Gas-Phase Hydrogen/Deuterium Exchange Reactions of Fulvic Acids: An Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectral Study

Abstract

A gas-phase hydrogen/deuterium (H/D) exchange reaction technique to determine the number of active hydrogens (NOAH) in fulvic acid ions using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry is described. First, fulvic acid precursor ions are isolated by stored waveform inverse Fourier transform dipolar excitation. Second, ion−molecule reactions of the selected fulvic acid ions with neutral H/D exchange reagent gases are monitored. The number of incorporated deuterium isotopes in the product ion provides the NOAH for the precursor ion. Previously characterized northern hardwood (NHFA) and red spruce (NCFA) fulvic acid samples were analyzed in this study. Selected ions of both fulvic acid samples undergo H/D exchange with D2O, ND3, and CD3OD reagent gases. The extent of H/D exchange of the fulvic acid ions increases with reagent gas basicity, D2O < CD3OD < ND3. For the first time, we were able to count the NOAH of selected fulvic acid molecules. The average maximum NOAH for NCFA and NHFA ions at m/z region 700−1000 Th is ∼7−9. For example, the singly charged NCFA positive ions at m/z 800 Th contain eight active hydrogens. There is no significant difference between NOAH for NCFA and NHFA. The proton affinities of fulvic acid ions at m/z range of 600−1000 Th do not vary significantly.