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The xylems of Alnus incana Moench. and Alnus glutinosa Gaertn. 8-15-year-old trees were subjected to comparative studies regarding the molecular weight (MW) of hemicelluloses. Extraction of hemicelluloses according to the sequence 1% NaOH-10% KOH-18% NaOH, followed by direct analysis of each extract using the SEC/UV method, was applied aiming at simultaneous quantitative and MW analysis. Both the tree age and alder species were found to affect the behaviour of hemicelluloses in alkaline solutions as well as their MW. Under equal extraction conditions, the solubility of xylan increased for A. incana and decreased for A. glutinosa with the tree age. For glucuronoxylan and glucomannan, a cyclic MW oscillation within a certain range throughout the tree growth was found. Both the sinusoid- and impulse-type of MW changes were suggested. The MW of arabinogalactan and xyloglucan decreased to a certain invariant value in the tree age period under investigation.

The state of the art in the determination of cellulose molecular mass (MM) by the size-exclusion chromatography (SEC) method during the last 10 years was examined. The separation of cellulose/pulp during SEC under conditions usually applied, i.e. 0.5–1% LiCl in N,N-dimethylacetamide (DMA) at 80°C on styrene-divinylbenzene packing, was recognized as pseudo-exclusion. The salt sensitivity, over-salting and polyelectrolyte interactions have been proposed as the secondary phenomena accompanying the separation of polysaccharides in DMA with 0.5–1% LiCl added. Chemical heterogeneity as well as the conformation and rigidity of the polymer backbone are the governing factors of the distorting effect. It has been concluded that the SEC technique requires further improvement aiming at optimising the ionic strength and pH of the mobile phase and determining the Mark-Houwink constants.