Figure 44. Pyrolysis mass spectrum of Bemileia vastatrix (coffee rust). Conditions: sample 5 yg; Tc 510°C; E^ 14 eV.
given type of infection. Recent studies of virus-infected leaf tissues by Py-MS confirmed the existence of characteristic changes in the pyrolysis profiles in some cases where the morphological lesions were indistinguishable and the infection could be firmly diagnosed only by lengthy virus culturing techniques (ref. 176).
An important class of problems in agricultural science which can be addressed by Py-MS techniques is the elucidation of the composition and structure of soil humic compounds such as humic acids and fulvic acids, and the relationship with soil fertility and plant development. Analytical pyrolysis studies of these compounds were first reported by Nagar (ref. 177) using Py-GC. Later studies by other authors included Py-GC (ref. 178), Py-GC-MS (refs. 180 - 182) and direct Py-MS (refs. 179, 183 - 186). Hal ma et al. (ref. 51) and Bracewel 1 et al. (ref. 183) showed the possibility of characterising the organic material in soil by directly pyrolysing whole soil samples, obviating the lengthy and ill-defined extraction and fractionation steps for isolating humic substances. Progression of the stage of humification can readily be detected, as the pyrolysis mass spectrum shows the gradual loss of methoxyphenols (lignin), the loss of some polysaccharide products, and increase of aromatic components and alkyl-pyrroles (ref. 186). Py-MS investigations of seasonal changes in fulvic acid patterns from lake water were carried out by De Haan et al. (ref. 187). As examples of pyrolysis mass spectra of these types of complex organic materials, some typical spectra of soil and soil fractions are given in Figure 45.
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