Carbohydrate components in lignins

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A) Methanolysis

Hemicellulose is the second most abundant polysaccharide constituent of lignocellulosic biomass, which is present in almost all terrestrial plant cell walls and also found as the major chemical “impurity” in technical lignins (Vuorinen & Alén, 1999).

Hemicellulose is an amorphous polymer with a much lower degree of polymerization compared to cellulose which is therefore more susceptible to hydrothermal extraction and/or hydrolysis. This is also the reason why hemicellulose and its degradation products accumulate in different biorefinery streams, effluents, or black liquors. Hemicelluloses usually accompany isolated lignins, no matter whether from a laboratory preparation or an industrial process.

Depending on the source, hemicelluloses are comprised of D–xylose, D–glucose, D–galactose, D–mannose, L–arabinose, and D–glucuronic acid to name but a few major monosaccharidic constituents. The composition and structure of hemicelluloses varies significantly depending on the origin of the lignocellulosic biomass and the conditions of its processing. Knowledge of the hemicellulose content and composition provides information about raw materials, processing conditions, product composition or side stream composition. Moreover, monitoring and optimization of biomass processing based on chemical changes of the hemicellulose fraction is possible.

ALICE core facility offers an accurate and robust determination of the hemicellulose content and the hemicellulose composition (monosaccharide analysis) by means of methanolysis (Sundberg et al. 1996). The protocol requires just milligram quantities of dry sample, which are digested under carefully optimized conditions in an anhydrous methanolic solution of hydrochloric acid, fowwoed by GC-MS-FID quantification of the liberated methyl glycosides. The method has a high selectivity towards hemicelluloses, but covers also the amorphous parts of celluloses. The advantage of methanolysis ocer acidic hydrolysis is the preservation and quantification of uronic acids, which are otherwise degraded under total hydrolysis conditions.

Sample amount required: 100 mg.

References

Sundberg, A., Sundberg, K., Lillandt, C., & Holmhom, B. (1996).
Determination of hemicelluloses and pectins in wood and pulp fibres by acid methanolysis and gas chromatography.
11(4), 216-2019. doi: 10.3183/npprj-1996-11-04-p216-219

Vuorinen, T., & Alén, R. (1999).
Carbohydrates.
In E. Sjöström & R. Alén (Eds.), Analytical Methods in Wood Chemistry, Pulping, and Papermaking (pp. 37-75). Berlin, Heidelberg: Springer Berlin Heidelberg.

B) Fully automated high-performance thin layer chromatography (HPTLC)

Biorefinery streams quite often contain some amounts of carbohydrates and their degradation products with low degree of polymerization.

When those compounds need to be characterized, high-performance liquid chromatography (HPLC) is the traditional method of choice which however, soon comes to its limits when encountering the overly complex product mixtures typical of biorefineries. ALICE core facility offers a much better solution for the characterization of low molecular weight carbohydrates and carbohydrate degradation products, namely high-performance thin layer chromatography (HPTLC), in a fully automated setup. The method is much faster than HPLC or HPLC-derived approaches, it offers the same accuracy and precision, and – most importantly - it is largely insensitive towards (insoluble and insoluble) impurutues and thus does not require sophisticated sample purification (Oberlerchner et al., 2018). Matrix compounds, which are usually a major part of biomass, do not interfere with the analysis. The method is highly selective, has good repeatability and has limits of detection and quantification in the nanogram range. Therefore, also common minor compounds of biorefinery streams – such as glucuronic acid, galacturonic acid, rhamnose, cellobiose and hydroxymethylfurfural – can easily be determined. The visual fingerprint offers information on the samples’ constituents in the case of unknown samples. Also, hydrolysis of biomass can be followed by the oligosaccharide patterns.

Sample amount required: 200 mg.