HUMUS TYPE DISCRIMINATION USING PATTERN RECOGNITION OF THE MASS SPECTRA OF VOLATILE PYROLYSIS PRODUCTS

Mull and mor humus can be distinguished in freely drained British surface soils by the pyrolysis products derived from whole soil samples. About i milligram of soil is pyrolysed at 770 °C in a Curie point apparatus and a sample of the volatile decomposition products is allowed to enter a low-resolution mass spectrometer. A numerical value (the ‘Discriminator Value’) which will discriminate between the two humus types is then calculated from the spectrum of each sample. This is performed by multiplying the intensity of each mass peak by a weight factor and then summing the weighted intensities up to mass 110 The weight factors are obtained empirically by taking sets of spectra from clearly distinguished mull and mor samples and applying a computerized learning machine procedure. Once obtained, they are relatively easily applied to spectra from other soils. The method places all samples on a numerical scale on which mull types appear positive up to about 20 × 10⁸ and mor types negative down to about -15 × 10³, intermediate types being recognized in about the middle of the range. It is indicated that the most important pyrolysis products active in the discrimination process are the olefin hydrocarbons, particularly ethylene, which are produced in greater abundance from mor humus.