Fulvic acids as transition state of organic matter: Indications from high resolution mass spectrometry

Fulvic acids are one of the largest classes of dissolved organic matter, but they are poorly defined and of unclear origin. Three fulvic acid isolates of different origin were analyzed by size-exclusion chromatography coupled to electrospray ionization-Fourier-transform ion cyclotron resonance (FTICR)-mass spectrometry, and molecular formulas for 700-1900 species in these isolates were derived. All three isolates show the same pattern in the elemental composition of their molecules and a large congruence in the molecular sets. It is proposed that the elemental and structural regularity of fulvic acid molecules does not indicate one common precursor material and formation process, but that this regularity is due to both the strong reworking of source materials in the environment and the valency of the three elements (C, H, O) from which most fulvic acid molecules are formed. Potential molecular formulas of fulvic acids were predicted for a mass range of 60 amu based on a few presumptions. A good agreement was found between the predicted and the detected molecular formulas, and it is concluded that (poly-)carboxylic acids with very limited number of hydroxy groups are the major compound class in fulvic acid isolates. It appears that fulvic acids are metastable molecules that characterize a state of transition of diverse precursor compounds during their oxidation.