ABSTRACT
The mass and size mass distributions of humic and fulvic acids have been a matter of controversy for many years. Depending on the analytical technique, the method of sample preparation and the conditions during the measurement, mean masses between low-end values of 300-600 Da and high-end values of up to about 100,000 Da have been reported [1]. Recent work on fulvic acids by different mass spectrometry techniques consistently revealed mass distributions peaking at m/z between 200 and 400 [2-5]. Taken as a whole, these results comply with the idea that humic substances have a macromolecular (or supramolecular) structure brought about by the association (or self-assembly) of relatively small heterogeneous components to form large or even very large aggregates in aqueous solution [6-9]. The small molecules are assumed to be held together by dispersive forces [8].
