160 m/z 180
Figure 49. Pyrolysis mass spectra of peat samples, showing predominant polysaccharide patterns plus typical phenolic series derived from sphagnol (m/z 94, 108, 120, and 134; upper trace) and grass lignins (m/z 124, 138, 150, 154, and 164: lower trace). Conditions: samples 25 yg; Tc 510°C; Eel 15 eV.
Lignin-derived signals can even be found in much older geochemical formations, such as brown coals or oil shales. In this context, it is interesting to note that lignin-type fragment series have been observed in the pyrolysates of fossil wood, petrified under various conditions, e.g. the silicified wood of the Petrified Forest National Park in Arizona (ref. 193). The German brown coal sample shown in Figure 50 (b) exhibits a strong dominance of phenolic signals which closely resemble the spectrum of recent lignins (see Figure 10). Since this brown coal, a Herzogenrath lignite of Miocene age, is mainly composed of redwood residues, the lignin originally must have been accompanied by large amounts of cellulose and hemicel1uloses (see Figure 50 (a)). Apparently, the latter have disappeared selectively, leaving only the lignin-like materials. When coalification progresses further these lignin-like materials rapidly loose their methoxyl groups by demethylation, resulting in the formation of dihydroxybenzene moieties. A prominent dihydroxybenzene peak at m/z 110 is already visible in the lignite spectrum in Figure 50 (a) and is still a dominant feature in the subbituminous coal spectrum in Figure 51 (a). However, as shown in Figure 51 the dihydroxybenzene series strongly decreases with increasing degree of coalification ("rank"), followed by the phenols and sulphur-containing ion signals.
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