Molecular size and mass distributions of humic substances measured by AFFFF and TOF-SIMS

The molecular size and mass distributions of different natural and synthetic humic substances (HSs: one fulvic acid (FA) and four humic acids (HAs)) were determined by asymmetrical flow field-flow fractionation (AFFFF) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Investigated were the following natural HSs: Aldrich HA, Gohy-573 FA and HA and synthetic HSs: M1 HA and M42 HA. The individual HSs show characteristic molecular size/mass distributions with the following trends in size (AFFFF): M42 HA > Gohy-573 HA > M1 HA > Aldrich HA > Gohy-573 FA or mass (TOF-SIMS): Gohy-573 HA > Aldrich HA > Gohy-573 FA > M42 HA ∝ M1 HA. These results show that molecular size is not always correlated with molecular mass and show generally higher values. Reasons may be association in aqueous solution and/or analyte-membrane interactions, but changes in ionization probabilities and fragmentation probabilities of weak associated molecules with higher masses can also not be excluded. (orig.)