Three different molecular weight fractions of fulvic acid from Tjeukemeer, sampled monthly between January and September 1978, were studied by Curie-point pyrolysis-mass spectrometry. The winter samples apparently differed markedly in composition from the summer samples. These differences, which were particularly striking in the pyrograms of the high molecular weight fraction, can be explained in terms of fragment molecules attributable to polysaccharides, proteins and/or phenolic polymers.

Because of increased interest in mineralogical analysis of soils a rapid, generally applicable method to separate clay minerals is needed, and such a method is described here. The technique is a modification of the heavy liquid density gradient centrifugation method which is commonly applied in mineralogical practice. Modifications are:(1) The use of suitable surface active agents to overcome the flocculation problem.(2) The use of the ultra centrifuge at about 33,000 g to hasten sedimentation.(3) The use of a linear density gradient which supplies in addition a quick identification of the mineral composition. Preliminary experiments, evaluating different variables (e.g. cation form, ultrasonic treatment, influence of surfactant), reveal the scope and limitations of the procedure presented.Clay minerals of different density can be separated. However, if the clay minerals of a mixture have overlapping densities, or if they contain a series of mixed-layer minerals, only a broad fractionation is possible. This is shown by X-ray diffraction and electron microscope studies of isolated zones (or parts of them) which develop in the centrifuged density gradient columns containing various pre-treated natural clays or clay mixtures.

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SUMMARY. Curie point pyrolysis mass‐spectrometry of freshwater fulvic acids from Tjeukemeer indicates the presence of polysaccharides, amino sugars, proteinaceous material and phenols. These results resemble those of the pyrolysis of soil fulvic acids. However, freshwater fulvic acids may be richer in aliphatic components than those from soil.

A method is outlined and apparatus described to prepare rapidly and conveniently a reproduceable linear density gradient of any low-to-high value. The apparatus consists of two prismatic connected vessels of triangular shape one of which is inverted and tilted to a pre-calculated degree to account for the density difference of the liquids in the vessels. The liquids emerge, by gravitational levelling, from a common efflux tube connecting the vessels. 30 ml of density gradient can be withdrawn in 5 min. The linearity of the etflux is theoretically derived and shown experimentally using a number of organic liquids.