Low Molecular Weight Components in an Aquatic Humic Substance As Characterized by Membrane Dialysis and Orbitrap Mass Spectrometry

Suwannee River fulvic acid (SRFA) was dialyzed through a 100–500 molecular weight cutoff dialysis membrane, and the dialysate and retentate were analyzed by UV–visible absorption and high-resolution Orbitrap mass spectrometry (MS). A significant fraction (36% based on dissolved organic carbon) of SRFA passed through the dialysis membrane. The fraction of SRFA in the dialysate had a different UV–visible absorption spectrum and was enriched in low molecular weight molecules with a more aliphatic composition relative to the initial SRFA solution. Comparison of the SRFA spectra collected by Orbitrap MS and Fourier transform ion cyclotron resonance MS (FT-ICR MS) demonstrated that the mass accuracy of the Orbitrap MS is sufficient for determination of unique molecular formulas of compounds with masses