Dissolved organic matter (DOM) plays an important role in the global carbon cycle, and an in-depth analysis of its chemical composition is fundamental to the study of its environmental and biogeochemical behavior and significance. DOM is a complex mixture of organic substances, and determining its molecular composition is a long-standing challenge in the field of analytical chemistry. Chromatography is the most efficient method for characterizing the chemical composition of complex mixtures. However, the molecular composition of DOM is so complex that it is imposable to resolve each indivadual compound in DOM with chromatographic techniques. Currently, DOM is usually characterized in molecular composition by ultra-high resolution mass spectrometry.

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 study, the application of high resolution 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 has advantages for molecular characterization of DOM. Comparing with FT-ICR MS, the Orbitrap MS has a comparable mass resolution power, an acceptable isotope ratio, a more reasonable mass distribution in the lower mass end, and a more rapid signal acquisition. Moreover, Orbitrap MS has good repeatability and reproducibility and is more stable than the FT-ICR MS.

In general, Orbitrap MS is suitable for molecular characterization of DOM in a variety of aquatic systems, which can provide important methodological support for scientific research in the field of geochemistry and has a good application prospect. As for the next step, it is necessary to establish a standard that can be accepted by different laboratories.