Elucidation of Aluminum−Fulvic Acid Interactions by Gas-Phase Hydrogen/Deuterium (H/D) Exchange and Electrospray Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (ESI FT-ICR)

A gas-phase hydrogen/deuterium (H/D) exchange reaction technique using electrospray ionization Fourier transform ion cyclotron resonance (ESI FT-ICR) mass spectrometry to study the interactions between aluminum and fulvic acid (FA) species is described. The gas-phase hydrogen/deuterium exchange reactions and metal complexation (HYDERMEC) approach can be used to study both pure and impure samples. Upon metal complexation, the hydrogen atoms at the binding sites of the molecule are replaced with the metal and hence are not available for H/D exchange reactions. The HYDERMEC method provides the difference between the number of labile hydrogens that are available for H/D exchange reactions in a molecule before and after metal complexation. The HYDERMEC results for a linear oligopeptide composed of three amino acids, arginyl-glycyl-aspartic acid (Arg-Gly-Asp or RGD), and fulvic acid samples are discussed. The oligopeptide and its metal complexes were analyzed directly and without any ICR ion isolation. The electrosprayed fulvic acid ions were selectively isolated in the ICR cell by stored waveform inverse Fourier transform (SWIFT) dipolar excitation. The selected fulvic acid ions were reacted with deuterium exchange reagent molecules (e.g., CD 3 OD) in the gas-phase. An ESI FT-ICR mass spectrometer was used to analyze precursor and/or product ions. The number of incorporated deuterium isotopes in the product ion provides the number of active hydrogens (NOAH) for the precursor ion. The selected ions of all fulvic acid samples used in this study undergo H/D exchange with CD 3 OD neutral molecules. The average maximum NOAH for northern coniferous (NCFA), northern hardwood (NHFA), and southern hardwood (SHFA) soil fulvic acid ions at m / z region 700−1000 Th is ∼7−9 per molecule. Once aluminum ions react with the fulvic acid species, the number of observed exchangeable protons decreases. There is no significant difference at pH ≅ 2.5 between NCFA, SHFA, and NHFA in their NOAH and thus presumably in their complexation with aluminum ions. The HYDERMEC results show that not all of the functional groups with labile hydrogens participate in Al−FA complexation. Also, comparison of the results for various m / z ranges suggests that in terms of available labile hydrogens and Al complexation, species at m / z range 600−1000 Th behave similarly.