Figure 45. Pyrolysis mass spectra of whole soil, soil humic fractions and extracted soil from the A1 horizon of a brown soil on granite (Typic Xerochrept) as described by Saiz-Jimenez et at. (refs. 185, 188). Note the dominance of carbohydrate-type signals in the unextracted soil sample (a). Most of these carbohydrates are removed in the first water-extraction step and form the highly complex polysaccharide fraction (b) which exhibits typical hexose (m/z 102, 126), pentose (m/z 114), deoxyhexose (m/z 128) and hexosamine (m/z 125, 137, 151) signals. The fulvic acid pattern (c) appears to consist of a mixture of furan-type fragments (e.g. m/z 68, 82, 96, 110) and aromacic fragments (e.g. m/z 78, 94, 120, 124, 138, 150), several of which are 1ignin-derived molecular units. Compared with that of fulvic acid, the humic acid pattern (d) shows even more prominent aromatic signals, in addition to pyrrole (m/z 67, 81 , 95) and indole (m/z 117, 131) series'. The pattern of extracted soil (e) shows a high degree of similarity to the humic acid pattern indicating the presence of unextractable humic acid-like materials, known as humins. Also note that characteristic lignin-type fragments at m/z 124, 150 and 164 are abundant in extracted soil. Throughout all spectra, sulphur-containing ion signals are more or less apparent at m/z 34 (H2S), 48 (CH3SH) and 64 (SO2), although these appear to be equally pronounced in the fulvic and humic acid fractions.
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