Molecular characterization of natural organic matter using complementary liquid chromatography-mass spectrometry and gas chromatography-mass spectrometry techniques

초록

Given the impact of natural organic matter (NOM) on water quality and treatment, understanding its properties is essential in water chemistry. However, the heterogeneous nature of NOM presents analytical challenges. To address these, we employed liquid chromatography-Orbitrap mass spectrometry (LC-Orbitrap MS) and pyrolysis-gas chromatography-MS (Py-GCMS) for the molecular characterization of NOM samples: Suwannee River NOM (SRNOM), Suwannee River humic acid (SRHA), Suwannee River fulvic acid (SRFA), and Aldrich humic acid (HAA). LC-Orbitrap MS, a high-resolution, non-destructive technique, was complemented by Py-GCMS, which characterizes high-molecular-weight (HMW), thermally stable compounds. Differences and complementarities between the methods were examined by comparing results with and without solid-phase extraction (SPE). SPE reduced the total number of detected molecular features by 33.8-68.4%, illustrating a clear trade-off between noise reduction and loss of fractions not retained by the extraction cartridge. LC-Orbitrap MS with SPE provided insight into low-molecular-weight components (<500 Da), revealing higher lipid, protein, and lignin contents than in the non-SPE samples. Conversely, Py-GCMS without SPE detected a broader molecular size and chemical range, including HMW compounds such as lignins and polycyclic aromatic hydrocarbon oxides (PAOs). A comparison of biopolymer group composition revealed that Py-GCMS without SPE followed the trend lignins > carbohydrates (polysaccharides (PS), amino sugars) > proteins > lipids, whereas LC-Orbitrap MS with SPE followed the trend lignins > proteins approximate to lipids > carbohydrates. These results highlight the distinct molecular coverage of each technique and underscore the value of integrating multiple analytical approaches for a comprehensive understanding of NOM.

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