Careful calculation of the small differences between peaks in the ultra-high resolution FT-ICR mass spectra also revealed what appeared to be regular variations in the compositions of individual fulvic acids, and the regularity in these variations were confirmed by sorting all formulas according to the Kendrick Mass Defect (KMD). This sorting led to the conclusion that all SRFA molecules observed in the ESI FT-ICR mass spectra belonged to one of 266 homologous series; that is, the molecules belonged to a family of compounds that differed from each other only in the number of -CH2-groups. The homologous series identified differed from each other in degree of saturation and through simple substitutions of O for CH4 or CH for N.