| Under proper reaction conditions, bromination of wood and pulps is specific for lignin, and the amount of bromine incorporated is proportional to the lignin content. A method was, therefore, developed for the quantitative determination of lignin distribution in wood cell walls. The method involves the use of thin sections (0.5 (mu)m or less) and determination of lignin concentrations in various morphological regions by electron microscopy coupled with an energy dispersive X-ray analysis (EDXA).; Evaluation of backscattering, absorption and fluorescence effects on the quantitative assay of lignin distribution indicated that these effects may all be ignored, if relative comparisons are made. Thus the relative ratio of measured Br-L X-ray intensities provides a bromine concentration ratio, which is proportional to the ratio of lignin concentrations in different morphological regions.; A study of lignin distribution in Douglas-fir (Pseudotsuga menziesii Franco) and loblolly pine (Pinus taeda L.) with this technique showed a uniform lignin distribution across the secondary walls of Douglas-fir tracheids, whereas both S(,1) and S(,3) layers of loblolly pine tracheids revealed higher lignin concentrations than the S(,2) layer. Although cell corner middle lamella and compound middle lamella have the highest lignin concentrations, because of its larger volume, the latewood tracheid secondary wall in both Douglas-fir and loblolly pine had 84 to 87% of the total tracheid lignin. The corresponding value was 76 to 77% in earlywood tracheids.; The lignification process in different morphological regions of loblolly pine tracheids studied with the EDXA technique revealed that, prior to S(,2) layer formation, lignification was initiated in the cell corner middle lamella and compound middle lamella regions. Subsequently a rapid lignin deposition was observed in both regions, whereas lignification of the secondary wall was a more gradual process. Furthermore, middle lamella lignification was essentially completed during the early stages of secondary wall lignification. Lignification process within the secondary wall revealed that the S(,1) layer was extensively lignified first. Then, lagging behind cell wall formation, S(,2) layer lignification initiated adjacent to the S(,1) layer and extended to the inner portion of the cell wall. After completion of S(,2) layer lignification, the S(,3) layer continued to increase its lignin concentration at least in the tangential walls. Upon completion, the cell walls revealed a higher lignin concentration in both the S(,1) and S(,3) layers than in the S(,2) layer.; Delignification studies of pulps prepared by soda-anthraquinone, soda and kraft processes by the EDXA technique revealed that soda/AQ pulping was much more selective in removing lignin from the middle lamella and cell corner regions than either soda or kraft pulping. However, up to the 50% delignification point, more lignin was removed from the secondary wall by soda and kraft, compared to soda/AQ pulping. Thus, in order to reach the fiber liberation point, less of the lignin originally present in Douglas-fir wood was removed by soda/AQ than by either soda or kraft pulping.; The observations with conventional electron microscopy confirmed the results obtained for the topochemistry study of delignification with the EDXA technique. |
