Gas-phase hydrogen/deuterium exchange reactions of fulvic acids: An electrospray ionization Fourier transform ion cyclotron resonance mass spectral study

A gas-phase hydrogen/deuterium (H/D) exchange reaction:technique to determi ne the number of active hydrogens (NOAH) in fulvic acid ions using electros pray ionization Fourier transform ion cyclotron resonance mass spectrometry is described. First, fulvic acid precursor ions are isolated by stored wav eform inverse Fourier transform dipolar excitation. Second, ion-molecule re actions of the selected fulvic acid ions with neutral H/D exchange reagent gases are monitored. The number of incorporated deuterium isotopes in the p roduct ion provides the NOAH for the precursor ion, Previously characterize d northern hardwood (NHFA) and red spruce (NCFA) fulvic acid samples were a nalyzed 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 exchan ge 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 fulv ic acid molecules. The average maximum NOAH for NCFA and NHFA ions at m/z r egion 700-1000 Th is similar to 7-9. For example, the singly charged NCFA p ositive ions at m/z 800 Th contain eight active hydrogens. There is no sign ificant 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.