlapped in routine 13C NMR spectra (Gellerstedt and Robert, 1987). These advanced NMR methods showed the possibility of expanding our understanding of the structure of technical lignins. How­ever, more comprehensive studies and cross-validation of the advanced methodologies with independent methods are needed before these methods can be routinely used.