Acetyl group distribution in acetylated wood investigated by microautoradiography

Sapwood of Scots pine (Pinus silvestris L.) was acetylated with C-14- and H-3-labelled acetic anhydride. The distribution of acetyl groups was investigated with microautoradiography and microautoradiographs were evaluated with ESEM, Environmental Scanning Electron Microscopy. The investigation showed that the impregnation of wood with radioisotope-labelled substances provides a good opportunity to investigate the location of substances covalently bonded to the wood material. Introduced C-14-labelled acetyl groups show an even distribution in the wood cell wall, with no discernible concentration gradients at acetylation levels of about 5, 15 and 20 % weight gain. H-3-labelled acetyl groups show an even distribution in the wood cell wall at lj and 20 % weight gain, with no discernible concentration gradients. At the 5 % weight gain level, however, an uneven distribution of H-3-labelled acetyl groups over the cell wall is observed. Nevertheless, the unevenness is random and no concentration gradient is discernible at this level. H-3 with a relatively high resolution, 0.5-1 mum, compared to C-14 with a resolution of 2-5 mum, gives more accurate information about where exactly the acetyl groups are situated in the wood cell wall. Acetic anhydride was evenly distributed when a full impregnation procedure was used. The chemical and physical properties of acetic anhydride allow a uniform penetration into the pine cell wall and a complete acetylation takes place when the specimens are heated.