3.1.2. Gel permeation chromatography (GPC)
It has been widely known that during alkaline extraction of
lignocellulosic material, both high molecular weight lignin and
hemicellulose were released into black liquor, thus the molecular
weight distribution was determined by gel permeation chromatography
(GPC). Fig. 2 showed that highest molecular weight of
lignin with lowest polydispersity was obtained from dissolution in
deionized water though thermo-physical process at elevated temperature
(90 C). Lignin isolated from 50% (v/v) ethanol at 90 C for
4 h had higher polydispersity than that from deionized water at the
same condition. From the GPC chromatogram, higher amount of
lignin containing smaller molecules were released in the presence
of 50% (v/v) ethanol compared with water. Isolated lignin by 40%
(w/w) NaOH in 50% (v/v) ethanol (organosolv lignin) had the
similar GPC profile as standard lignin from Kraft process (low sulfonate
content) from SigmaeAldrich, but isolated lignin from this
organosolv process having a narrower range of molecular weights
of lignin than that of standard lignin.
In the present study, lignin isolated from 40% (w/w) NaOH in
deionized water yielded high amount of both large and small
molecules of lignin indicated by two zones of absorption peaks
observed in Fig. 2; the first region ranging between 50 and 75 mL
eluent volume and the second region ranging between 85 and
100 mL eluent volume. Owing the highest amount of hemicellulose
contamination of lignin isolated from aqueous 40% (w/w) NaOH
solution as shown in Table 1, it can be referred that the low molecular
weight of lignin detected was related to the relative
decrease of associated polysaccharides contents, because most of
the dissolved lignin in alkaline treatments was free of linkages with
polysaccharide. In other words, the high alkaline concentration
could cleave the bonds between hemicelluloses and lignin (Mao,
Zhang, & Xu, 2012). The findings in the present study gave