The characterization of humic substances through electrospray ionization Fourier transform-ion cyclotron resonance mass spectrometry

Electrospray Ionization Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry (ESI FT-ICR MS) is currently the only technique capable of fully resolving individual humic molecules. Despite this obvious benefit, concerns about the applicability of the technique to the characterization of humic substances remain.; One of the largest concerns is that the gas-phase ions observed in ESI FT-ICR MS may not be representative of the entire sample in its natural state. Furthermore it remains to be demonstrated that valuable new insight into humic substance composition and their role in the environment can be derived from accurate mass measurements.; Here it was confirmed that the technique appears to be somewhat selective for a certain fraction of humic substances. It was discovered that this fraction is most likely low-MW, degraded lignin. This selectivity largely limits the technique to the characterization of one specific fraction. Fortunately, degraded lignin is a significant component of humic substances and also one that is of great environmental interest, especially in terms of metal-humic complexation. It was further demonstrated that ultra-high mass accuracy combined with ultra-high mass resolving power can provide insight into the molecular composition of humic substances and their precursors. Based on recurring mass spacing patterns and ultra-high mass accuracy, it was possible to determine molecular formulae for humic molecules between approximately 300 and 1000 Da. Interestingly, all identified molecular formulae could be expressed with four equations based on the measured parameter z*. Finally, plausible structures for an individual humic molecule were assigned based on the molecular formulae, tandem mass spectral data and functional group data derived from other techniques (such as NMR and IR).