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(n)). 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(n) 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.

Because mass spectrometers provide their own dispersion and resolution of analytes, electrospray ionization mass spectrometry (ESI-MS) has become a workhorse for the characterization of complex mixtures from aerosols to crude oil. Unfortunately, ESI mass spectra commonly contain multimers, adducts and fragments. For the characterization of complex mixtures of unknown initial composition, this presents a significant concern. Mixed-multimer formation could potentially lead to results that bare no resemblance to the original mixture. Conversely, ESI-MS has continually reflected subtle differences between natural organic matter mixtures that are in agreement with prediction or theory. Knowing the real limitations of the technique is therefore critical to avoiding both over-interpretation and unwarranted skepticism. Here, data were collected on four mass spectrometers under a battery of conditions. Results indicate that formation of unrepresentative ions cannot entirely be ruled out, but non-covalent multimers do not appear to make a major contribution to typical natural organic matter spectra based on collision-induced dissociation results. Multimers also appear notably reduced when a cooling gas is present in the accumulation region of the mass spectrometer. For less complex mixtures, the choice of spray solvent can make a difference, but generally spectrum cleanliness (i.e. representativeness) comes at the price of increased selectivity.

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