A method for evaluating lignin mobility distributions obtained by capillary zone electrophoresis

The profile of the mobility distribution of lignin-containing samples depen ds on type of sample. To facilitate comparison, a procedure for determining the average mobility (mu(av)), i.e. the average charge density, of lignin is presented. The procedure is applied to black liquor (Sb), isolated disso lved lignin (Sd) and isolated residual (Sr) lignin samples, obtained from f low-through kraft cooks of softwood. The mu(av) of the isolated lignin samp les is compared with the concentration of phenol and carboxyl groups and re lative molecular size. As the cook proceeds the mu(av) for a particular typ e of lignin sample increases, reflecting an increase in average charge dens ity. The mu(av), measured at pH 12, decreases in the order Sd>Sb>Sr, except at the end of the cook, when the average charge densities of the Sb and Sr samples are similar. Associations between lignin and carbohydrate fragment s are proposed to cause the lower mobility of black liquor compared to isol ated dissolved lignin. Characterisations performed at pH 10 indicate that t he isolated dissolved lignin samples may have a higher pK(a) in the middle of the cook than the other samples.