Chromatographic Reduction of Isobaric and Isomeric Complexity of Fulvic Acids To Enable Multistage Tandem Mass Spectral Characterization

Humic substances and related material commonly grouped under the designation of natural organic matter (NOM) are of interest in fields ranging from marine chemistry and geochemistry to industry, agriculture, and pharmacology. High-field Fourier transform ion cyclotron resonance mass spectrometry enables resolution and identification of elemental compositions of up to thousands of components from a single mass spectrum. Here, we introduce an offline prefractionation to reduce the number of species of the same nominal (nearest-integer) mass, allowing for isolation of ions of one or a few m/z values, from which structural information can be obtained by low-resolution multistage tandem mass spectrometry (MS"). Alternatively, precharacterized fractions can be generated for other types of analysis. As an example, we demonstrate significant reduction of isomeric and isobaric complexity for Suwannee River fulvic acid (SRFA). The combined MS and MS" analyses support the hypothesis that early eluting material comprises older, highly oxidized SRFA, whereas later eluting material is younger, retaining some similarity with precursor material.