Electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been widely used for molecular characterization of dissolved organic matter (DOM). However, ESI FT-ICR MS generally has poor repeatability and reproducibility because of its inherent ionization mechanism and structural characteristics, which severely hindered its application in quantitative analysis of complex mixtures. In this article, we developed an in-house standard method for molecular characterization of DOM by ESI FT-ICR MS. Instead of obtaining reproducible results by determining the instrument parameters, we adopted an approach of object control on the mass spectrum to solve the problem of poor reproducibility. The mass peak shape, resolution, and relative intensity distribution of a natural organic matter standard were adjusted by optimizing the operating conditions to obtain a repeatable result. The quality control sample was run 26 times by the different operators in a 6month-long period to evaluate the reproducibility. Results showed that the relative standard deviation (%) of repeatability and reproducibility are 1.02 and 2.35 for average H/C, respectively. The in-house standard method has been validated and successfully used for the characterization of more than 4000 DOM samples, which is transferable to other laboratories.

Molecular composition of dissolved organic matter (DOM) is a hot topic in subjects such as environmental science and geochemistry. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been applied to molecular composition characterization of DOM successfully. However, high instrument and maintenance costs have constrained its wider application. A high-resolution Orbitrap mass spectrometer (Orbitrap MS) can provide approximately 500,000 resolving power (at m/z 200), which is potentially capable of characterizing the molecular composition of DOM. In this paper, the application of highresolution Orbitrap MS was evaluated by comparing with FT-ICR MS in the aspect of resolution, mass distribution, detection dynamic range, and isotopic peak intensity ratio. The impact of instrument parameters of Orbitrap MS was further investigated, which includes ionization, ion transfer, and mass detection. The result shows that the high-resolution Orbitrap MS is capable or even preferable for molecular characterization of DOM. However, the peak intensity distributions are dependent on the instrument parameters, which could affect the environmental impact assessment caused by the sample itself. The result indicates that development of a universal and comparable method is of great demand.

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More than 90% of marine dissolved organic matter (DOM) is biologically recalcitrant. This recalcitrance has been attributed to intrinsically refractory molecules or to low concentrations of molecules, but their relative contributions are a long-standing debate. Characterizing the molecular composition of marine DOM and its bioavailability is critical for understanding this uncertainty. Here, using different sorbents, DOM was solidphase extracted from coastal, epipelagic, and deep-sea water samples for molecular characterization and was subjected to a 180day incubation. 1 H nuclear magnetic resonance spectroscopy and ultra-high-resolution mass spectrometry (UHRMS) analyses revealed that all of the DOM extracts contained refractory carboxyl-rich alicyclic molecules, accompanied with minor biolabile components, for example, carbohydrates. Furthermore, dissolved organic carbon concentration analysis showed that a considerable fraction of the extracted DOM (86−95%) amended in the three seawater samples resisted microbial decomposition throughout the 180-day heterotrophic incubation, even when concentrated threefold. UHRMS analysis revealed that DOM composition remained mostly invariant in the 180-day deep-sea incubations. These results underlined that the dilution and intrinsic recalcitrance hypotheses are not mutually exclusive in explaining the recalcitrance of oceanic DOM, and that the intrinsically refractory DOM likely has a relatively high contribution to the solid-phase extractable DOM in the ocean.

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